 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. The Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Sixth London Edition by Charles Darwin. Chapter No. 9, Hybridism. Section 2 of 2. Origin and Causes of the Sterility of First Crosses and of Hybrids. At one time it appeared to me probable, as it has to others, that the Sterility of First Crosses and of Hybrids might have been slowly acquired through the natural selection of slightly lessened degrees of fertility, which, like any other variation, spontaneously appeared in certain individuals of one variety when crossed with those of another variety. For it would clearly be advantageous to two varieties, or incipient species, if they could be kept from blending, on the same principle that, when man is selecting at the same time two varieties, it is necessary that he should keep them separate. In the first place it may be remarked that species inhabiting distinct regions are often sterile when crossed. Now it could clearly have been of no advantage to such separated species to have been rendered mutually sterile, and consequently this could not have been affected through natural selection. But it may perhaps be argued that if a species was rendered sterile with some one compatriot, sterility with other species would follow as a necessary contingency. In the second place it is almost as much opposed to the theory of natural selection as to that of special creation, that in reciprocal crosses the male element of one form should have been rendered utterly impotent on a second form, while at the same time the male element of this second form is enabled freely to fertilize the first form. For this peculiar state of the reproductive system could hardly have been advantageous to either species. In considering the probability of natural selection having come into action in rendering species mutually sterile the greatest difficulty will be found to lie in the existence of many graduated steps from slightly lessened fertility to absolute sterility. It may be admitted that it would profit an incipient species if it were rendered in some slight degree sterile when crossed with its parent form or with some other variety. For thus fewer bastardized and deteriorated offspring would be produced to commingle their blood with the new species in process of formation. But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that high degree which is common with so many species and which is universal with species which have been differentiated to a generic or family rank will find the subject extraordinarily complex. After mature reflection it seems to me that this could not have been affected through natural selection. Take the case of any two species which when crossed produced few and sterile offspring. Now what is there which could favour the survival of those individuals which happened to be endowed in a slightly higher degree with mutual infertility and which thus approached by one small step towards absolute sterility. Yet an advance of this kind if the theory of natural selection be brought to bear must have incessantly occurred with many species for a multitude are mutually quite barren. With sterile neuter insects we have reason to believe that modifications in their structure and fertility have been slowly accumulated by natural selection from an advantage having been thus indirectly given a community to which they belonged over other communities of the same species. But an individual animal not belonging to a social community if rendered slightly sterile when crossed with some other variety would not thus itself gain any advantage or indirectly give any advantage to the other individuals of the same variety thus leading to their preservation. But it would be superfluous to discuss this question in detail for with plants we have conclusive evidence that the sterility of crossed species must be due to some principle quite independent of natural selection. Both Gardner and Collreuter have proved that in genera including numerous species a series can be formed from species which when crossed yield fewer and fewer seeds to species which never produce a single seed but yet are affected by the pollen of certain other species for the German swells. It is here manifestly impossible to select the more sterile individuals which have already ceased to yield seeds so that this acme of sterility when the German alone is affected cannot have been gained through selection and from the laws governing the various grades of sterility being so uniform throughout the animal and vegetable kingdoms we may infer that the cause whatever it may be is the same or nearly the same in all cases. We will now look a little closer at the probable nature of the differences between species which induce sterility in first crosses and in hybrids. In the case of first crosses the greater or less difficulty in affecting a union and in obtaining offspring apparently depends on several distinct causes. There must sometimes be a physical impossibility in the male element reaching the ovule as would be the case with a plant having a pistol too long for the pollen tubes to reach the ovarium. It has also been observed that when the pollen of one species is placed on the stigma of a distantly allied species though the pollen tubes protrude they do not penetrate the stigmatic surface. Again the male element may reach the female element but be incapable of causing an embryo to be developed as seems to have been the case with some of Tourette's experiments on fukai. No explanation can be given of these facts any more than why certain trees cannot be grafted on others. Lastly an embryo may be developed and then perish at an early period. This latter alternative has not been sufficiently attended to but I believe from observations communicated to me by Mr. Hewitt who has had great experience in hybridizing pheasants and fowls that the early death of the embryo is a very frequent cause of sterility in first crosses. Mr. Salter has recently given the results of an examination of about 500 eggs produced from various crosses between three species of gallus and their hybrids. The majority of these eggs had been fertilized and in the majority of the fertilized eggs the embryos had either been partially developed and then perished or had become nearly mature but the young chickens had been unable to break through the shell. Of the chickens which were born more than four-fifths died within the first few days or at least weeks without any obvious cause apparently from mere inability to live so that from the 500 eggs only 12 chickens were reared. With plants hybridized embryos probably often perish in a like manner at least it is known that hybrids raised from very distinct species are sometimes weak and dwarfed and perish at an early age of which fact Max Mature has recently given some striking cases with hybrid willows. It may be here worth noticing that in some cases of parthenogenesis the embryos within the eggs of silk moths which had not been fertilized pass through their early stages of development and then perish like the embryos produced by a cross between distinct species. Until becoming acquainted with these facts I was unwilling to believe in the frequent early death of hybrid embryos for hybrids when once born are generally healthy and long-lived as we see in the case of the common mule. Hybrids however are differently circumstanced before and after birth. When born and living in a country where their two parents live they are generally placed under suitable conditions of life but a hybrid partakes of only half of the nature and constitution of its mother. It may therefore before birth as long as it is nourished within its mother's womb or within the egg or seed produced by the mother be exposed to conditions in some degree unsuitable and consequently be liable to perish at an early period more especially as all very young beings are eminently sensitive to injurious or unnatural conditions of life but after all the cause more probably lies in some imperfection in the original act of impregnation causing the embryo to be imperfectly developed rather than in the conditions to which it is subsequently exposed. In regard to the sterility of hybrids in which the sexual elements are imperfectly developed the case is somewhat different. I have more than once alluded to a large body of facts showing that when animals and plants are removed from their natural conditions they are extremely liable to have their reproductive systems seriously affected. This in fact is the great bar to the domestication of animals. Between the sterility thus super induced and that of hybrids there are many points of similarity. In both cases the sterility is independent of general health and is often accompanied by excess of size or great luxuriance. In both cases the sterility occurs in various degrees. In both the male element is the most liable to be affected but sometimes the female more than the male. In both the tendency goes to a certain extent with systematic affinity for whole groups of animals and plants are rendered impotent by the same unnatural conditions and whole groups of species tend to produce sterile hybrids. On the other hand, one species in a group will sometimes resist great changes of conditions with unimpaired fertility and certain species in a group will produce unusually fertile hybrids. No one can tell till he tries whether any particular animal will breed under confinement or any exotic plant seed freely under culture nor can he tell till he tries whether any two species of a genus will produce more or less sterile hybrids. Lastly, when organic beings are placed during several generations under conditions not natural to them they are extremely liable to vary which seems to be partly due to their reproductive systems having been specially affected though in a lesser degree than when sterility ensues. So it is with hybrids for their offspring in successive generations are eminently liable to vary as every experimentalist has observed. Thus we see that when organic beings are placed under new and unnatural conditions and when hybrids are produced by the unnatural crossing of two species the reproductive system independently of the general state of health is affected in a very similar manner. In the one case the conditions of life have been disturbed so often in so slight a degree as to be inappreciable by us. In the other case or that of hybrids the external conditions have remained the same but the organisation has been disturbed by two distinct structures and constitutions including of course the reproductive systems having been blended into one. For it is scarcely possible that two organisations should be compounded into one without some disturbance occurring in the development or periodical action or mutual relations of the different parts and organs one to another or to the conditions of life. When hybrids are able to breed in to say they transmit to their offspring from generation to generation the same compounded organisation and hence we need not be surprised that their sterility though in some cases variable does not diminish it is even apt to increase this being generally the result as before explained of too close interbreeding. The above view of the sterility of hybrids being caused by two conditions being compounded into one has been strongly maintained by Max Wichura. It must however be owned that we cannot understand on the above or any other view several facts with respect to the sterility of hybrids for instance the unequal fertility of hybrids produced with reciprocal crosses or the increased sterility in those hybrids which occasionally and exceptionally resemble closely either pure parent. Nor do I pretend that the foregoing remarks go to the root of the matter no explanation is offered why an organism when placed under unnatural conditions is rendered sterile all that I have attempted to show is that in two cases in some respects allied sterility is the common result in the one case from the conditions of life having been disturbed in the other case from the organization having been disturbed by two organizations being compounded into one. A simple parallelism holds good with an allied yet very different class of facts. It is an old and almost universal belief founded on a considerable body of evidence which I have elsewhere given that slight changes in the conditions of life are beneficial to all living things. We see this acted on by farmers and gardeners in their frequent exchanges of seed, tubers etc from one soil or climate to another and back again. During the convalescence of animals great benefit is derived from almost any change in the habits of life. Again both with plants and animals there is the clearest evidence that the loss between individuals of the same species which differ to a certain extent gives vigor and fertility to the offspring and that close interbreeding continued during several generations between the nearest relations if these be kept under the same conditions of life almost always leads to decreased size, weakness or sterility. Hence it seems that on the one hand slight changes in the conditions of life benefit all organic beings and on the other hand that slight crosses that is crosses between the males and females of the same species which have been subjected to slightly different conditions or which have been slightly varied give vigor and fertility to the offspring. But as we have seen organic beings long habituated to certain uniform conditions under a state of nature when subjected as under confinement to a considerable change in their conditions very frequently are rendered more or less sterile and we know that a cross between two forms that have become widely or specifically different produce hybrids which are almost always in some degree sterile. I am fully persuaded that this double parallelism is by no means an accident or an illusion he who is able to explain why the elephant and a multitude of other animals are incapable of breeding when kept under only partial confinement in their native country will be able to explain the primary cause of hybrids being so generally sterile. He will at the same time be able to explain how it is that the races of some of our domesticated animals which have often been subjected to new and not uniform conditions are quite fertile together although they are descended from distinct species which would probably have been sterile if aboriginally crossed. The above two parallel series of facts seem to be connected together by some common but unknown bond which is essentially related to the principle of life. This principle, according to Mr. Herbert Spencer being that life depends on or consists in the incessant action and reaction of various forces which as throughout nature are always tending towards an equilibrium and when this tendency is slightly disturbed by any change the vital forces gain in power. Reciprocal dimorphism and trimorphism This subject may be here briefly discussed and will be found to throw some light on hybridism. Several plants belonging to distinct orders present two forms which exist in about equal numbers and which differ in no respect except in their reproductive organs. One form having a long pistol with short stamens the other a short pistol with long stamens the two having differently sized pollen grains With trimorphic plants there are three forms likewise differing in the lengths of their pistils and stamens in the size and colour of the pollen grains and in some other respects and as in each of the three forms there are two sets of stamens the three forms possess altogether six sets of stamens and three kinds of pistils These organs are so proportioned in length to each other that half the stamens in two of the forms stand on a level with the stigma of the third form Now I have shown and the result has been confirmed by other observers that in order to obtain full fertility with these plants it is necessary that the stigma of the one form should be fertilised by the pollen taken from the stamens of corresponding height in another form so that with dimorphic species two unions which may be called legitimate are fully fertile and two which may be called illegitimate are more or less infertile With trimorphic species six unions are legitimate or fully fertile and twelve are illegitimate or more or less infertile The infertility which may be observed in various dimorphic and trimorphic plants when they are illegitimately fertilised that is by pollen taken from stamens not corresponding in height with the pistil differs much in degree up to absolute and utter sterility just in the same manner as occurs in crossing distinct species As the degree of sterility in the latter case depends in an eminent degree on the conditions of life being more or less favourable So I have found it with illegitimate unions It is well known that if pollen of a distinct species be placed on the stigma of a flower and its own pollen be afterwards even after a considerable interval of time placed on the same stigma its action is so strongly prepotent that it generally annihilates the effect of the foreign pollen So it is with the pollen of the several forms of the same species therefore legitimate pollen is strongly prepotent over illegitimate pollen when both are placed on the same stigma I ascertained this by fertilising several flowers first illegitimately and 24 hours afterwards legitimately with pollen taken from a peculiarly coloured variety and all the seedlings were similarly coloured This shows that the legitimate pollen though applied 24 hours subsequently had wholly destroyed or prevented the action of the previously applied illegitimate pollen Again, as in making reciprocal crosses between the same two species there is occasionally a great difference in the result so the same thing occurs in trimorphic plants For instance, the mid-styled form of lytherum salicaria was illegitimately fertilised with the greatest ease by pollen from the longer stamens of the short-styled form and yielded many seeds but the latter form did not yield a single seed when fertilised by the longer stamens of the mid-styled form In all these respects and in others which might be added the forms of the same undoubted species when illegitimately united behave in exactly the same manner as do two distinct species when crossed This led me carefully to observe during four years many seedlings raised from several illegitimate unions The chief result is that these illegitimate plants as they may be called are not fully fertile It is possible to raise from dimorphic species both long-styled and short-styled illegitimate plants and from trimorphic plants all three illegitimate forms These can then be properly united in a legitimate manner When this is done there is no apparent reason why they should not yield as many seeds as did their parents when legitimately fertilised but this is not the case They are all infertile in various degrees some being so otterly and incurably sterile that they do not yield during four seasons a single seed or even seed capsule The sterility of these illegitimate plants when united with each other in a legitimate manner may be strictly compared with that of hybrids when crossed in to say If, on the other hand, a hybrid is crossed with either pure parent species the sterility is usually much lessened and so it is when an illegitimate plant is fertilised by a legitimate plant In the same manner as the sterility of hybrids does not always run parallel with the difficulty of making the first cross between the two parent species so that sterility of certain illegitimate plants was unusually great while the sterility of the union from which they were derived was by no means great With hybrids raised from the same seed capsule the degree of sterility is innately variable so it is in a marked manner with the illegitimate plants Lastly, many hybrids are profuse and persistent flowers while other and more sterile hybrids produce few flowers and are weak, miserable dwarfs Exactly similar cases occur with the illegitimate offspring of various dimorphic and trimorphic plants Altogether there is the closest identity in character and behaviour between illegitimate plants and hybrids It is hardly an exaggeration to maintain that illegitimate plants are hybrids produced within the limits of the same species by the improper union of certain forms while ordinary hybrids are produced from an improper union between so-called distinct species We have also already seen that there is the closest similarity in all respects between first illegitimate unions and first crosses between distinct species This will perhaps be made more fully apparent by an illustration We may suppose that a botanist found two well-marked varieties and such occur of the long-styled form of the trimorphic lytherum salicaria and that he determined to try by crossing whether they were specifically distinct He would find that they yielded only about one-fifth of the proper number of seed and that they behaved in all the other above specified respects as if they had been two distinct species But to make the case sure he would raise plants from his supposed hybridised seed and he would find that the seedlings were miserably dwarfed and utterly sterile and that they behaved in all other respects like ordinary hybrids He might then maintain that he had actually proved in accordance with the common view that his two varieties were as good and as distinct species as any in the world but he would be completely mistaken The facts now given on dimorphic and trimorphic plants are important They show us first that the physiological test of lessened fertility both in first crosses and in hybrids is no safe criterion of specific distinction Secondly, because we may conclude that there is some unknown bond which connects the infertility of illegitimate unions with that of their illegitimate offspring and we are led to extend the same view to first crosses and hybrids Thirdly, because we find, and this seems to me of special importance that two or three forms of the same species may exist and may differ in no respect whatever either in structure or in constitution relatively to external conditions and yet be sterile when united in certain ways For we must remember that it is the union of the sexual elements of individuals of the same form for instance of two long-styled forms which results in sterility while it is the union of the sexual elements proper to two distinct forms which is fertile Hence the case appears at first sight exactly the reverse of what occurs in the ordinary unions of the individuals of the same species and with crosses between distinct species It is however doubtful whether this is really so but I will not enlarge on this obscure subject We may however infer as probable from the consideration of dimorphic and trimorphic plants that the sterility of distinct species when crossed and of their hybrid progeny depends exclusively on the nature of their sexual elements and not on any difference in their structure or general constitution We are also led to this same conclusion by considering reciprocal crosses in which the male of one species cannot be united or can be united with great difficulty with the female of a second species while the converse cross can be affected with perfect facility That excellent observer, Gartner likewise concluded that species when crossed are sterile owing to differences confined to their reproductive systems fertility of varieties when crossed and of their mongrel offspring not universal It may be urged as an overwhelming argument that there must be some essential distinction between species and varieties in as much as the latter however much they may differ from each other in external appearance cross with perfect facility and yield perfectly fertile offspring With some exceptions presently to be given I fully admit that this is the rule but the subject is surrounded by difficulties for looking to varieties produced under nature if two forms hitherto reputed to be varieties be found in any degree sterile together they are at once ranked by most naturalists as species for instance the blue and red pimpenel which are considered by most botanists varieties are said by Gartner to be quite sterile when crossed and he consequently ranks them as undoubted species if we thus argue in a circle the fertility of all varieties produced under nature will assuredly have to be granted if we turn to varieties produced or supposed to have been produced under domestication we are still involved in some doubt for when it is stated for instance certain South American indigenous domestic dogs do not readily unite with European dogs the explanation which will occur to everyone and probably the true one is that they are descended from aboriginally distinct species nevertheless the perfect fertility of so many domestic races differing widely from each other in appearance for instance those of the pigeon or of the cabbage is a remarkable fact more especially when we reflect how many species there are which though resembling each other most closely are utterly sterile when intercrossed several considerations however render the fertility of domestic varieties less remarkable in the first place it may be observed that the amount of external difference between two species is no sure guide to their degree of mutual sterility so that similar differences in the case of varieties would be no sure guide it is certain that with species the cause lies exclusively in differences in their sexual constitution now the varying conditions to which domesticated animals and cultivated plants have been subjected have had so little tendency towards modifying the reproductive system in a manner leading to mutual sterility that we have good grounds for committing the directly opposite doctrine of palace namely that such conditions generally eliminate this tendency so that the domesticated descendants of species which in their natural state probably would have been in some degree sterile when crossed become perfectly fertile together with plants so far is cultivation from giving a tendency towards sterility between distinct species that in several well authenticated cases already alluded to certain plants have been affected in an opposite manner for they have become self-impetent while still retaining the capacity of fertilizing and being fertilized by other species if the palaceian doctrine of the elimination of sterility through long continued domestication be admitted and it can hardly be rejected it becomes in the highest degree improbable that similar conditions long continued should likewise induce this tendency though in certain cases with species having a peculiar constitution sterility might occasionally be thus caused thus as I believe we can understand why with domesticated animals varieties have not been produced which are mutually sterile and why with plants only a few such cases immediately to be given have been observed the real difficulty in our present subject is not as it appears to me why domestic varieties have not become mutually infertile when crossed but why this has so generally occurred with natural varieties as soon as they have been permanently modified in a sufficient degree to take rank as species we are far from precisely knowing the cause nor is this surprising seeing how profoundly ignorant we are in regard to the normal and abnormal action of the reproductive system but we can see that species owing to their struggle for existence with numerous competitors will have been exposed during long periods of time to more uniform conditions than have domestic varieties and this may well make a wide difference in the result for we know how commonly wild animals and plants when taken from their natural conditions subjected to captivity are rendered sterile and the reproductive functions of organic beings which have always lived under natural conditions would probably in like manner be eminently sensitive to the influence of an unnatural cross domesticated productions on the other hand which as shown by the mere fact of their domestication were not originally highly sensitive to changes in their conditions of life and which can now generally resist with undiminished fertility repeated changes of conditions might be expected to produce varieties which would be little liable to have their reproductive powers injuriously affected by the act of crossing with other varieties which had originated in a like manner I have as yet spoken as if the varieties of the same species were invariably fertile when intercrossed but it is impossible to resist the evidence of the existence of a certain amount of sterility in the few following cases which I will briefly abstract the evidence is at least as good as that from which we believe in the sterility of a multitude of species the evidence is also derived from hostile witnesses who in all other cases consider fertility and sterility as safe criterion of specific distinction Gardener kept during several years a dwarf kind of maize with yellow seeds and a tall variety with red seeds growing near each other in his garden and although these plants have separated sexes they never naturally crossed he then fertilized thirteen flowers of the one kind with pollen of the other but only a single head produced any seed and this one head produced only five grains manipulation in this case could not have been injurious as the plants have separated sexes no one I believe has suspected that these varieties of maize are distinct species and it is important to notice that the hybrid plants thus raised were themselves perfectly fertile so that even Gardener did not venture to consider the two varieties as specifically distinct Girouda Bouserang crossed three varieties of gourd which like the maize had separated sexes and he asserts that their mutual fertilization is by so much the less easy as their differences are greater how far these experiments may be trusted I know not but the forms experimented on are ranked by Sagare who mainly founds his classification by the test of infertility as varieties and Nordau has come to the same conclusion the following case is far more remarkable and seems at first incredible but it is the result of an astonishing number of experiments made during many years on nine species of bascum by so good an observer and so hostile a witness as Gardener namely that the yellow and white varieties when crossed produce less seed than the similarly coloured varieties of the same species however he asserts that when yellow and white varieties of one species are crossed with yellow and white varieties of a distinct species more seed is produced by the crosses between the similarly coloured flowers than between those which are differently coloured Mr. Scott also has experimented on the species and varieties of bascum and although unable to confirm Gardener's results on the crossing of the distinct species he finds that the dissimilarly coloured varieties of the same species yield fewer seeds in the proportion of 86 to 100 than the similarly coloured varieties yet these varieties differ in no respect except in the colour of their flowers and one variety can sometimes be raised from the seed of another Collreuter whose accuracy has been confirmed by every subsequent observer has proved the remarkable fact that one particular variety of the common tobacco was more fertile than the other varieties when crossed with a widely distinct species he experimented on five forms which are commonly reputed to be varieties and which he tested by the severest trial namely by reciprocal crosses and he found their mongrel offspring perfectly fertile but one of these five varieties when used either as the father or mother and crossed with the Nicotiana glutinosa always yielded hybrids not so sterile as those which were produced from the four other varieties when crossed with Nicotiana glutinosa hence the reproductive system of this one variety must have been in some manner and in some degree modified from these facts it can no longer be maintained that varieties when crossed are invariably quite fertile from the great difficulty of ascertaining the infertility of varieties in a state of nature for a supposed variety if proved to be infertile in any degree would almost universally be ranked as a species for man attending only to external characters in his domestic varieties and from such varieties not having been exposed for very long periods to uniform conditions of life from these several considerations we may conclude that fertility does not constitute a fundamental distinction between varieties and species when crossed the general sterility of crossed species may safely be looked at not as a special acquirement or endowment but as incidental on changes of an unknown nature in their sexual elements hybrids and mongrels compared independently of their fertility independently of the question of fertility the offspring of species and of varieties when crossed may be compared in several other respects Gartner whose strong wish it was to draw a distinct line between species and varieties could find very few and as it seems to me quite unimportant differences between the so-called hybrid offspring of species and the so-called mongrel offspring of varieties and on the other hand they agree most closely in many important respects I shall here discuss this subject with extreme brevity the most important distinction is that in the first generation mongrels are more variable than hybrids but Gartner admits that hybrids from species which have long been cultivated are often variable in the first generation and I have myself seen striking instances of this fact Gartner further admits that hybrids between very closely allied species are more variable than those from very distinct species and this shows that the difference in the degree of variability graduates away when mongrels and the more fertile hybrids are propagated for several generations an extreme amount of variability in the offspring in both cases is notorious but some few instances of both hybrids and mongrels long retaining a uniform character could be given the variability however in the successive generations of mongrels is perhaps greater than in hybrids the greater variability in mongrels than in hybrids does not seem at all surprising for the parents of mongrels are varieties and mostly domestic varieties very few experiments have been tried on natural varieties and this implies that there has been recent variability which would often continue and would augment that arising from the act of crossing the slight variability of hybrids in the first generation in contrast with that in the succeeding generations is a curious fact and deserves attention for it bears on the view which I have taken of one of the courses of ordinary variability namely that the reproductive system from being eminently sensitive to change conditions of life fails under these circumstances to perform its proper function of producing offspring closely similar in all respects to the parent form now hybrids in the first generation are descended from species excluding those long cultivated which have not had their reproductive systems in any way affected and they are not variable but hybrids themselves have their reproductive systems seriously affected and their descendants are highly variable but to return to our comparison of mongrels and hybrids Gartner states that mongrels are more liable than hybrids to revert to either parent form but this if it be true is certainly only a difference in degree moreover Gartner expressly states that the hybrids from long cultivated plants are more subject to reversion than hybrids from species in their natural state and this probably explains the singular difference in the results arrived at by different observers thus Max Wichura doubts whether hybrids ever revert to their parent forms and he experimented on uncultivated species of willows while Norda on the other hand insists in the strongest terms on the almost universal tendency to reversion in hybrids and he experimented chiefly on cultivated plants Gartner further states that when any two species although most closely allied to each other are crossed with the third species the hybrids are widely different from each other whereas if two very distinct varieties of one species are crossed with another species the hybrids do not differ much but this conclusion as far as I can make out is founded on a single experiment and seems directly opposed to the results of several experiments made by Collroyter such alone are the unimportant differences which Gartner is able to point out between hybrid and mongrel plants on the other hand the degrees and kinds of resemblance in mongrels and in hybrids to their respective parents more especially in hybrids produced from nearly related species follow according to Gartner the same laws when two species are crossed one has sometimes a prepotent power of impressing its likeness on the hybrid so I believe it to be with varieties of plants and with animals one variety has this prepotent power over another variety hybrid plants produced from a reciprocal cross generally resemble each other closely and so it is with mongrel plants from a reciprocal cross both hybrids and mongrels can be reduced to either pure parent form by repeated crosses in successive generations with either parent these several remarks are apparently applicable to animals but the subject is here much complicated partly owing to the existence of secondary sexual characters but more especially owing to prepotency in transmitting likeness running more strongly in one sex than in the other both when one species is crossed with another and when one variety is crossed with another variety for instance I think those authors are right who maintained that the ass has a prepotent power over the horse so that both the mule and the hiny resemble more closely the ass than the horse but that the prepotency runs more strongly in the male than in the female ass so that the mule which is an offspring of the male ass and mare is more like an ass than is the hiny which is the offspring of the female ass and stallion much stress has been laid by some authors on the supposed fact that it is only with mongrels that the offspring are not intermediate in character but closely resemble one of their parents but this does sometimes occur with hybrids yet I grant much less frequently than with mongrels looking to the cases which I have collected of crossbred animals closely resembling one parent the resemblances seem chiefly confined to characters almost monstrous in their nature and which have suddenly appeared such as albinism, melanism deficiency of tail or horns or additional fingers and toes and do not relate to characters which have been slowly acquired through selection a tendency to sudden reversions to the perfect character of either parent would also be much more likely to occur with mongrels which are descended from varieties often suddenly produced and semi monstrous in character than with hybrids which are descended from species slowly and naturally produced on the whole I entirely agree with Dr. Prosper Lukas who after arranging an enormous body of facts with respect to animals comes to the conclusion that the laws of resemblance of the child to its parents are the same whether the two parents differ little or much from each other namely in the union of individuals of the same variety or of different varieties or of distinct species independently of the question of fertility and sterility in all other respects there seems to be a general and close similarity in the offspring of crossed species and of crossed varieties if we look at species as having been specially created and at varieties as having been produced by secondary laws this similarity would be an astonishing fact but it harmonizes perfectly with the view that there is no essential distinction between species and varieties summary of chapter first crosses between forms sufficiently distinct to be ranked as species and their hybrids are very generally but not universally sterile the sterility is of all degrees and is often so slight that the most careful experimentalists have arrived at diametrically opposite conclusions in ranking forms by this test the sterility is innately variable in individuals of the same species and is eminently susceptible to action of favorable and unfavorable conditions the degree of sterility does not strictly follow systematic affinity but is governed by several curious and complex laws it is generally different and sometimes widely different in reciprocal crosses between the same two species it is not always equal in degree in a first cross and in the hybrids produced from this cross in the same manner as in grafting trees the capacity in one species or variety to take on another is incidental on differences generally of an unknown nature in their vegetative systems so in crossing the greater or less facility of one species to unite with another is incidental on unknown differences in their reproductive systems there is no more reason to think that species have been specially endowed with various degrees of sterility to prevent their crossing and blending in nature than to think that trees have been specially endowed with various and somewhat analogous degrees of difficulty in being grafted together in order to prevent there in arching in our forests the sterility of first crosses and of their hybrid progeny has not been acquired through natural selection in the case of first crosses it seems to depend on several circumstances in some instances in chief part on the early death of the embryo in the case of hybrids it apparently depends on their whole organization having been disturbed by being compounded from two distinct forms the sterility being closely allied to that which so frequently affects pure species when exposed to new and unnatural conditions of life he who will explain these latter cases will be able to explain the sterility of hybrids this view is strongly supported by a parallelism of another kind namely that firstly slight changes in the conditions of life add to the vigor and fertility of all organic beings and secondly that the crossing of forms which have been exposed to slightly different conditions of life or which have varied favours the size, vigor and fertility of their offspring the facts given on the sterility of the illegitimate unions of dimorphic and trimorphic plants and of their illegitimate progeny perhaps render it probable that some unknown bond in all cases connects the degree of fertility of first unions with that of their offspring the consideration of these facts on dimorphism as well as of the results of reciprocal crosses clearly leads to the conclusion that the primary cause of the sterility of crossed species is confined to differences in their sexual elements but why in the case of distinct species the sexual elements should so generally have become more or less modified leading to their mutual infertility we do not know but it seems to stand in some close relation to species having been exposed for long periods of time to nearly uniform conditions of life it is not surprising that the difficulty in crossing any two species and the sterility of their hybrid offspring should in most cases correspond even if due to distinct causes for both depend on the amount of difference between the species which are crossed nor is it surprising that the facility of effecting a first cross and the fertility of the hybrids thus produced and the capacity of being grafted together though this latter capacity evidently depends on widely different circumstances should all run to a certain extent parallel with the systematic affinity of the forms subjected to experiment for systematic affinity includes resemblances of all kinds first crosses between forms known to be varieties or sufficiently alike to be considered as varieties and their mongrel offspring are very generally but not as is often stated invariably fertile nor is this almost universal and perfect fertility surprising when it is remembered how liable we are to argue in a circle with respect to varieties in a state of nature and when we remember that the greater number of varieties have been produced under domestication by the selection of mere external differences and that they have not been long exposed to uniform conditions of life it should also be especially kept in mind that long continued domestication tends to eliminate sterility and is therefore little likely to induce this same quality independently of the question of fertility in all other respects there is the closest general resemblance between hybrids and mongrels in their variability in their power of absorbing each other by repeated crosses and in their inheritance of characters from both parent forms finally then although we are as ignorant of the precise cause of the sterility of first crosses and of hybrids as we are why animals and plants removed from their natural conditions become sterile yet the facts given in this chapter do not seem to me opposed to the belief that species aboriginally existed as varieties End of Chapter 9 by Charles Darwin Contents of this chapter include On the poorness of our paleontological collections On the intermittence of geological formations On the denudation of granitic areas On the absence of intermediate varieties in any one formation On the sudden appearance of groups of species On their sudden appearance in the lowest known fossiliferous strata Antiquity of the habitable earth In the sixth chapter I enumerated the chief objections which might be justly urged against the views maintained in this volume Most of them have now been discussed One, namely the distinctness of specific forms and they're not being blended together by innumerable transitional lengths is a very obvious difficulty I assigned reasons why such lengths do not commonly occur at the present day under the circumstances apparently most favorable for their presence, namely on an extensive and continuous area with graduated physical conditions I endeavored to show that the life of each species depends in a more important manner on the presence of other already defined organic forms than on climate and therefore that the really governing conditions of life do not graduate away quite insensibly like heat or moisture I endeavored also to show that intermediate varieties from existing in lesser numbers than the forms which they connect will generally be beaten out and exterminated during the course of further modification and improvement The main cause, however, of innumerable intermediate lengths not now occurring everywhere throughout nature depends on the very process of natural selection through which new varieties continually take the places of and supplant their parent forms But just in proportion as this process of extermination has acted on an enormous scale so must the number of intermediate varieties which have formerly existed be truly enormous why then is not every geological formation and every stratum full of such intermediate lengths Geology assuredly does not reveal any such finely graduated organic chain and this perhaps is the most obvious and serious objection which can be urged against my theory The explanation lies, as I believe in the extreme imperfection of the geological record In the first place it should always be borne in mind what sort of intermediate forms must on the theory have formerly existed I have found it difficult when looking at any two species to avoid picturing to myself forms directly intermediate between them But this is a wholly false view We should always look for forms intermediate between each species and a common but unknown progenitor and the progenitor will generally have differed from all its modified descendants to give a simple illustration The fantail and powder pigeons are both descended from the rock pigeon If we possessed all the intermediate varieties which have ever existed we should have an extremely close series between both and the rock pigeon but we should have no varieties directly intermediate between the fantail and powder None for instance combining a tail somewhat expanded with a crop somewhat enlarged the characteristic features of these two breeds moreover have become so such modified that if we had no historical or indirect evidence regarding their origin it would not have been possible to have determined from a mere comparison of their structure with that of the rock pigeon sea levia whether they had descended from this species or from some other allied species such as sea onus So with natural species if we look to forms very distinct for instance to the horse and tapir we have no reason to suppose that links directly intermediate between them ever existed but between each and an unknown common parent the common parent will have had in its whole organization much general resemblance to the tapir and to the horse but in some points of structure may have differed considerably from both even perhaps more than they differ from each other hence in all such cases we should be unable to recognize the parent form of any two or more species even if we closely compared the structure of the parent with that of its modified descendants unless at the same time we had a nearly perfect chain of the intermediate links it is just possible by the theory that one of two living forms might have descended from the other for instance a horse from a tapir and in this case direct intermediate links will have existed between them but such a case would imply that one form had remained for a very long period unaltered thus its descendants had undergone a vast amount of change and the principle of competition between organism and organism between child and parent will render this a very rare event for in all cases the new and improved forms of life tend to supplant the old and unimproved forms by the theory of natural selection all living species have been connected with the parent species of each genus by differences not greater than we see in the natural and domestic varieties of the same species at the present day and these parent species now generally extinct have in their turn been similarly connected with more ancient forms and so on backwards always converging to the common ancestor of each great class so that the number of intermediate and transitional links between all living and extinct species must have been inconceivably great but assuredly if this theory be true such have lived upon the earth on the lapse of time as inferred from the rate of deposition and extent of denudation independently of our not finding fossil remains of such infinitely numerous connecting links it may be objected that time cannot have sufficed for so great an amount of organic change all changes having been affected slowly it is hardly possible for me to recall to the reader who is not a practical geologist the facts leading the mind feebly to comprehend the lapse of time he who can read Sir Charles Lyell's grand work on the principles of geology which the future historian will recognize as having produced a revolution in natural science and yet does not admit how vast have been the past periods of time may at once close this volume not that it suffices to study the principles of geology or to read special treatises by different observers on separate formations and to mark how each author attempts to give an inadequate idea of the duration of each formation or even of each stratum we can best gain some idea of past time by knowing the agencies at work and learning how deeply the surface of the land has been denuded and how much sediment has been deposited as Lyell has well remarked the extent and thickness of our sedimentary formations are the result and the measure of the denudation which the earth's crust has elsewhere undergone therefore a man should examine for himself the great piles of superimposed strata and watch the rivulets bringing down mud and the waves wearing away the sea-cliffs in order to comprehend something about the duration of past time the monuments of which we see all around us it is good to wander along the coast when formed of moderately hard rocks and mark the process of degradation the tides in most cases reach the cliffs only for a short time twice a day and the waves eat into them only when they are charged with sand or pebbles for there is good evidence that pure water affects nothing in wearing away rock at last the base of the cliff is undermined huge fragments fall down and these remaining fixed have to be worn away atom by atom until after being reduced in size they can be rolled about by the waves and then they are more quickly ground into pebbles, sand or mud but how often do we see along the bases of retreating cliffs rounded boulders all thickly clothed by marine productions showing how little they are abraded and how seldom they are rolled about moreover if we follow for a few miles any line of rocky cliff which is undergoing degradation we find that it is only here and there along a short length or round a promontory that the cliffs are at the present time suffering the appearance of the surface and the vegetation show that elsewhere years have elapsed since the waters washed their base we have however recently learned from the observations of Ramsey in the van of many excellent observers of Jukes, Geike, Kroll and others that sub aerial degradation is a much more important agency than coast action or the power of the waves the whole surface of the land is exposed to the chemical action of the air and of the rain water it's dissolved carbonic acid and in colder countries to frost the disintegrated matter is carried down even gentle slopes during heavy rain and to a greater extent than might be supposed especially in arid districts by the wind it is then transported by the streams and rivers which when rapid deepen their channels and triterate the fragments on a rainy day even in a gently undulating country we see the effects of sub aerial degradation in the muddy rills which flow down every slope missers Ramsey and Whitaker have shown and the observation is a most striking one that the great lines of escarpment in the Wealdian district and those ranging across England which formerly were looked at as ancient sea coasts cannot have been thus formed for each line is composed of one in the same formation while our sea cliffs are everywhere formed by the intersection of various formations this being the case I'm compelled to admit that the escarpments owe their origin and chief part to the rocks of which they are composed having resisted sub aerial denudation better than the surrounding surface this surface consequently has been gradually lowered with the lines of harder rock left projecting nothing impresses the mind with a vast duration of time according to our ideas of time more forcibly than the conviction thus gained that sub aerial agencies which apparently have so little power and seem to work so slowly have produced great results when thus impressed with the slow rate at which the land is worn away through sub aerial and littoral action it is good in order to appreciate the past duration of time to consider on the one hand the masses of rock which have been removed over many extensive areas and on the other hand the thickness of our sedimentary formations I remember having been much struck when viewing volcanic islands which have been worn by the waves and paired all round into perpendicular cliffs of one or two thousand feet in height for the gentle slope of the lava streams due to their formerly liquid state showed at a glance how far the hard rocky beds had once extended into the open ocean the same story is told still more plainly by faults those great cracks along which the strata have been up heaved on one side or thrown down on the other to the height or depth of thousands of feet for since the crust cracked and it makes no great difference whether the upheaval was sudden or as most geologists now believe was slow and affected by many starts the surface of the land has been so completely planed down that no trace of these vast dislocations is externally visible the craven fault, for instance extends for upward of 30 miles and along this line the vertical displacement of the strata from 600 to 3,000 feet Professor Ramsey has published an account of a down throw in Anglesey of 2300 feet and he informs me that he fully believes that there is one in Marionuthshire of 12,000 feet yet in these cases there is nothing on the surface of the land to show such prodigious movements the pile of rocks on either side of the crack having been smoothly swept away on the other hand in all parts of the world the piles of sedimentary strata are of wonderful thickness in the Cordillera I estimated one mass of conglomerate at 10,000 feet and although conglomerates have probably been accumulated at a quicker rate than finer sediments yet from being formed of worn and rounded pebbles each of which bears the stamp of time they are good to show how slowly the mass must have been heaped together Professor Ramsey has given me the maximum thickness from actual measurement of the successive formations in different parts of Great Britain and this is the result Paleozoic strata not including igneous beds 57,154 feet secondary strata 13,190 feet tertiary strata 2240 feet making altogether 72,584 feet that is very nearly 13 and 3 quarters British miles some of these formations which are represented in England by thin beds are thousands of feet in thickness on the continent moreover between each successive formation we have in the opinion of most geologists blank periods of enormous length so that the lofty pile of sedimentary rocks in Britain gives but an inadequate idea of the time which has elapsed in their accumulation the consideration of these various facts impresses the mind almost in the same manner as does the vain endeavor to grapple with the idea of eternity nevertheless this impression is partly false Mr. Kroll in an interesting paper remarks that we do not air informing too great a conception of the length of geological periods but in estimating them by years when geologists look at large complicated phenomena and then at the figures representing several million years the two produce a totally different effect on the mind and the figures are at once pronounced too small in regard to sub aerial denudation Mr. Kroll shows by calculating the known amount of sediment annually brought down by certain rivers relatively to their areas of drainage that 1000 feet of solid rock as it became gradually disintegrated would thus be removed from the mean level of the whole area in the course of 6 million years this seems an astonishing result and some considerations lead to the suspicion that it may be too large but if halved or quartered it is still very surprising few of us however know what a million really means Mr. Kroll gives the following illustrations take a narrow strip of paper 83 feet 4 inches in length and stretch it along the wall of a large hall then mark off at one end and a tenth of an inch this tenth of an inch will represent 100 years and the entire strip a million years but let it be borne in mind in relation to the subject of this work what 100 years implies represented as it is by a measure utterly insignificant in a hall of the above dimensions several eminent breeders during a single lifetime have so largely modified some of the higher animals which propagate their kind much more slowly than most of the lower animals that they have formed what well deserves to be called a new sub breed few men have attended with due care to any one strain for more than half a century so that 100 years represents the work of two breeders in succession it is not to be supposed that species in a state of nature ever changed so quickly as domestic animals under the guidance of methodical selection the comparison would be in every way fairer with the effects which follow from unconscious selection that is the preservation of the most useful or beautiful animals with no intention of modifying the breed but by this process of unconscious selection various breeds have been sensibly changed in the course of two or three centuries species however probably change much more slowly and within the same country only a few change at the same time this slowness follows from all the inhabitants of the same country being already so well adapted to each other that new places in the polity of nature do not occur until after long intervals due to the occurrence of physical changes of some kind or through the immigration of new forms moreover variations or individual differences of the right nature by which some of the inhabitants might be better fitted to their new places under the altered circumstance would not always occur at once unfortunately we have no means of determining according to the standard of years how long a period it takes to modify a species but to the subject of time to return on the poorness of paleontological collections now let us return to our richest museums and what a paltry display we behold that our collections are imperfect is admitted by everyone the remark of that admirable paleontologist Edward Forbes should never be forgotten namely that very many fossil species are known and named from single and often broken specimens or from a few specimens on some one spot only a small portion of the surface of the earth has been geologically explored and no part with sufficient care as the important discoveries made every year in Europe prove no organism wholly soft can be preserved shells and bones decay and disappear when left on the bottom of the sea where sediment is not accumulating we probably take a quite erroneous view when we assume that sediment is being deposited over nearly the whole bed of the sea it's sufficiently quick to embed and preserve fossil remains throughout an enormously large proportion of the ocean the bright blue tint of the water bespeaks its purity the many cases on record of a formation conformably covered after an immense interval of time by another and later formation without the underlying bed having suffered in the interval anywhere and tear seem explicable only on the view of the bottom of the sea not rarely lying for ages in an unaltered condition the remains which do become embedded if in sand or gravel will when the beds are upraised generally be dissolved by the percolation of rain water charged with carbonic acid some of the many kinds of animals which live on the beach between high and low water mark seem to be rarely preserved for instance the several species of the cthamelinnae a subfamily of sessile syrupedes coat the rocks all over the world in infinite numbers they are all strictly littoral with the exception of a single Mediterranean species which inhabits deep water and this has been found fossil in Sicily whereas not one other species has hitherto been found in any tertiary formation yet it is known that the genus cthamelis existed during the chalk period lastly many great deposits requiring a vast length of time for their accumulation are entirely destitute of organic remains without being able to assign any reason one of the most striking instances is that of the flesh formation which consists of shale and sandstone several thousand occasionally even 6000 feet in thickness and extending for at least 300 miles from Vienna to Switzerland and although this great mass has been most carefully searched no fossils except a few vegetable remains have been found with respect to the terrestrial productions that existed during the secondary and Paleozoic periods it is superfluous to state that our evidence is fragmentary in an extreme degree for instance until recently not a land shell was known belonging to either of these vast periods with the exception of one species discovered by Sir C. Lyell and Dr. Dawson in the Carboniferous Strat of North America but now land shells have been found in the Leas in Lyell's manual which bring home the truth how accidental and rare is their preservation far better than pages of detail nor is their rarity surprising when we remember how larger proportion of the bones of tertiary mammals have been discovered either in caves or in lacostrine deposits and that not a cave or true lacostrine bed is known belonging to the age of our secondary or Paleozoic formations but the imperfection in the geological record largely results from another more important cause than any of the foregoing namely from the several formations being separated from each other by wide intervals of time this doctrine has been emphatically admitted by many geologists and paleontologists who, like E. Forbes entirely disbelieve in the change of species when we see the formations tabulated in written works or when we follow them in nature believing that they are closely consecutive but we know for instance from Sir R. Murchison's great work on Russia what wide gaps there are in that country between the superimposed formations so it is in North America and in many other parts of the world the most skillful geologist if his attention had been confined exclusively to these large territories would never have suspected that during the periods which were blank and barren in his own country great piles of sediment attached with new and peculiar forms of life had elsewhere been accumulated and if in every separate territory hardly any idea can be formed of the length of time which is elapsed between the consecutive formations we may infer that this could nowhere be ascertained the fragment and great changes in the mineralogical composition of consecutive formations generally implying great changes in the geography of the surrounding lands whence the sediment was derived accord with the belief of vast intervals of time having elapsed between each formation we can I think see why the geological formations of each region are almost invariably intermittent that is have not followed each other in close sequence scarcely any fact struck me more when examining many hundred miles of the South American coasts which have been upraised several hundred feet within the recent period than the absence of any recent deposits sufficiently extensive to last short geological period along the whole west coast which is inhabited by a peculiar marine fauna tertiary beds are so poorly developed that no record of several successive and peculiar marine faunas will probably be preserved to a distant age a little reflection will explain why along the rising coast of the western side of South America no extensive formations with recent or tertiary remains can anywhere be found though the supply of sediment is great from the enormous de-radiation of the coast rocks and from the muddy streams entering the sea the explanation no doubt is that the littoral and sub-littoral deposits are continually worn away as soon as they are brought up by the slow and gradual rising of the land within the grinding action of the coast waves we may I think conclude that sediment must be accumulated in extremely thick solid or extensive masses in order to withstand the incessant action of the waves when first upraised and during subsequent oscillations of level as well as the subsequent sub-areal degradation such thick and extensive accumulations of sediment may be formed in two ways either in profound depths of the sea in which case the bottom will not be inhabited by so many in such varied forms of life as the more shallow seas and the mass when upraised will give an imperfect record of the organisms which existed in the neighborhood during the period of its accumulation or sediment may be deposited to any thickness and extent over a shallow bottom if it continues slowly to subside in this latter case as long as the rate of subsidence and supply of sediment nearly balance each other the sea will remain shallow and favorable for many in varied forms and thus a rich fossiliferous formation thick enough when upraised to resist a large amount of denudation may be formed since that nearly all are ancient formations which are throughout the greater part of their thickness rich in fossils have thus been formed during subsidence since publishing my views on this subject in 1845 I have watched the progress of geology and have been surprised to note how author after author in treating of this or that great formation has come to the conclusion that it was accumulated during subsidence I may add that the only ancient tertiary formation on the west coast of South America which has been bulky enough to resist such degradation as it has as yet suffered but which will hardly last to a distant geological age was deposited during a downward oscillation of level and thus gained considerable thickness all geological facts tell us plainly that each area has undergone numerous slow oscillations of level and apparently these oscillations have affected wide spaces consequently formations rich in fossils and sufficiently thick and extensive to resist subsequent degradation will have been formed over wide spaces during periods of subsidence but only where the supply of sediment was sufficient to keep the sea shallow and to embed and preserve the remains before they had time to decay on the other hand as long as the bed of the sea remained stationary thick deposits cannot have been accumulated in the shallow parts which are the most favorable to life still less can this have happened during the alternate periods of elevation or to speak more accurately the beds which were then accumulated will generally have been destroyed by being upraised and brought within the limits of the coast action these remarks apply chiefly to littoral and sub-littoral deposits in the case of an extensive and shallow sea such as that within a large part of the Malay archipelago where the depth varies from 30 or 40 to 60 fathoms a widely extended formation might be formed during a period of elevation and yet not suffer excessively from denudation during its slow upheaval but the thickness of the formation could not be great for owing to the elevatory movement it would be less than the depth in which it was formed nor would the deposit be much consolidated nor be capped by overlying formations so that it would run a good chance of being worn away by atmospheric degradation and by the action of the sea and by the oscillations of level it has, however, been suggested by Mr. Hopkins that if one part of the area after rising and before being denuded subsided the deposit formed during the rising movement though not thick might afterwards become protected by fresh accumulations and thus be preserved for a long period Mr. Hopkins also expresses his belief that sedimentary beds of considerable horizontal extent have rarely been completely destroyed and all geologists accepting the few who believe that our present metamorphic shists and plutonic rocks once formed the primordial nucleus of the globe will admit that these latter rocks have been stripped of their covering to an enormous extent for it is scarcely possible that such rocks could have been solidified and crystallized while uncovered but if the metamorphic action occurred at profound depths of the ocean the former protecting mantle of rock then that Gnice, Mykoshist, Granite, Diorite, etc. were once necessarily covered up how can we account for the naked and extensive areas of such rocks in many parts of the world except on the belief that they have subsequently been completely denuded of all overlying strata that such extensive areas do exist cannot be doubted the granitic region of Parime is described by Humboldt as being at least 19 times as large as Switzerland south of the Amazon Bué colors an area composed of rocks of this nature as equal to that of Spain, France, Italy, part of Germany and the British Isles all conjoined this region has not been carefully explored but from the concurrent testimony of travelers the granitic area is very large thus Von Eschwege gives a detailed section of these rocks stretching from Rio de Janeiro for 260 geographical miles inland in a straight line and I traveled for 150 miles in another direction and saw nothing but granitic rocks numerous specimens collected along the whole coast from Rio de Janeiro to the mouth of the Plata a distance of 1,100 geographical miles were examined by me and they all belonged to this class inland along the whole northern bank of the Plata I saw besides modern tertiary beds only one small batch of an anamorphosed rock which alone could have formed a part of the original capping of the granitic series turning to a well-known region namely to the United States and Canada as shown in Professor H.D. Rogers' beautiful map I have estimated the areas by cutting out and weighing the paper and I find that the metamorphic excluding the semi-metamorphic and granite rocks exceed in the proportion of 19 to 12.5 the whole of the newer paleozoic formations in many regions the metamorphic and granite rocks would be found much more widely extended than they appear to be if all the sedimentary beds were removed which rest unconformably on them and which could not have formed part of the original mantle under which they were crystallized hence it is probable that in some parts of the world whole formations have been completely denuded with not a wreck left behind one remark is here worth a passing notice during periods of elevation the area of the land under the adjoining shoal parts of the sea will be increased and new stations will often be formed all circumstances favorable as previously explained for the formation of new varieties and species but during such periods there will generally be a blank in the geological record on the other hand during subsidence the inhabited area and number of inhabitants will decrease accepting on the shores of a continent when first broken up into an archipelago and consequently during subsidence though there will be much extinction few new varieties or species will be formed and it is during these very periods of subsidence that the deposits which are richest in fossils have been accumulated End of Chapter 10, Part A