 This is a LibreVox recording. All LibreVox recordings are in the public domain. For more information and to find out how you can volunteer, please visit LibreVox.org. Recorded by CHIP in Tampa, Florida on January 26, 2006. The origin of species by means of natural selection or the preservation of favorite races in the struggle for life. 6. London Edition by Charles Darwin Chapter 12 Second Section Dispersal during the Glacial Period The identity of many plants and animals on mountain summits separated from each other by hundreds of miles of lowlands where alpine species could not possibly exist is one of the most striking cases known of the same species living at different points without the apparent possibility of their having migrated from one point to another. It is indeed a remarkable fact to see so many plants of the same species living on the snowy regions of the Alps or Pyrenees and in the extreme northern parts of Europe. It is far more remarkable that the plants on the white mountains in the United States of America are all the same with those of Labrador and nearly all the same as we hear from Asa Gray with those on the loftiest mountains in Europe. Even as long ago as 1747 such facts led Gamelon to conclude that the same species must have been independently created at many distinct points and we might have remained in this same belief had not Agassiz and others called vivid attention to the Glacial Period, which as we shall immediately see affords a simple explanation of these facts. We have evidence of almost every conceivable kind, organic and inorganic, that within a very recent geological period central Europe and North America suffered under an arctic climate. The ruins of a house burnt by fire do not tell their tale more plainly than do the mountains of Scotland and Wales with their scoured flanks, polished surfaces and perched boulders of the many icy streams with which their valleys were lately filled. So greatly has the climate of Europe changed that in northern Italy gigantic moraines left by old glaciers are now clothed by the vine and maize. Throughout a large part of the United States erratic boulders and scoured rocks plainly reveal a former cold period. The former influence of the glacial climate on the distribution of the inhabitants of Europe as explained by Edward Forbes is substantially as follows, but we shall follow the changes more readily by supposing a new glacial period slowly to come on and then pass away as formerly occurred. As the cold came on and as each more southern zone became fitted for the inhabitants of the north, these would take the places of the former inhabitants of the temperate regions. The latter at the same time would travel further and further southward unless they were stopped by barriers in which case they would perish. The mountains would become covered with snow and ice and their former alpine inhabitants would descend to the plains. By the time that the cold had reached its maximum we should have an arctic flora and fauna covering the central parts of Europe as far south as the Alps and the Pyrenees and even stretching into Spain. The now temperate regions of the United States would likewise be covered by arctic plants and animals and these would be nearly the same as those of Europe for the present circumpolar inhabitants which we suppose to have travelled everywhere southward are remarkably uniform throughout the world. As the warmth returned the arctic forms would retreat northward closely followed up in their retreat by the productions of the more temperate regions and as the snow melted from the bases of the mountains the arctic forms would seize on the cleared and thawed ground always ascending as the warmth increased and the snow still further disappeared higher and higher whilst their brethren were pursuing their northern journey. Hence when the warmth had fully returned the same species which had lately lived together on the European and North American lowlands would again be found in the arctic regions of the old and new worlds and on many isolated mountain summits far distant from each other. Thus we can understand the identity of many plants at points so immensely remote as the mountains of the United States and those of Europe. We can thus also understand the fact that the alpine plants of each mountain range are more especially related to the arctic forms living due north or nearly due north of them. For the first migration when the cold came on and the re-migration on the returning warmth would generally have been due south and north. The alpine plants for example of Scotland as remarked by Mr. H. C. Watson and those of the Pyrenees as remarked by Ramon are more especially allied to the plants of northern Scandinavia. Those of the United States to Labrador, those of the mountains of Siberia to the arctic regions of that country. These views grounded as they are on the perfectly well ascertained occurrence of a former glacial period seem to me to explain in so satisfactory a manner the present distribution of the alpine and arctic productions of Europe and America that when in other regions we find the same species on distant mountain summits we may almost conclude without other evidence that a colder climate formerly permitted their migration across the intervening lowlands now become too warm for their existence. As the arctic forms moved first southward and afterwards backward to the north in unison with the changing climate they will not have been exposed during their long migrations to any great diversity of temperature and as they all migrated in a body together their mutual relations will not have been much disturbed. Hence in accordance with the principles inculcated in this volume these forms will not have been liable to much modification but with the alpine productions left isolated from the moment of returning warmth first at the bases and ultimately on the summits of the mountains the case will have been somewhat different for it is not likely that all the same arctic species will have been left on mountain ranges far distant from one another and have survived there ever since. They will also in all probability have become mingled with ancient alpine species which must have existed on the mountains before the commencement of the glacial epoch and which during the coldest period will have been temporarily driven down to the plains. They will also have been subsequently exposed to somewhat different climatical influences. Their mutual relations will have been in some degree disturbed consequently they will have been liable to modification and they have been modified for if we compare the present alpine plants and animals of the several great European mountain ranges with one another though many of the species remain identically the same some exist as varieties some as doubtful forms or subspecies and some as distinct yet closely allied species representing each other on several ranges. In the foregoing illustration I have assumed that at the commencement of the imaginary glacial period the arctic productions were as uniform round the polar regions as they are at the present day. But it is also necessary to assume that many sub-arctic and some few temperate forms were the same round the world for some of the species which now exist on the lower mountain slopes and on the plains of North America and Europe are the same. And it may be asked how I account for this degree of uniformity of the sub-arctic and temperate forms around the world at the commencement of the real glacial period. At the present day the sub-arctic and northern temperate productions of the old and new worlds are separated from each other by the whole Atlantic Ocean and by the northern part of the Pacific. During the glacial period when the inhabitants of the old and new worlds lived further southward than they do at the present they must have been still more completely separated from each other by wider spaces of ocean so that it may well be asked how the same species could then or previously have entered the two continents. The explanation I believe lies in the nature of the climate before the commencement of the glacial period. At this the newer Pliocene period the majority of the inhabitants of the world were specifically the same as now and we have good reason to believe that the organisms which now live under latitude 60 degrees lived during the Pliocene period further north under the polar circle in latitude 66 to 67 degrees. And that the present arctic productions then lived on the broken land still nearer to the pole. Now if we look at a terrestrial globe we see under the polar circle that there is almost continuous land from western Europe through Siberia to eastern North America. And this continuity of the circumpolar land with the consequent freedom under a more favorable climate for intermigration will account for the supposed uniformity of the subarctic and temperate productions of the old and new worlds at a period anterior to the glacial epoch. Believing from reasons before alluded to that our continents have long remained in nearly the same relative position, though subjected to great oscillations of level, I am strongly inclined to extend the above view and to infer that during some earlier and still warmer period such as the older Pliocene period a large number of the same plants and animals inhabited the almost continuous circumpolar land and that these plants and animals both in the old and new worlds began slowly to migrate southwards as the climate became less warm long before the commencement of the glacial period. We now see as I believe their descendants mostly in a modified condition in the central parts of Europe and the United States. On this view we can understand the relationship with very little identity between the productions of North America and Europe, a relationship which is highly remarkable considering the distance of the two areas and their separation by the whole Atlantic Ocean. We can further understand the singular fact remarked on by several observers that the productions of Europe and America during the later tertiary stages were more closely related to each other than they are at the present time. For during these warmer periods the northern parts of the old and new worlds will have been almost continuously united by land serving as a bridge since rendered impassable by cold for the intermigration of their inhabitants. During the slowly decreasing warmth of the Pliocene period as soon as the species in common which inhabited the old and new worlds migrated south of the polar circle they will have been completely cut off from each other. This separation as far as the more temperate productions are concerned must have taken place long ages ago as the plants and animals migrated southward. They will have become mingled in the one great region with the native American productions and would have had to compete with them and in the other great region with those of the old world. Consequently we have here everything favorable for much modification, for far more modification than with the Alpine productions, left isolated within a much more recent period on the several mountain ranges and on the arctic lands of Europe and North America. Hence it has come that when we compare the now living productions of the temperate regions of the new and old worlds we find very few identical species, though Asa Gray has lately shown that more plants are identical than was formerly supposed. But we find in every great class many forms which some naturalists rank as geographical races and others as distinct species and a host of closely allied or representative forms which are ranked by all naturalists as specifically distinct. As on the land so in the waters of the sea a slow southern migration of a marine fauna which through the Pliocene or even a somewhat earlier period was nearly uniform along the continuous shores of the polar circle will account on the theory of modification for many closely allied forms now living in marine areas completely sundered. Thus I think we can understand the presence of some closely allied, still existing and extinct tertiary forms on the eastern and western shores of temperate North America, and still more striking fact of many closely allied crustaceans as described in Dana's admirable work, some fishes and other marine animals inhabiting the Mediterranean and the seas of Japan. These two areas being now completely separated by the breadth of a whole continent and by wide expanses of ocean. These cases of close relationship in species either now or formerly inhabiting the seas of the western and eastern shores of North America, the Mediterranean and Japan and the temperate lands of North America and Europe are inexplicable on the theory of creation. We cannot maintain that such species have been created alike in correspondence with the nearly similar physical conditions of the areas for if we compare for instance certain parts of South America with parts of South Africa or Australia we see conditions closely similar in all their physical conditions yet their inhabitants utterly dissimilar. Alternate glacial periods in the North and South. But we must return to our more immediate subject. I am convinced that forms of you may be largely extended. In Europe we meet with the plainest evidence of the glacial period from the western shores of Britain to the Euro range and southward to the Pyrenees. We may infer from the frozen mammals and nature of the mountain vegetation that Siberia was similarly affected. In the Lebanon according to Dr. Hooker perpetual snow formerly covered the central axis and fed glaciers which rolled 400 feet down the valleys. The same observer has recently found great moraines at a low level of the Atlas range in North Africa. Along the Himalaya at points 900 miles apart glaciers have left their marks of the former low descent. And in Sakeem Dr. Hooker saw nays growing on ancient and gigantic moraines. Southward of the Asiatic continent on the opposite side of the equator we know from the excellent researches of Dr. J. Haast and Dr. Hector that in New Zealand immense glaciers formerly descended to a low level and the same plants found by Dr. Hooker on widely separated mountains in this island tell the same story of a former cold period. From the facts communicated to me by the Reverend W. B. Clark it appears also that there are traces of former glacial action on the mountains of the southeastern corner of Australia. Looking to America in the northern half ice-born fragments of rock have been observed on the eastern side of the continent as far south as latitude 36 and 37 degrees and on the shores of the Pacific where the climate is now so different as far south as latitude 46 degrees. Erratic boulders have also been noticed on the Rocky Mountains. In the Cordillera of South America nearly under the equator glaciers once extend far below their present level. In Central Chile I have examined a great mound of detritus with vast boulders crossing the Portillo Valley which there can hardly be a doubt once formed a huge moraine. And Mr. D. Forbes informs me that he has found in various parts of the Cordillera from latitude 13 through 30 degrees south at about the height of 12,000 feet deeply furrowed rocks resembling those with which he was familiar in Norway and likewise great masses of detritus including grooved pebbles. Along this whole space of the Cordillera true glaciers do not now exist even at much more considerable heights. Further south on both sides of the continent from latitude 41 degrees to the southernmost extremity we have the clearest evidence of formal glacier action in numerous immense boulders transported far from their parent source. From these several facts namely from the glacial action having extended all around the northern and southern hemispheres from the period having been in a geological sense recent in both hemispheres from its having lasted in both during a great length of time as may be inferred from the amount of work affected and lastly from glaciers having recently descended to a low level along the whole line of the Cordillera. Yet at one time appeared to me that we could not avoid the conclusion that the temperature of the whole world had been simultaneously lowered during the glacial period. But now Mr. Kroll in a series of admirable memoirs has attempted to show that a glacial condition of climate is the result of various physical causes brought into operation by an increase in the eccentricity of the Earth's orbit. All of these causes tend toward the same end but the most powerful appears to be the indirect influence of the eccentricity of the orbit upon oceanic currents. According to Mr. Kroll cold periods regularly recur every 10 to 15,000 years and these at long intervals are extremely severe owing to certain contingencies of which the most important as Sir C. Lyell has shown is the relative position of the land and water. Mr. Kroll believes that the last great glacial period occurred about 240,000 years ago and endured with slight alterations of climate for about 160,000 years. With respect to more ancient glacial periods several geologists are convinced from direct evidence that such occurred during the Miocene and Eocene formations not to mention still more ancient formations. But the most important result for us arrived at by Mr. Kroll is that whenever the northern hemisphere passes through a cold period the temperature of the southern hemisphere is actually raised with the winters rendered much milder chiefly through changes in the direction of the ocean currents. So conversely it will be with the northern hemisphere while the southern passes through a glacial period. This conclusion throws so much light on geographical distribution that I am strongly inclined to trust in it but I will first give the facts which demand explanation. In South America Dr. Hooker has shown that besides many closely allied species between 40 and 50 of the flowering plants of Tierra del Fuego forming no inconsiderable part of its scanty flora are common to North America and Europe enormously remote as these areas in opposite hemispheres are from each other. On the lofty mountains of equatorial America a host of peculiar species belonging to European genera occur. On the organ mountains of Brazil some few temperate European some Antarctic and some Andean genera were found by Gardner which do not exist in the low interbeening hot countries. On the Silla of Caracas the illustrious Humboldt long ago found species belonging to the genera characteristic of the Cordillera. In Africa several forms characteristic of Europe and some few representatives of the flora of the Cape of Good Hope occur on the mountains of Abyssinia. At the Cape of Good Hope a very few European species believed not to have been introduced by man and on the mountains several representative European forms are found which have not been discovered in the intertropical parts of Africa. Dr. Hooker has also lately shown that several of the plants living in the upper parts of the lofty island of Fernando Poo and on the neighboring Cameroon Mountains in the gulf of Guinea are closely related to those on the mountains of Abyssinia and likewise to those of temperate Europe. It also now appears as I hear from Dr. Hooker that some of these same temperate plants have been discovered by the Reverend R. T. Low on the mountains of the Cape Verde Islands. This extension of the same temperate forms almost under the equator across the whole continent of Africa and to the mountains of the Cape Verde archipelago is one of the most astonishing facts ever recorded in the distribution of plants. On the Himalaya and on the certain isolated mountain ranges of the peninsula of India on the heights of Ceylon and on the volcanic cones of Java many plants occur either identically the same or representing each other and at the same time representing plants of Europe not found in the intervening hot lowlands. A list of the genera of plants collected on the loftier peaks of Java raises a picture of the collection made on a hillock in Europe. Still more striking is the fact that peculiar Australian forms are represented by certain plants growing on the summits of the mountains of Borneo. Some of these Australian forms as I hear from Dr. Hooker extend along the heights of the peninsula of Malacca and are thinly scattered on the one hand over India and on the other hand as far north as Japan. On the southern mountains of Australia Dr. F. Muller has discovered several European species other species not introduced by man occur on the lowlands and a long list can be given as I am informed by Dr. Hooker of European genera found in Australia but not in the intermediate toroid regions. In the admirable introduction to the flora of New Zealand by Dr. Hooker analogous and striking facts are given in regards to the plants of that large island hence we see that certain plants growing on the temperate plains of the north and south are either the same species or varieties of the same species. It should however be observed that these plants are not strictly arctic forms for as Mr. H. C. Watson has remarked in receding from polar toward equatorial latitudes the alpine or mountain flora really become less and less arctic. Besides these identical and closely aligned forms many species inhabiting the same widely sundered areas belonging to genera not found in the intermediate tropical lowlands. These brief remarks apply to plants alone but some few analogous facts could still be given in regards to terrestrial animals. In marine productions similar cases likewise occur as an example I may quote a statement by that highest authority professor Dana that it is certainly a wonderful fact that New Zealand should have a closer resemblance in its crustacea to Great Britain its antipode than to any other part of the world. Sir J. Richardson also speaks of the reappearance on the shores of New Zealand Tasmania etc. of northern forms of fish. Dr. Hooker informs me that 25 species of algae are common to New Zealand and to Europe but they have not been found in the intermediate tropical seas. From the foregoing facts namely the presence of temperate forms on the highlands across the whole of equatorial Africa and along the peninsula of India to Ceylon and the Mele archipelago and in less well marked manners across the wide expanse of tropical South America it appears almost certain that some former period no doubt during the most severe part of a glacial period that the lowlands of these great continents were everywhere tenanted under the equator by a considerable number of temperate forms. At this period the equatorial climate at that level of the sea was probably about the same with what is now experienced at a height of from five to six thousand feet under the same latitude or perhaps even rather cooler. During this the coldest period the lowlands under the equator must have been closed with a mingled tropical and temperate vegetation like that described by Hooker as growing luxuriously at the height of from four to five thousand feet on the lower slopes of the Himalaya but with perhaps a still greater preponderance of temperate forms. So again in the mountainous island of Fernando Poo in the Gulf of Guinea Mr. Mann found temperate European forms beginning to appear at the height of about five thousand feet. On the mountains of Panama at the height of only two thousand feet Dr. Seaman found the vegetation like that of Mexico with forms of the torrid zone harmoniously blended with those of the temperate. Now let us see whether Mr. Cole's conclusion that the northern hemisphere suffered from the extreme cold of the great glacial period the southern hemisphere was actually warmer throws any clear light on the present apparently inexplicable distribution of various organisms in the temperate parts of both hemispheres and on the mountains of the tropics. The glacial period as measured by years must have been very long and when we remember over what vast spaces some naturalized plants and animals have spread within a few centuries this period will have been ample for any amount of migration. As the cold became more and more intense we know that arctic forms invaded the temperate regions and from the facts just given there can hardly be a doubt that some of the more vigorous dominant and widest spreading temperate forms invaded the equatorial lowlands. The inhabitants of these hot lowlands would have at the same time migrated to the tropical and subtropical regions of the south for the southern hemisphere was at this period warmer. On the decline of the glacial period as both hemispheres gradually recovered their former temperature the northern temperate forms living on the lowlands under the equator would have been driven from their former homes or have been destroyed. Being replaced by the equatorial forms returning from the south. Some however of the northern temperate forms would almost certainly have ascended any adjoining highland where if sufficiently lofty they would have long survived like the arctic forms on the mountains of Europe. They might have survived even if the climate were not perfectly fitted for them for the change of temperature must have been very slow and plants undoubtedly possess a certain capacity for a climatization as shown by their transmitting to their offspring different constitutional powers of resisting heat and cold. In the regular course of events the southern hemisphere would in its turn be subjected to a severe glacial period with the northern hemisphere rendered warmer and then the southern temperate forms would invade the equatorial lowlands. The northern forms which had before been left on the mountains would now descend and mingle with the southern forms. The latter when the warmth return would return to their former homes leaving some few species on the mountains and carrying southward with them some of the northern temperate forms which had descended from their mountain fastness. Thus we should have some few species identically the same in the northern and southern temperate zones and on the mountains of the intermediate tropical regions. But the species left during a long time on these mountains or in opposite hemispheres would have to compete with many new forms and would be exposed to somewhat different physical conditions. Hence they would be eminently liable to modification and would generally now exist as varieties or as representative species and this is the case. We must also bear in mind the occurrence in both hemispheres of former glacial periods for these will account in accordance with the same principles for the many quite distinct species inhabiting the same widely separated areas and belonging to genera not now found in the intermediate torrid zones. It is a remarkable fact strongly insisted on by Hooker in regard to America and by Alphonse de Candole in regard to Australia that many more identical or slightly modified species have migrated from the north to the south than in a reversed direction. We see however a few southern forms on the mountains of Borneo and Abyssinia. I suspect that this preponderant migration from the north to the south is due to the greater extent of land in the north and to the northern forms having existed in their own homes in greater numbers and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power than the southern ones. And thus when the two sets became co-mingled in the equatorial regions during the alternations of the glacial periods the northern forms were the more powerful and were able to hold their places on the mountains and afterwards migrate southwards with the southern forms but not so the southern in regard to the northern forms. In the same manner at the present day we see that very many European productions cover the ground in La Plata, New Zealand and to a lesser degree in Australia and have beaten the natives whereas extremely few southern forms have become naturalized in any part of the northern climate sphere though hides, wool and other objects likely to carry seeds have been largely imported to Europe during the last two or three centuries from La Plata and during the last forty or fifty years from Australia. The Nilgiri Mountains in India however offer a partial exception for here as I hear from Dr. Hooker Australian forms are rapidly sowing themselves and becoming naturalized. Before the last great glacial period no doubt the intertropical mountains were stocked with more endemic alpine forms but these have almost everywhere yielded to the more dominant forms generated in the larger areas and more efficient workshops of the north. In many islands the native productions are nearly equaled or even outnumbered by those which have become naturalized and this is the first step toward their extinction. Mountains are islands on the land and their inhabitants have yielded to those produced within the larger areas of the north just in the same way as the inhabitants of real islands have everywhere yielded and are still yielding to continental forms naturalized. The same principles apply to the distribution of terrestrial animals and to marine productions in the northern and southern temperate zones and on the intertropical mountains. When during the height of the glacial period the ocean currents were widely different from what they are now some of the inhabitants of the temperate seas might have reached the equator. Of these a few would perhaps at once be able to migrate southwards by keeping into the cooler currents while others might remain and survive in the colder depths until the southern atmosphere was in its turn subjected to a glacial climate and permitted their further progress. In nearly the same manner as according to Forbes isolated species inhabited by Arctic productions exist to the present day in the deeper parts of the northern temperate seas. I am far from supposing that all the difficulties in regard to the distribution and affinities of the identical and allied species which now live so widely separated in the northern south than sometimes on the intermediate mountain ranges are removed on the views above given. The exact lines of migration cannot be indicated. We cannot say why certain species and not others have migrated, why certain species have been modified and have given rise to new forms while others have remained unaltered. We cannot hope to explain such facts until we can say why one species and not another becomes naturalized by man's agency in a foreign land, why one species ranges twice or thrice as far and is twice or thrice as common as other species within their own homes. Various special difficulties also remain to be solved. For instance, the occurrence as shown by Dr. Hooker of the same plants so enormously remote as Kyrgylline land in New Zealand and Fuegoia, but icebergs, as suggested by Lyell, might have been concerned in their dispersal. The existence at these and other distant points of the southern hemisphere of species which, though distinct, belong to genera exclusively confined to the south is a more remarkable case. Some of these species are so distinct that we cannot suppose that there has been a time since the commencement of the last glacial period for their migration and subsequent modification to the necessary degree. These facts seem to indicate that distinct species belonging to the same genera have migrated in radiating lines from a common center, and I am inclined to look in the southern as in the northern hemisphere to a former and warmer period before the commencement of the last glacial period when the Antarctic lands now covered with ice supported a highly peculiar and isolated flora. It may be suspected that before this flora was exterminated during the last glacial epoch a few forms had been already widely dispersed to various points of the southern hemisphere by occasional means of transport and by the aid as halting places of now sunken islands. Thus the southern shores of America, Australia and New Zealand might have become slightly tinted by the same peculiar forms of life. Sir C. Lyell in a striking passage has speculated, in language almost identical with mine, on the effects of great alternations of climate throughout the world on geographic distribution. And we have now seen that Mr. Kroll's conclusion that successive glacial periods in the one hemisphere coincide with warmer periods in the opposite hemisphere together with the admission of the slow modification of species explains a multitude of facts in the distribution of the same and allied forms of life in all parts of the globe. The living waters have flowed during one period from the north and during another from the south and in both cases have reached the equator, but the stream of life has flowed with greater force from the north than in the opposite direction and has consequently more freely inundated the south. As the tide leaves its drift in horizontal lines, rising higher on the shores where the tide rises highest, so have the living waters left their living drift on our mountain summits in a line gently rising from the Arctic lowlands to a great latitude under the equator. The various beings thus left stranded may be compared with savage races of man driven up and surviving in the mountain fastness of almost every land which serves as a record full of interest to us of the former inhabitants of the surrounding lowlands. So ends Chapter 12 of The Origin of Species by Charles Darwin. This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information and to find out how you can volunteer, please visit LibriVox.org. Recorded by Chip in Tampa, Florida on February 1, 2006. The Origin of Species by means of natural selection or the preservation of favorite races in the struggle for life. Sixth London Edition by Charles Darwin. Chapter 13 Geographical Distribution Continued Distribution of freshwater productions on the inhabitants of oceanic islands, absence of Batrachians and of terrestrial mammals, on the relation of the inhabitants of islands to those of the nearest mainland, on colonization from the nearest sources with subsequent modification, and summary of the last and present chapters. Freshwater Productions As lakes and river systems are separated from each other by barriers of land, it might have been thought that freshwater productions would not only have ranged widely within the same country and as the sea is apparently a still more formidable barrier that they would never have extended to distant countries. But the case is exactly the reverse. Not only have many freshwater species belonging to different classes an enormous range, but allied species prevail in a remarkable manner throughout the world. When first collecting in the fresh waters of Brazil, I well remembered feeling much surprise at the similarity of the freshwater insects, shells, etc., and at the dissimilarity of the surrounding terrestrial beings compared with those of Britain. But the wide-ranging power of freshwater productions can, I think in most cases, be explained by their having become fitted in a manner highly useful to them for short and frequent migrations from pond to pond or from stream to stream within their own countries, and liability to wide dispersal would follow from this capacity as an almost necessary consequence. We can here consider only a few cases of these, some of the most difficult to explain are presented by fish. It was formerly believed that the same freshwater species never existed unto continents distant from each other. But Dr. Gunther has lately shown that the Galaxius Antenuatus inhabits Tasmania, New Zealand, the Falkland Islands, and the mainland of South America. This is a wonderful case, and probably indicates dispersal from an Antarctic center during a formerly warm period. This case, however, is rendered in some degree less surprising by the species of this genus having the power of crossing by some unknown means, considerable spaces of open ocean. Thus there is one species common to New Zealand and to the Auckland Islands, although separated by a distance of about 230 miles. On the same continent freshwater fish often range widely, and as if capriciously, for in two adjoining river systems some of the species may be the same, and some wholly different. It is probable that they are occasionally transported by what may be called accidental means. Thus fishes still alive are not very rarely dropped at distant points by whirlwinds, and it is known that the ova retain their vitality for a considerable time after removal from the water. Their dispersal may, however, be mainly attributed to changes in the level of land within the recent period, causing rivers to flow into one another. Instances also could be given that this having occurred during floods without any change of level. The wide differences of the fish on the opposite sides of most mountain ranges, which are continuous and consequently must from an early period have completely prevented the inosculation of the river systems on the two sides, leads to the same conclusion. Some freshwater fish belong to very ancient forms, and in such cases there will have been ample time for great geographical changes, and consequently time and means for much migration. Moreover, Dr. Gunther has been recently led by several conclusions to infer that with fishes the same forms have a long endurance. Saltwater fish can, with care, be slowly accustomed to live in freshwater, and according to Valenciennes there is hardly a single group of which all the members are confined to freshwater, so that a marine species belonging to a freshwater group might travel far along the shores of the sea, and could, it is probable, become adapted without too much difficulty to the fresh waters of a distant land. Some species of freshwater shells have very wide ranges, and allied species which, on our theory, are descended from a common parent, and must have proceeded from a single source, prevail throughout the world. Their distribution at first perplexed me, much as their ova are not likely to be transported by birds, and the ova as well as the adults are immediately killed by saltwater. I could not even understand how some naturalized species have spread rapidly throughout the same country, but two facts which I have observed, and many others no doubt will be discovered, throw some light on this subject. When ducks suddenly emerge from a pond covered with duckweed, I have twice seen these little plants adhering to their backs, and it has happened to me in removing a little duckweed from one aquarium to another that I have unintentionally stocked the one with freshwater shells from the other. But another agency is perhaps more effectual. I suspended the feet of a duck in an aquarium, where many ova of freshwater shells were hatching, and found that numbers of the extremely minute and just-hatched shells crawled on the feet, and clung to them so firmly that, when taken out of the water, they could not be jarred off, though at a somewhat more advanced age they would voluntarily drop off. These just-hatched mollusks, though aquatic in their nature, survived on the duck's feet in damp air from twelve to twenty hours, and in this length of time a duck or heron might fly at least six or seven hundred miles, and if blown across the sea to an oceanic island, or to any other distant point would be sure to alight on a pool or rivulet. Sir Charles Lyell informs me that a dictius has been caught with an ankylos, a freshwater shell like a limpet, firmly adhering to it. And a water beetle of the same family, a colombides, once flew aboard the beagle, when forty-five miles distant from the nearest land. How much farther it might have been blown by a favoring gale? No one can tell. With respect to plants, it has long been known what enormous ranges many freshwater and even marsh species have, both over continents and to the most remote oceanic islands. This is strikingly illustrated, according to Alphonse de Condole, in those large groups of terrestrial plants which have very few aquatic members, for the latter seem immediately to acquire, as if in consequence, a wide range. I think favorable means of dispersal explain this fact. I have mentioned before that earth occasionally adheres in some quantity to the feet and beaks of birds, wading birds which frequent these muddy edges of ponds, if suddenly flushed, would be the most likely to have muddy feet. Birds of this order wander more than those of any other, and are occasionally found on the most remote and barren islands of the open ocean. They would not be likely to alight on the surface of the sea, so that any dirt their feet would not have been washed off, and when gaining the land they would be sure to fly to their natural freshwater haunts. I do not believe that botanists are aware how charged the mud of ponds is with seeds. I have tried several little experiments, but will here give only the most striking case. I took in February three tablespoons full of mud from three different ponds, beneath water, on the edge of a little pond. This mud, when dry, weighed only seven and three-quarter ounces. I kept it covered up in my study for six months, pulling up and counting each plant as it grew. The plants were of many kinds, and altogether five hundred thirty-seven in number, and yet the viscid mud was all contained in a breakfast cup. Considering these facts, I think it would be an inexplicable circumstance if waterbirds did not transport the seeds of freshwater plants to unstocked ponds and streams situated at very distant points. This same agency may have come into play with the eggs of some of the smaller freshwater animals. Other and unknown agencies probably also have played a part. I have stated that freshwater fish eat some kinds of seeds, though they reject many other kinds after having swallowed them. Even small fish swallow seeds of moderate size as of the yellow water lily and the tumigaton. Herons and other birds, century after century, have gone on daily devouring fish. They then take flight and go on to other waters or are blown across the sea, and we have seen that seeds retain their power of germination when rejected many hours afterwards in pellets or in the excrement. When I saw the great size of the seeds of the fine water lily, the nebulum, I remembered Alphonse de Candle's remarks on the distribution of this plant, and I thought that the means of its dispersal must remain inexplicable. But Audubon states that he found the seeds of the great southern water lily, probably according to Dr. Hooker, the nebulum luteum, in a heron's stomach. Now this bird must often have flown with its stomach thus well stocked to distant ponds, and then, getting a hearty meal of fish, analogy makes me believe that it would have rejected the seeds in the pellet in a fit state for germination. In considering these several means of distribution, it should be remembered that when a pond or stream is first formed, for instance on a rising islet, it will be unoccupied, and a single seed or egg will have a good chance of succeeding, although there will always be a struggle for life between the inhabitants of the same pond. However, few in kind, yet as the number even in a well stocked pond is small in comparison with the number of species inhabiting an equal area of land, the competition between them will probably be less severe than between terrestrial species. Consequently, an intruder from the waters of a foreign country would have a better chance of seizing on a new place than is the case in terrestrial colonists. We should also remember that many freshwater productions are low in the scale of nature, and have reason to believe that such beings become modified more slowly than the high, and this will give time for the migration of aquatic species. We should not forget the probability of many freshwater forms having formerly ranged continuously over immense areas and then having become extinct at intermediate points. But the wide distribution of freshwater plants and of the lower animals, whether retaining the same identical form or in some degree modified, apparently depends in main part on the wide dispersal of their seeds and eggs by animals, more especially by freshwater birds, which have the powers of flight and naturally travel from one place of water to another. On the Inhabitance of Oceanic Islands We now come to the last of the three classes of facts which I have selected as presenting the greatest amount of difficulty with respect to distribution on the view that not only all of the individuals of the same species have migrated from some one area, but that allied species, although now inhabiting the most distant points, have preceded from a single area the birth pace of their early virginity. I have already given my reasons for disbelieving in continental extensions within a period of existing species on so enormous a scale that all the many islands of the several oceans were thus stocked with their present terrestrial inhabitants. This view removes many difficulties, but it does not accord with all the facts in regard to the productions of the islands. In the following remarks I shall not confine myself to the mere question of dispersal, but shall consider some other cases bearing on the truth of the two theories of independent creation and of descent with modification. The species of all kinds, which inhabit oceanic islands, are few in number compared with those on equal continental areas. Alphos de Candol admits this for plants and Wallaston for insects. New Zealand, for instance, with its lofty mountains and diversified stations extending over 780 miles of latitude, together with the outlying islands of Auckland, Campbell and Chatham, contain altogether only 960 kinds of flowering plants. If we compare this moderate number with the species which swarm over equal areas in southwestern Australia, or at the Cape Good Hope, we must admit that some cause independently of different physical conditions has given rise to so great a difference in number. Even the Uniform County of Cambridge has 847 plants, and the little island of Angelsea 764. But a few ferns and a few introduced plants are included in these numbers, and the comparison in some other respects is not quite fair. We have evidence that the barren island of Asansion aboriginally possessed less than half a dozen flowering plants, and yet many species have now become naturalized on it, as they have in New Zealand and on every other oceanic island which can be named. In St. Helena there is reason to believe that naturalized plants and animals have nearly, or quite, exterminated many native populations. He who admits the doctrine of the creation of each separate species will have to admit that a sufficient number of the best adapted plants and animals were not created for oceanic islands, for man has unintentionally stocked them far more fully and perfectly than did nature. Although in oceanic islands the species are few in number, the proportion of endemic kinds, i.e. those found nowhere else in the world, is often extremely large. If we compare, for instance, the number of endemic land shells in Madeira, or of endemic birds in the Galapagos archipelago, with the number found on any continent, and then compare the area of the island with that of the continent, we shall see that this is true. This fact might have been theoretically expected for, as already explained, species occasionally arriving after long intervals of time in the new and isolated district and having come to compete with new associates would be eminently liable to modification and would often produce groups of modified descendants. But it by no means follows that, because in an island nearly all of the species of one class are peculiar, those of another class or of another section of the same class are peculiar, and this difference seems to depend partly on the species which are not modified having immigrated in a body, so that their mutual relations have not been much disturbed, and partly on the frequent arrival of unmodified immigrants from the mother country with which the insular forms have been intercrossed. It should be borne in mind that the offspring of such crosses would certainly gain in vigor so that even an occasional cross would produce more effect than might have been anticipated. I will give a few illustrations of the foregoing remarks. In the Galapagos Islands there are twenty-six land birds. Some of these twenty-one or perhaps twenty-three are peculiar, whereas of the eleven marine birds only two are peculiar, and it is obvious that marine birds could arrive at these islands much more easily and frequently than land birds. Bermuda, on the other hand, which lies at about the same distance from North America as the Galapagos do from South America, and which has a very peculiar soil, does not possess a single endemic land bird, and we know from J. M. Jones' admirable account of Bermuda that very many North American birds occasionally or even frequently visit this island. Almost every year as I am informed by Mr. E. V. Harcourt, many European and African birds are blown to Madeira. This island is inhabited by ninety-nine kinds of which one alone is peculiar, though very closely related to a European form, and three or four other species are confined to this island and to the Canaries, although that the islands of Bermuda and Madeira have been stocked from neighbouring continents with birds, which for long ages have there struggled together and have become mutually co-adapted. Hence when settled in their new homes each kind will have been kept by the others to its proper place and habits, and will consequently have been but little liable to modification. The tendency to modification will also have been checked by intercrossing with the unmodified immigrants, often arriving from the mother country. Madeira again is inhabited by a wonderful number of peculiar land-shells, whereas not one species of seashell is peculiar to its shores. Now, though we do not know how seashells are dispersed, yet we can see that their eggs or larvae perhaps attached to seaweed or floating timber or to the feet of wading birds might be transported across three or four hundred miles of open sea far more easily than land-shells. The different orders of insects inhabiting Madeira present nearly parallel cases. Oceanic islands are sometimes deficit in animals of certain whole classes, and their places are occupied by other classes. Thus in the Galapagos Islands reptiles and in New Zealand gigantic wingless birds take or recently took the place of mammals. Although New Zealand is here spoken of as an oceanic island, it is in some degree doubtful whether it should be so ranked. It is of large size, and it is not separated from Australia by a profoundly deep sea. From its geological character and the direction of its mountain ranges, the Reverend W. B. Clark has lately maintained that this island, as well as New Caledonia, should be considered as a pertinence of Australia. Turning to plants, Dr. Hooker has shown that in the Galapagos Islands the proportional numbers of the different orders are very different from what they are elsewhere. All such differences in number and the absence of certain whole groups of animals and plants are generally accounted for by supposed differences in the physical conditions of the islands. But this explanation is not a little doubtful. Facility of immigration seems to be fully as important as the nature of the conditions. Many remarkable little facts could be given with respect to the inhabitants of oceanic islands. For instance, in certain islands, not tenanted by a single mammal, some of the endemic plants have beautifully hooked seeds, yet few relations are more manifest than that hooks serve for the transportation of seeds in the wool or fur of quadrupeds. But a hooked seed might be carried to an island by another means. The plant there becoming modified would form an endemic species, still retaining its hooks, which would form a useless appendage, like the shriveled wings under the soldered wing covers of many insular beetles. Again, the islands often possess trees or bushes belonging to orders which elsewhere include only herbaceous trees. Now trees, as Alphons de Candol has shown, generally have, whatever their cause may be, confined ranges. Hence trees would be little likely to reach distant oceanic islands, and an herbaceous plant which had no chance of successfully competing with the many fully developed trees growing on a continent might, when established on an island, gain an advantage over other herbaceous plants by growing taller and taller and overtopping them. In this case, natural selection would tend to add to the stature of the plant, to whatever order it belonged, and thus first convert it into a bush, and then into a tree. Absence of Betrations and Terrestrial Mammals on Oceanic Islands With respect to the absence of whole orders of animals on oceanic islands, Boris St. Vincent long ago remarked that betrations—frogs, toads, nudes, etc.—are never found on any of the main islands with which the great oceans are studded. I have taken pains to verify this assertion and have found it true, with the exception of New Zealand, New Caledonia, and the Ataman Islands, and perhaps the Solomon Islands and the Seychelles. But I have already remarked that it is doubtful whether New Zealand and New Caledonia ought to be classed as oceanic islands, and this is still more doubtful with respect to the Andaman and Solomon groups and the Seychelles. This general absence of frogs, toads, and nudes on so many true oceanic islands cannot be accounted for by their physical conditions. Indeed, it seems that islands are peculiarly fitted for these animals, for frogs have been introduced into Madeira, the Azores, and Mauritius, and have multiplied so as to become a nuisance. But as these animals and their own spawn are immediately killed, with the exception, as far as known, of one Indian species, by seawater, there would be great difficulty in their transportal across the sea, and therefore we can see why they do not exist on strictly oceanic islands. Mammals offer another and similar case. I have carefully searched the oldest voyages and have not found a single incidence, free from doubt, of a terrestrial mammal excluding domesticated animals kept by the natives. Inhabiting an island situated above 300 miles from a continent or great continental island, and many islands situated at a much less distance are equally barren. The Falkland Islands, which are inhabited by a wolf-like fox, comes nearest to an exception, but this group cannot be considered as oceanic as it lies on a bank in connection with the mainland of a distance of about 280 miles. Moreover, icebergs formerly brought boulders to its western shores, and they may have formerly transported foxes, as now frequently happens in the Arctic regions. Yet it cannot be said that small islands will not support at least small mammals, for they occur in many parts of the world on very small islands, and when lying close to a continent, and hardly an island can be named on which our smaller quarter-beds have not become naturalized and greatly multiplied. It cannot be said, on the ordinary view of creation, that there has not been time for the creation of mammals. Many oceanic islands are sufficiently ancient, as shown by the stupendous degradation they have suffered, and by their tertiary strata. There has also been time for the production of endemic species belonging to other classes, and on continents it is known that new species of mammals appear and disappear at a quicker rate than other and lower animals. Although terrestrial mammals do not occur on oceanic islands, aerial mammals do occur on almost every island. New Zealand possesses two bats found nowhere else in the world. Norfolk Island, the VT Archipelago, the Bonin Islands, the Caroline and Marianne Archipelagos, and Mauritius all possess their peculiar bats. Why, it may be asked, has this supposed creative force produced bats and no other mammals on remote islands? On my view this question can easily be answered, for no terrestrial mammal can be transported across a wide space of sea, but bats can fly across. Bats have been seen wandering by day far over the Atlantic Ocean, and two North American species, either regularly or occasionally, visit Bermuda at a distance of 600 miles from the mainland, and I hear from Mr. Tomes, who has specially studied this family, that many species have enormous ranges and are found on continents and far distant islands. Hence we have only to suppose that such wandering species have been modified in their new homes in relation to their new position, and we can understand the presence of endemic bats on oceanic islands with the absence of all other terrestrial animals. Another interesting relation exists between the depth of the sea separating islands from each other, or from the nearest continent, and the degree of affinity of their mammalian inhabitants. Mr. Windsor Earl has made some striking observations on this head, since greatly extended by Mr. Wallace's admirable researches in regard to the great melee archipelago, which is transversed near the Celebes by a space of deep ocean, and this separates two widely distinct mammalian faunas. On either side the islands stand on a moderately shallow submarine bank, and these islands are inhabited by the same or by closely allied quadrupeds. I have not as yet had time to follow up on this subject in all quarters of the world, but as far as I have gone the relation holds good. For instance Britain is separated by a shallow channel from Europe, and the mammals are the same on both sides, so it is with all the islands near the shores of Australia. The West Indian islands on the other hand stand on a deeply submerged bank, nearly one thousand fathoms in depth, and here we find American forms, but the species and even the genera are quite distinct. As the amount of modification which animals of all kinds undergo partly depends on the lapse of time, and as the islands which are separated from each other or from the mainland by shallow channels are more likely to have been continuously united within a recent period than the islands separated by deeper channels, we can understand how it is that a relation exists between the depth of the sea separating two mammalian faunas and the degree of their affinity, a relation which is quite inexplicable on the theory of independent acts of creation. The foregoing statements in regard to the inhabitants of oceanic islands, namely the fewness of the species with a large proportion consisting of endemic forms, the members of certain groups but not those of other groups in the same class having been modified, the absence of certain whole orders as of the bacterians and of terrestrial animals notwithstanding the presence of aerial bats, the singular proportions of certain orders of plants, herbaceous forms having been developed into trees, etc., seemed to me to accord better with the belief in the efficiency of occasional means of transport carried on during a long course of time than with the belief in the former connection of all oceanic islands with the nearest continent. For on this latter view it is probable that the various creatures would have immigrated more uniformly and from the species having entered as a body their mutual relations would not have been much disturbed and consequently they would either not have been modified or all of the species in a more equitable manner. I do not deny that there are many and serious difficulties in understanding how many of the inhabitants of the more remote islands, whether still retaining some of the specific form or subsequently modified, have reached their present homes, but the probability of other islands having once existed as a halting place of which not a wreck now remains must not be overlooked. I will specify one difficult case. Almost all oceanic islands, even the most isolated and smallest, are inhabited by land-shells, generally by endemic species but sometimes by species bound elsewhere, striking instances of which have been given by Dr. A. A. Gould in relation to the Pacific. Now it is notorious that land-shells are easily killed by sea-water. Their eggs, at least such as I have tried, sink in it and are killed. Yet there must be some unknown but occasionally efficient means for their transportal. Were the just hatched young sometimes adhered to the feet of birds roosting on the ground and thus get transported? It occurred to me that land-shells, when hibernating and having a membranous diaphragm over the mouth of the shell, might be floated in chinks of drifted lumber across moderately wide arms of the sea. And I find that several species in this state withstand uninjured an immersion in sea-water during seven days. One shell, the helix pomacea, after having been thus treated and again hibernating, was put into sea-water for twenty days and perfectly recovered. During this length of time the shell might have been carried by a marine current of average swiftness to a distance of 660 geographical miles. As this helix has a thick cal-seris uber-culum, I removed it, and when it had formed a new membranous one I again immersed it for fourteen days in sea-water, and again it recovered and crawled away. Baron Ochapitan has since tried similar experiments. He placed one hundred land-shells, belonging to ten species, in a box pierced with holes and immersed it for a fortnight in the sea. Out of the hundred shells, twenty-seven recovered. The presence of an uber-culum seems to have been of importance, as out of twelve specimens of cyclostoma elegans, which is thus furnished, eleven survived. It is remarkable, seeing how well the helix pomacea resisted with me the salt water, that not one of fifty-four specimens belonging to four other species of helix tried by Ochapitan recovered. It is, however, not at all probable that land-shells have often been thus transported. The feet of birds offer a more probable method. On the relations of the inhabitants of islands to those of the nearest mainland. The most striking and important fact for us is the affinity of the species which inhabit islands to those of the nearest mainland, without being actually the same. Numerous species could be given. The Galapagos archipelago, situated under the equator, lies at a distance of between five hundred and six hundred miles from the shores of South America. Here almost every product of the land and the water bears the unmistakable stamp of the American continent. There are twenty-six land-birds. Of these, twenty-one or perhaps twenty-three are ranked as distinct species, and would commonly been assumed to have been there created. Yet the close affinity of most of these birds to American species is manifest in every character of their habits, gestures, and tones of voice. So it is with the other animals, and with a large proportion of the plants, as shown by Dr. Hooker in his admirable flora of this archipelago. The naturalist, looking at the inhabitants of these volcanic islands in the Pacific, distant several hundred miles from the continent, feels that he is standing on American land. Why should this be so? Why should the species which are supposed to have been created in the Galapagos archipelago, and nowhere else, bear so plainly the stamp of affinity to those created in America? There is nothing in the conditions of life, in the geological nature of the islands, in their height or climate, or in the proportions in which the several classes are associated together which closely resembles the conditions of the South American coast. In fact, there is a considerable dissimilarity in all of these respects. On the other hand, there is a considerable degree of resemblance in the volcanic nature of the soil, in the climate, height and size of the islands between the Galapagos and Cape Verde archipelagos, but what an entire and absolute difference in their inhabitants. The inhabitants of the Cape Verde islands are related to those of Africa, like those of the Galapagos to America. Facts such as these admit of no sort of explanation on the ordinary view of independent creation, whereas on the view here maintained it is obvious that the Galapagos islands would be likely to receive colonists from America, whether by occasional means of transport, or, though I do not believe in this doctrine, by formerly continuous land, and the Cape Verde islands from Africa such colonists would be liable to modification, the principle of inheritance still betraying their original birthplace. Many analogous facts could be given. Indeed, it is an almost universal rule that the endemic productions of islands are related to those of the nearest continent, or of the nearest large island. The exceptions are few, and most of them can be explained. Thus, although Kyrgyllian land stands nearer to Africa than to America, the plants are related, and that very closely, as we know from Dr. Hooker's account, to those of America. But on the view that this island has been mainly stocked by seeds brought within earth and stones on icebergs, drifted by the prevailing currents, this anomaly disappears. New Zealand, in its endemic plants, is much more closely related to Australia, the nearest mainland, than to any other region, and this is what might have been expected, but it is also plainly related to South America, which, although the next nearest continent is so enormously remote, that the fact becomes an anomaly. But this difficulty partially disappears on the view that New Zealand, South America, and other southern lands, has been stocked in part from a nearly indeterminate though distant point, namely from the Antarctic lands, when they were clothed with vegetation during a warmer tertiary period, before the commencement of the last glacial period. The affinity, which, though feeble, I am assured by Dr. Hooker is real, between the flora of the southwestern corner of Australia and of the Cape of Good Hope, is a far more remarkable case, but this affinity is confined to the plants, and will, no doubt, someday be explained. The same law which has determined the relationship between the inhabitants of islands and the nearest mainland is sometimes displayed on a small scale, but in a most interesting manner within the limits of the same archipelago. Thus each separate island of the Galapagos Archipelago is tenanted, and the fact is a marvellous one by many different species, but these species are related to each other in a very much closer manner than the inhabitants of the American continent, or of any other quarter of the world. This is what might have been expected for islands situated so near to each other that they would almost necessarily receive immigrants from the same original source and from each other. But how is it that many of the immigrants have been differently modified, though only in a small degree, in islands situated within sight of each other, having the same geological nature, the same height, climate, etc. This long appeared to me a great difficulty, but it arises in chief part from the deeply seated error of considering the physical conditions of a country as the most important, whereas it cannot be disputed that the nature of the other species with which it has to compete is at least as important, and generally a far more important element of success. Now if we look to the species which inhabit the Galapagos Archipelago, and are likewise found in other parts of the world, we find that they differ considerably in the several islands. This difference might indeed have been expected if the islands had been stocked by occasional means of transport, a seed for instance of one plant having been brought to one island and that of another plant to another island, though all proceeding from the same general source. Hence when in former times an immigrant first settled on one of the islands, or when it subsequently spread from one to another, it would undoubtedly be exposed to different conditions in the different islands, for it would have to compete with a different set of organisms. A plant for instance would find the ground best-mitted for it occupied by a somewhat different species in the different islands, and would be exposed to the attacks of somewhat different enemies. If then it varied, natural selection would probably favour different varieties in the different islands. Some species however might spread and yet retain the same character throughout the group, just as we see some species spreading widely throughout a continent and remaining the same. The really surprising fact in this case of the Galapagos Archipelago, and in a lesser degree in some analogous cases, is that each new species, after being formed on any one island, did not spread quickly to the other islands. But the islands though within sight of each other are separated by deep arms of the sea, in most cases wider than the British Channel, and there is no reason to suppose that they have at any former period been continuously united. The currents of the sea are rapid and deep between the islands, and gales of wind are extraordinarily rare, so that islands are far more effectually separated from each other than they appear on a map. Nevertheless, some of the species, both of those found on other parts of the world and of those confined to the archipelago, are common to the several islands, and we may infer from the present manner of distribution that they have spread from one island to the others. But we often take, I think, an erroneous view of the probability of closely allied species invading each other's territory when put into free intercommunication. Undoubtedly, if one species has any advantage over another, it will, in a very brief time, wholly or in parts supplant it, but if both are equally well fitted for their own places, both will probably hold their separate places for almost any length of time. Being familiar with this fact that many species, naturalized through mad's agency, have spread with astonishing rapidity over wide areas, we are apt to infer that most species would thus spread. But we should remember that the species which become naturalized in new countries are not generally closely allied to the aboriginal inhabitants, but are very different forms, belonging in a large proportion of cases as shown by Alphons de Gandol to distinct genera. In the Galapagos Archipelago, many even of the birds, though so well adapted for flying from island to island, differ on the different islands. Thus there are three closely allied species of mockingthrush, each confined to its own island. Now let us suppose the mockingthrush of Chatham Island to be blown to Charles Island, which has its own mockingthrush. Why should it succeed in establishing itself there? We may safely infer that Charles Island is well stocked with its own species, for annually more eggs are laid and young birds hatched than can possibly be reared, and we may infer that the mockingthrush peculiar to Charles Island is at least as well fitted for its home as the species peculiar to Chatham Island. Sir C. Lyell and Mr. Walliston have communicated to me a remarkable fact bearing on this subject, namely that Madeira and the adjoining islet of Porto Santo possess many distinct yet representative species of land snails, some of which live in crevices of stone, and although large quantities of stone are annually transported from Porto Santo to Madeira, yet this latter island has not become colonized by the Porto Santo species. Nevertheless, both islands have been colonized by some European land-shells, which no doubt had some advantage over the indigenous species. From these considerations I think we need not greatly marvel at the endemic species which inhabit the several islands of the Galapagos archipelago, not having at all spread from island to island. On the same continent also, preoccupation has probably played an important part in checking the commingling of the species which inhabit different districts with nearly the same physical conditions. Thus the southeast and southwest corners of Australia have nearly the same physical conditions and are united by continuous land, yet they are inhabited by a vast number of distinct mammals, birds, and plants, so it is according to Mr. Bates, with his butterflies and other animals, inhabiting the great open and continuous valley of the Amazons. The same principle which governs the general character of the inhabitants of oceanic islands, namely the relation to the source whence the colonists could have most easily derived, together with their subsequent modification, is of the widest application throughout nature. We see this on every mountain summit, in every lake and marsh, for alpine species, accepting in as far as the same species have become widely spread during the glacial epoch, are related to those of the surrounding lowlands. Thus we have in South America alpine hummingbirds, alpine rodents, alpine plants, etc., all strictly belonging to American forms, and it is obvious that a mountain, as it became slowly upheaved, would be colonized from the surrounding lowlands. So it is that the inhabitants of lakes and marshes, accepting in so far as the great facility of transport, has allowed the same forms to prevail throughout large portions of the world. We see the same principle in the character of most of the blind animals inhabiting the caves of America and Europe. Other analogous facts could be given. It will, I believe, be found universally true that wherever in two regions let them be ever so distant, many closely allied or representative species occur. There will likewise be found some identical species, and wherever more closely allied species occur there will be found many forms which some naturalists rank as distinct species and others as mere varieties. These doubtful forms showing us the steps in the process of modification. The relation between the power and extent of migration in certain species, either at the present or at some former period, and the existence at remote points of the world of closely allied species, is shown in another and more general way. Mr. Gould remarked long ago that in those genera of birds which range over the world many of the species have very wide ranges. I can hardly doubt that this rule is generally true, although difficult of proof. Among mammals we see it strikingly displayed in bats and in a lesser degree in the filidae and kenidae. We see the same rule in the distribution of butterflies and beetles. So it is with most of the inhabitants of fresh water, for many of the genera in the most distinct classes range all over the world, and many of the species have enormous ranges. It is not meant that all but that some of the species have very wide ranges in the genera which range very widely, nor is it meant that the species in such genera have on average a very wide range, for this will largely depend on how far the process of modification has gone. For instance, two varieties of the same species inhabit America and Europe, and thus the species has an immense range, but if a variation were to be carried a little further, the two varieties would be ranked as distinct species, and their range would be greatly reduced. Still less is it meant that the species which have the capacity of crossing barriers and ranging widely, as in the case of certain powerfully winged birds, will necessarily range widely, for we should never forget that to range widely implies not only the power of crossing barriers, but the more important power of being victorious in distant lands in the struggle for life with foreign associates. But according to the view that all the species of a genus, though distributed to the most remote points of the world, are descendant from a single progenitor, we ought to find, and I believe as a general rule we do find, that at least some of the species range very widely. We should bear in mind that many genera in all classes are of ancient origin, and the species in this case will have had ample time for dispersal and subsequent modification. There is also reason to believe, from geological evidence, that within each class the lower organisms change at a slower rate than the higher. Consequently they will have had a better chance of ranging widely, and of still retaining the same specific character. This fact, together with that of the seeds and eggs of most lowly organized forms, being very minute and better fitted for distant transportal, probably accounts for a law which has long been observed and which has lately been discussed by Alphonse de Gandole in regard to plants, namely that the lower any group of organisms stands, the more widely it ranges. The relations just discussed, namely lower organisms ranging more widely than the higher, some of the species of widely ranging genera themselves ranging widely, such facts as alpine, locustron and marsh productions being generally related to those which live on the surrounding lowlands and drylands, the striking relationship between the inhabitants of islands and those of the nearest mainland, the still closer relationship of the distinct inhabitants of the islands of the same archipelago, are inexplicable on the ordinary view of the independent creation of each species, but are explicable if we admit colonization from the nearest or readyist source, together with the subsequent adaptation of the colonists to their new homes. Summary of the last and present chapters If we bear in mind, and this is a very important consideration, how often a species may have ranged continuously over a wide area and then have become extinct in the intermediate tracts, the difficulty is not insuperable in believing that all the individuals of these same species, wherever found, are descended from common parents. And we are led to this conclusion, which has been arrived at by many naturalists under the designation of single centers of creation by various general considerations, more especially from the importance of barriers of all kinds, and from the analogical distribution of subgenera, genera, and families. With respect to distinct species belonging to the same genus, which on our theory have spread from one parent's source, if we make the same allowances as before for our ignorance, and remember that some forms of life have changed very slowly, enormous periods of time have been thus granted for their migration. The difficulties are far from insuperable, though in this case, as in that of the individuals of the same species, they are often great. As exemplifying the effects of climatical changes on distribution, I have attempted to show how important a part of the last glacial period has played, which affected even the equatorial regions and which, during the alternations of the cold in the north and the south, allowed the productions of opposite hemispheres to mingle and left some of them stranded on the mountain summits in all parts of the world. As showing how diversified are the means of occasional transport, I have discussed at some little length the means of dispersal of freshwater productions. If the difficulties be not insuperable in admitting that, in the long course of time, all the individuals of the same species and likewise of the several species belonging to the same genus have proceeded from some one source, then all the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms. We can thus understand the high importance of barriers, whether of land or water, in not only separating but in apparently forming the several zoological and botanical provinces. We can thus understand the concentration of related species within the same areas and how it is that under northern latitudes, for instance in South America, the inhabitants of the plains and mountains of the forests, marshes and deserts are linked together in so mysterious a manner and are likewise linked to the extinct beings which formerly inhabited the same continent. Bearing in mind the mutual relation of organism to organism is of the highest importance. We can see why two areas, having nearly the same physical considerations, should often be inhabited by very different forms of life, for according to the length of time which has elapsed since the colonists entered one of the regions, or both according to the nature of the communication which allowed certain forms and not others to enter, either in greater or lesser numbers. According or not, as those which entered happen to come into more or less direct competition with each other and with the aborigines, and according, as the immigrants were capable of varying more or less rapidly, there would ensue in the two or more regions independently of their physical conditions infinitely diversified conditions of life. There would be an almost endless amount of organic action and reaction, and we should find some groups of beings greatly and some only slightly modified. Some developed in great force, some existing in scanty numbers, and this we do find in the several great geographical provinces of the world. On these same principles we can understand, as I have endeavored to show, why oceanic islands should have few inhabitants, but that of these a large proportion should be endemic or peculiar, and why, in relation to the means of migration, one group of beings should have all its species peculiar, and another group, even within the same class, should have all its species the same. We can see why whole groups of organisms, as petrations and terrestrial animals, should be absent from oceanic islands, whilst the most isolated islands should possess their own peculiar species of aerial mammals or bats. We can see why, in islands, there should be some relation between the presence of mammals in a more or less modified condition and the depth of the sea between such islands and the mainland. We can clearly see why all of the inhabitants of an archipelago, though specifically distinct on the several islets, should be closely related to each other and should likewise be related, but less closely, to those of the nearest continent or other source whence immigrants might have been derived. We can see why, if there exist very closely allied or representative species in two areas, however distant from each other, some identical species will almost always there be found. As the late Edwin Forbes often insisted, there is a striking parallelism in the laws of life throughout time and space, the laws governing the succession of forms in past times being nearly the same with those governing in the present time, the differences in different areas. We see this in many facts. The endurance of each species and group of species is continuous in time, for the apparent exceptions to the rule are so few that they may fairly be attributed to our not having as yet discovered in our intermediate deposit certain forms which are absent in it, but which occur above and below. So in space it certainly is the general rule that the area inhabited by a single species or by a group of species is continuous, and the exceptions which are not rare may, as I have attempted to show, be accounted for by former migrations under different circumstances or through occasional means of transport or by the species having become extinct in the determinant tracts. Both in time and in space species and groups of species have their points of maximum development. Groups of species living through the same period of time or living within the same area are often characterized by trifling features in common as of sculpture or color. In looking to the long succession of past ages as in looking to distant provinces throughout the world we find that species in certain classes differ little from each other, while those in another class or only in a different section of the same order differ greatly from one another. In both time and space the lowly organized members of each class generally change less than the highly organized, but there are in both cases marked exceptions to the rule. According to our theory these several relations throughout time and space are intelligible, for whether we look to the allied forms of life which have changed during successive ages or to those which have changed after having migrated into different quarters, in both cases they are connected by the same bond of ordinary generation. In both cases the laws of variation have been the same and modifications have been accumulated by the same means of natural selection. So ends Chapter 13 of The Origin of Species by Charles Darwin.