 This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Recorded by Rainer Obgenrein. The Origin of Species by Means of Natural Selection or the Preservation of Favorite Races in the Struggle of Life. Sixth London Edition by Charles Darwin. Chapter number five. Laws of Variation. Contents of this chapter include Effects of Change Conditions. Use and Disuse combined with Natural Selection. Organs of Light and of Vision. Acclimatization. Correlated Variation. Compensation and Economy of Growth. False Correlations. Multiple, rudimentary and lowly organized structures variable. Parts developed in an unusual manner are highly variable. Specific characters more variable than generic. Secondary sexual characters variable. Species of the same genus vary in an analogous manner. Reversions to long lost characters. Summary. I have witherow sometimes spoken as if the variations so common and multi-form with organic beings under domestication. And in a lesser degree with those under nature were due to chance. This, of course, is a wholly incorrect expression. But it serves to acknowledge plainly our ignorance of the cause of each particular variation. Some authors believe it to be as much the function of the reproductive system to produce individual differences or slight deviations of structure as to make the child like its parents. But the fact of variations and monstrosities occurring much more frequently under domestication than under nature and the greater variability of species having wide ranges than of those with restricted ranges lead to the conclusion that variability is generally related to the conditions of life to which each species has been exposed during several successive generations. In the first chapter I attempted to show that change conditions act in two ways. Directly on the whole organization or on certain parts alone and indirectly through the reproductive system. In all cases there are two factors. The nature of the organism which is much the most important of the two and the nature of the conditions. The direct action of change conditions leads to definite or indefinite results. In the latter case the organization seems to become plastic and we have much fluctuating variability. In the former case the nature of the organism is such that it yields readily when subjected to certain conditions and all or nearly all the individuals become modified in the same way. It is very difficult to decide how far change conditions such as of climate, food, etc. have acted in a definite manner. There is a reason to believe that in the course of time the effects have been greater than can be proved by clear evidence. But we may safely conclude that the innumerable complex co-adaptations of structure which we see throughout nature between various organic beings cannot be attributed simply to such action. In the following cases the conditions seem to have produced some slight definite effect. E. Forbes asserts that shells at the southern limit and when living in shallow water are more brightly colored than those of the same species of further north or from a greater depth. But this certainly does not always hold good. Mr. Gould believes that birds of the same species are more brightly colored under a clear atmosphere than when living near the coast or on islands and Wollaston is convinced that residents near the sea affect the colors of insects. Mokin Tenden gives a list of plants which when growing near the seashore have their leaves in some degree fleshy, though not elsewhere fleshy. These lightly varying organisms are interesting in as far as they present characters analogous to those possessed by the species which are confined to similar conditions. When a variation is of the slightest use to any being we cannot tell how much to attribute to the accumulative action of natural selection and how much to the definite action of the conditions of life. Thus it is well known to furrious that animals of the same species have thicker and better fur the further north they live. But who can tell how much of this difference may be due to the warmest clad individuals having been favored and preserved during many generations and how much to the action of the severe climate for it would appear that climate has some direct action on the hair of our domestic quadrupeds. Instances could be given of similar varieties being produced from the same species under external conditions of life as different as can well be conceived and on the other hand of the similar varieties being produced under apparently the same external conditions. Again, innumerable instances are known to every naturalist of species keeping true or not varying at all although living under the most opposite climates. Such considerations as these inclined me to lay less wide on the direct action of the surrounding conditions than on a tendency to vary due to causes of which we are quite ignorant. In one sense the conditions of life may be said not only to cause variability either directly or indirectly but likewise to include natural selection for the conditions determine whether this or that variety shall survive. But when man is a selecting agent we clearly see that the two elements of change are distinct. Variability is in some manner excited but it is the will of man which accumulates the variations in certain direction and it is this letter agency which answers to the survival of the fittest under nature. Subchapter Effects of the increased use and disuse of parts as controlled by natural selection. From the facts alluded to in the first chapter I think there can be no doubt that use in our domestic animals has drenched and enlarged certain parts and disused diminished them and that such modifications are inherited. Under free nature we have no standard of comparison by which to judge of the effects of long-continued use or disuse for we know not the parent forms but many animals possess structures which can be best explained by the effects of disuse. As Professor Owen has remarked there is no greater anomaly in nature than a bird that cannot fly. Yet there are several in this state. The logger-headed duck in South America can only flap along the surface of the water and has its wings in nearly the same condition as a domestic alesbury duck. It is a remarkable fact that the younger birds according to Mr. Cunningham can fly while the adults have lost this power. As larger ground-feeding birds seldom take flight except to escape danger it is probable that the nearly wingless condition of several birds now inhabiting or which lately inhabited several oceanic islands tenanted by no beasts of prey has been caused by disuse. The ostrich indeed inhabits continents and is exposed to danger from which it cannot escape a flight but it can defend itself by kicking its enemies as efficiently as many quadrupeds. We may believe that the progenitor of the ostrich genus had habits like those of the bastard and that as the size and weight of its body were increased during successive generations its legs were used more and its wings less until they became incapable of life. Kirby has remarked and I have observed the same fact that the anterior tarsie or feet of many male dung-feeding beetles are often broken off. He examined seven tea specimens in his own collection and not one had even a relic left. In the United Apollos the tarsie are so habitually lost that the insect has been described as not having them. In some other genera they are present but in a rudimentary condition. In the Antiochus or sacred beetle of the Egyptians they are totally deficient. The evidence that accidental mutilations can be inherited is at present not decisive. But the remarkable cases observed by Brown-Sacard in guinea pigs of the inherited effects of operations should make us cautious in denying this tendency. Hence it will perhaps be safest to look at the entire absence of the anterior tarsie and Antiochus and their rudimentary condition in some other genera not at cases of inherited mutilations but as due to the effects of long continued disuse. For as many dung-feeding beetles are generally found with their tarsie lost this must happen early in life. Therefore the tarsie cannot be of much importance or be much used by these insects. In some cases we might easily put down to disuse modifications of structure which are holy or mainly due to natural selection. Mr. Walliston has discovered the remarkable fact that 200 beetles are of the 550 species but more or not known. Inhabiting Madera are so far deficient in wings that they cannot fly and that of the 29 endemic genera no less than 23 have all their species in this condition. Several facts, namely that beetles in many parts of the world are very frequently blown to sea and perish that the beetles in Madera as observed by Mr. Walliston lie much concealed until the wind lulls and the sun shines that the proportion of wingless beetles is larger on the exposed to zertas than in Madera itself and especially the extraordinary fact so strongly insisted on by Mr. Walliston that certain large groups of beetles elsewhere excessively numerous which absolutely require the use of their wings are here almost entirely absent. These several considerations make me believe that the wingless condition of so many Madera beetles is mainly due to the action of natural selection combined probably with this use or during many successive generations each individual beetle which flew least either from its wings having been ever so little less perfectly developed or from indolent habit will have had the best chance of surviving from not being blown out to sea and on the other hand those beetles which most readily took to flight would oftenest have been blown to sea and thus destroyed. The insects in Madera which are not ground feeders and which a certain flower feeding Coleoptera and Lepidoptera must habitually use their wings to gain their subsistence have as Mr. Walliston suspects their wings not at all reduced but even enlarged this is quite compatible with the action of natural selection for when a new insect first arrives on the island the tendency of natural selection to enlarge or to reduce the wings would depend on whether a greater number of individuals were saved by successfully battling with the winds or by giving up the attempt and rarely or never flying as with marinas shipped wrecked near a coast it would have been better for the good swimmers if they had been able to swim still further whereas it would have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck. The eyes of moles and some burrowing rodents are rudimentary in size and in some cases are quite covered by skin and fur this state of the eyes is probably due to gradually reduction from this use but aided perhaps by natural selection in South America a burrowing rodent the tucotucco or stenomous is even more subterranean in its habits than the mole and I was assured by a Spaniard who had often caught them and they were frequently blind one which I kept alive was certainly in this condition the cause as appeared on the section having been inflammation of the nictitating membrane as frequent inflammation of the eyes must be injurious to any animal and as eyes are certainly not necessary to animals having subterranean habits a reduction in their size the adhesion of the eyelids and growth of fur over them might in such case be an advantage and if so natural selection would aid the effects of this use it is well known that several animals belonging to the most different classes which inhabit the caves of Carniola and Kentucky are blind in some of the crabs the foot stalk for the eye remains though the eye is gone the stand for the telescope is there though the telescope with its glasses has been lost as it is difficult to imagine the eyes though useless could be in any way injurious to animals living in darkness their loss may be attributed to disuse in one of the blind animals namely the cave red Neotoma two of which were captured by Professor Silliman at above half a mile distance from the mouth of the cave and therefore not in the profoundest depth the eyes were illustrious and of large size and these animals as I am informed by Professor Silliman having been exposed for about a month to graduated light acquired a dim perception of objects it is difficult to imagine conditions of life more similar than deep limestone caverns under a nearly similar climate though that in accordance with the old view of the blind animals having been separately created for the American and European caverns very close similarity in their organization and affinities might have been expected this is certainly not the case if we look at the two whole fawners with respect to the insects alone Shiot has remarked we are accordingly prevented from considering the entire phenomenon in any other light than something purely local and the similarity which is exhibited in a few forms between the mammoth cave in Kentucky and the caverns in Carniola otherwise then as a very plain expression of that analogy which subsists generally between the fauna of Europe and North America on my view we must suppose that American animals having in most cases ordinary powers of vision slowly migrated by successive generations from the outer world into the deeper and deeper recesses of the Kentucky caves as did European animals into the caves of Europe we have some evidence of this gradiation of habit for as Shiot remarks we accordingly look upon the Saptarian fawners as small ramifications which have penetrated into the earth from the geographically limited fawners of the adjacent tracts at which they extended themselves into darkness have been accommodated to surrounding circumstances animals not far remote from ordinary forms prepare the transition from light to darkness next follow those that are constructed for twilight and last of all those destined for total darkness and whose formation is quite peculiar these remarks of Shiots it should be understood apply not to the same but to distinct species by the time that an animal had reached after numberless generations the deepest recesses these youths will on this view have more or less perfectly obliterated its eyes a natural selection will often have affected other changes such as an increase in the length of the antenna or pulpy as a compensation for blindness notwithstanding such modifications we might expect still to see in the cave animals of America affinities to the other inhabitants of that continent and in those of Europe to the inhabitants of the European continent and this is the case with some of the American cave animals as I hear from Professor Dana and some of the European cave insects are very closely allied to those of the surrounding country it would be difficult to give any rational explanation of the affinities of the blind cave animals to the other inhabitants of the two continents on the ordinary view of their independent creation that several of the inhabitants of the caves of the old and the new worlds should be closely related we might expect from the well-known relationship of most of their other productions as a blind species of Batusia is found in abundance on shady rocks far from caves the loss of vision in the cave species of this one genus has probably had no relation to its dark habitation for it is natural that an insect already deprived of vision should readily become adapted to dark caverns an other blind genus and Oftalmus offers this remarkable peculiarity that the species, as Mr. Murray observes have not as yet been found anywhere except in caves yet those which inhabit the several caves of Europe and America are distinct but it is possible that the progenitors of these several species while they were furnished with ice may formally have ranged over both continents and then have become extinct accepting in their presence occluded abodes far from feeling surprised that some of the cave animals should be very anomalous as Agassus has remarked in regard to the blind fish the Amliopsis and as it is a case with the blind Proteus with reference to the reptiles of Europe I am only surprised that more wrecks of ancient life have not been preserved owing to the less severe competition to which the scanty inhabitants of these darker bodes will have been exposed sub-chapter acclimatization habit is a redeterraous plant as in the period of flowering in the time of sleep and the amount of rain requisite for seeds to germinate, etc. and this leads me to say a few words on acclimatization as it is extremely common for distinct species belonging to the same genus to inhabit hot and cold countries if it be true that all the species of the same genus are descendant from a single parent form acclimatization must be readily affected during a long course of descent it is notorious that each species is adapted to the climate of its own home species from an arctic or even from a temperate region cannot endure a tropical climate or conversely so again many succulent plants cannot endure a damp climate but the degree of adaption of species to the climates under which they live is often overrated we may infer this from our frequently inability to predict whether or not an important plant will endure our climate and from the number of plants and animals brought from different countries which are here perfectly healthy we have reason to believe that species in a state of nature are closely limited in the ranges by the competition of other organic beings as much as or more than by adaption to the particular climates but whether or not this adaption is in most cases very close we have evidence with some few plants of their becoming to a certain extent naturally habituated to different temperatures that is they become acclimatized thus the pines and rododendrons collected by Dr. Hooker from the same species growing at different heights on the Himalayas were found to possess in this country different constitutional powers of resisting cold Mr. Thwaites informs me that he has observed similar facts in Cylon analogous observations have been made by Mr. H. C. Watson on European species of plants brought from the Azores to England and I could give other cases in regard to animals there were authentic instances could be adused of species having largely extended within historical times their range from warmer to colder latitudes and conversely but we do not positively know that these animals were strictly adapted to their native climate though in all ordinary cases we assume such to be the case nor do we know that they have subsequently become especially acclimatized to their new homes so as to be better fitted for them than they were at first as we may infer that are domestic animals were originally chosen by uncivilized men because they were useful and because they bred readily under confinement and not because they were subsequently found capable of far extended transportation the common and extraordinary capacity in our domestic animals of not only withstanding the most different climates but of being perfectly fertile a far severer test under them may be used as an argument of other animals now in a state of nature could easily be brought to bear wildly different climates we must not however push the foregoing argument too far on account of the probable origin of some of our domestic animals from several wild stocks the blood for instance of a tropical and arctic wolf may perhaps be mingled in our domestic breeds the red and mouse cannot be considered as domestic animals but they have been transported by men to many parts of the world and now have a far wider range than any other rodent for they live under the cold climate of a row in the north and the falcons in the south and on many an island in the torrent hence adaptation to any special climate may be looked at as a quality readily grafted on an innate wide flexibility of constitution common to most animals on this view the capacity of enduring the most different climates by man himself and by his domestic animals and the fact of the extinct elephant and the rhinoceros having formally endured a glacial climate whereas the living species are now all tropical or subtropical in their habits ought not to be looked at as anomalies but as examples of a very common flexibility of constitution brought under peculiar circumstances into action how much of the acclimatization of species to any peculiar climate is due to mere habit and how much to the natural selection of varieties having different innate constitutions and how much to both means combined is an obscure question that habit or custom has some influence I must believe both from analogy and from the incessant advice even in agricultural works even in the ancients of China to be very cautious in transporting animals from one district to another as it is not likely that man should have succeeded in selecting so many breeds and sub-breeds with constitution specially fitted for their own districts the result must I think to be due to habit on the other hand natural selection would inevitably tend to preserve those individuals which were born with constitutions best adapted to any country which they inhabited in treatises on many kinds of cultivated plants certain varieties are said to withstand certain climates better than others this is strikingly shown in works on fruit trees published in the united states in which certain varieties are habitually recommended for the northern and others for the southern states and as most of these varieties are of recent origin they cannot owe the constitutional differences to habit the case of the Jerusalem artichoke which is never propagated in England by seed and of which consequently the new varieties have not been produced has even been advanced as proving that acclimatization cannot be affected for it is now as tender as ever it was the case also of the kidney bean has been often cited for a similar purpose and with much greater weight but until someone will so during the score of generations his kidney beans so early that a very large proportion are destroyed by frost and then collect seed from the few survivors with care to prevent accidental crosses and then again get seed from the seedlings with the same precautions the experiment cannot be said to have been tried nor let it be supposed the differences and the constitution appear for an account has been published how much more hardy some seedlings are than others and of this fact I have myself observed striking instances on the whole we may conclude that habit or use and disuse have in some cases played a considerable part in the modification of the constitution and structure but that the effects have often been largely combined with and sometimes overmasked by the natural selection of innate variations sub-chapter correlated variation I mean by this expression that the whole organization is so tied together during its growth and development that when slight variations and any one part occur and are accumulated through natural selection other parts become modified this is a very important subject most imperfectly understood and no doubt wholly different classes of facts may be here easily confounded together we shall presently see that simple inheritance often gives the false appearance of correlation one of the most obvious real cases is that variations of structure arising in the young or lave naturally tend to affect the structure of the mature animal the several parts which are homologous and which at an early embryonic period are identical in structure and which are necessarily exposed to similar conditions seem eminently liable to vary in a like manner we see this in the right and the left sides of the body varying in the same manner in the front and the hind legs and even in the jaws and limbs varying together for the lower jaw is believed by some anatomists to be homologous with the limbs these tendencies I do not doubt may be mastered more or less completely by natural selection a family of stags once existed with an antler only on one side and if this had been of any great use to the breed it might probably have been rendered permanent by natural selection homologous parts as has been remarked by some authors tend to cohere this is often seen in monstrous plants and nothing is more common in the union of homologous parts and normal structures as in the union of the petals into a tube hard parts seem to affect the form of the adjoining soft parts it is believed by some authors that with birds the diversity in the shape of the pelvis causes the remarkable diversity in the shape of the kidneys others believe that the shade of the pelvis in the human mother influences by pressure the shape of the head of the child in snakes according to Schliegel the shape of the body and the manner of swallowing determine the position and form of several of the most important viscera the nature of the bond is frequently quite obscure Ms. Joffrey Sanctelaire has forcibly remarked certain malconformations frequently and that others rarely coexist without our being able to assign any reason what can be more singular than the relation and cats between complete whiteness and blue eyes with deafness or between the tortoise shell color and the female sex or in pigeons between the feathered feet and skin betwixt the outer toes or between the presence of more or less down on the young pigeon when first hatched with the future color of its plumage or again the relation between the hair and the teeth and the naked Turkish dog though here no doubt homology comes into play with respect to this latter case of correlation I think it can hardly be accidental that the two orders of mammals which are most abnormal in their dermal covering these cetacea, whales and indentita armadilloas, scaly anteaters, etc are likewise on the whole the most abnormal in their teeth but there are so many exceptions to this rule as Mr. Mirward has remarked that it has little value I know of no case better adapted to show the importance of the laws of correlation and variation independently of utility and therefore of natural selection than that of the difference between the outer and inner flows in some compositors and armadilloas plants everybody is familiar with the difference between the ray and central florets of for instance the daisy and this difference is often accompanied with a partial or complete abortion of the reproductive organs but in some of these plants the seeds also different shape and sculpture these differences have sometimes been attributed to the pressure of the involucra on the florets or to the mutio pressure and the shape of the seeds in some compositor continents this idea but with the umbilifery it is by no means as Dr. Hooker informs me the species with the densest heads which most frequently differ in their inner and outer flowers it might have been thought that the development of the ray petals by drawing nourishment from the reproductive organs causes their abortion but this will hardly be the sole case for in some compositor the seeds of the outer and inner florets differ without any difference in the corolla possibly these several differences may be connected with a different flow of nourishment towards the central and external flowers we know at least that with irregular flowers those nearest to the axis are subject to palloria that is to become abnormally symmetrical I may add as an instance of this fact and as a striking case of correlation that in many palagoniums the two upper petals and the central flower of the truss often lose their patches of darker color and when this occurs the anterior nectare is quite aborted the central flower thus becoming palloric or regular when the color is absent from only one of the two upper petals the nectare is not quite aborted but is much shortened with respect to the development of the corolla sprangles idea that the ray florets serve to attract insects whose agency is highly advantageous or necessary for the fertilization of these plants is highly probable and if so natural selection may have come into play but with respect to the seeds it seems impossible that their differences in shape which are not always correlated with any difference in the corolla can be in any way beneficial yet in the umbilithory these differences are of such apparent importance the seeds being sometimes of the spermers in the exterior flowers in colio spermers in the central flowers that the elder decondole founded his main divisions in the order on such characters hence modification of structure viewed by systematists as of high value may be wholly due to the laws of variation and correlation of being as far as we can judge of the slightest service to the species we may often falsely attribute to the correlated variation structures which are common to whole groups of species and which in truth are simply due to inheritance for an ancient progenitor may have acquired through natural selection some one modification in structure and after thousands of generations some other and independent modification and these two modifications having been transmitted to a whole group of descendants with diverse habits would naturally be sought to be in some necessary manner correlated some other correlations are apparently due to the manner in which natural selection can alone act for instance after condole has remarked that winged seeds are never found in fruits which do not open I should explain this rule by the impossibility of seeds gradually becoming winged through natural selection unless the capsules were open for in this case alone could the seeds which were a little better adapted to be waived by the wind gain an advantage over others less well fitted for wider spurs sub-chapter compensation and economy of growth the elder Jeffrey and Goethe propounded at about the same time their law of compensation or a balancement of growth or as Goethe expressed it in order to spend on one side nature is forced to economize on the other side I think this holds true to a certain extent with our domestic productions if nourishment flows to one part of their organic excess it rarely flows at least in excess to another part thus it is difficult to get a cow to give much milk and to fatten readily the same varieties of the cabbage do not yield abundant and nutritious foliage and copious supply of oil-bearing seeds when seeds in our fruits become atrophied gains largely in size and quality in our poultry a large tuft of feathers on the head is generally accompanied by a diminished comp and a large beard by diminished battles with species in a state of nature it can hardly be maintained that the law is of universal application but many good observers more especially botanists believe in this truth I will not however here give any instances for I see hardly any way of distinguishing between the effects on the one hand of a part being largely developed through natural selection and another and adjoining part being reduced by the same process or by this use and on the other hand the actual withdrawal of nutriment from one part owing to the excess of growth in an other and adjoining part I suspect also that some of the cases of compensation which have been advanced and likewise some other effects may be merged under a more general principle namely the natural selection is continually trying to economize in every part of the organization if under change conditions of life a structure before useful becomes less useful its diminution will be favored for it will profit the individual not to have its nut remand wasted in building up a useless structure I can thus only understand the fact with which I was much struck when examining syrupedes and of which many other instances could be given namely that when a syrupede is parasitic within an other syrupede and is thus protected it loses more or less completely its own shell or carepathy this is the case with a male imla and in a truly extraordinary manner with a proteopolis for the carepathy in all other syrupedes consists of the three highly important anterior segments of the head enormously developed and furnished with great nerves and muscles but in the parasitic and protected proteopolis the whole anterior part of the head is reduced to a merist rudiment attached to the basis of the prehensile antennae now the saving of a large and complex structure when rendered superfluous would be a decided advantage to each successive individual of the species for in the struggle of life to which every animal is exposed each would have a better chance of supporting itself by less nutrient being wasted thus as I believe natural selection will tend in the long run to reduce any part of the organization as soon as it becomes through changed habits superfluous without by any means causing some other part to be largely developed in a corresponding degree and conversely that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary compensation the reduction of some adjoining part multiple rudimentary and lowly organized structures are variable it seems to be a rule as remarked by is Jofre Saint-Helaire both with varieties and species that when any part or organ is repeated many times in the same individual as the vertebrae and snakes and the stainless and polyandrous flowers the number is variable whereas the number of the same part or organ when it occurs in lesser numbers is constant the same author as well as some botanists have further remarked multiple parts are extremely liable to varying structure as vegetative repetition to use professor Owens expression is a sign of low organization the foregoing statements accord with the common opinion of naturalists that beings which stand low in the scale of nature are more variable than those which are higher I presume that lowness here means that the several parts of the organization have been but did specialized for particular functions and as long as the same part has to perform diversified work we can perhaps see why it should remain variable that is why natural selection would not have preserved or ejected each little deviation to form so carefully as when the part has to serve for some one special purpose in the same way as a knife which has to cut all sorts of things maybe of almost any shape whilst the tool for some particular purpose must be of some particular shape natural selection it should never be forgotten can act solely through and for the advantage of each being rudimentary parts as is generally admitted are apt to be highly variable we shall have to refer to the subject and I will hear only that their variability seems to result from their uselessness and consequently from natural selection having had no power to eject deviations and their structure sub-chapter a part developed in any species in an extraordinary degree or manner in comparison with the same part in an allied species tends to be highly variable several years ago I was much struck by a remark to the above of the effect made by Mr. Waterhouse Professor Ohn also seems to have come to a nearly similar conclusion it is hopeless to attempt to convince any one of the truth of the above proposition without giving the long array of facts which I have collected and which cannot possibly be here introduced I can only state my conviction of my generality I am aware of several causes of error but I hope that I have made new allowances for them I should be understood that the rule by no means applies to any part however unusually developed unless it be unusually developed in one species or in a few species in comparison with the same part in many closely allied species thus the wing of the bat is the most abnormal structure in the class of mammals but the rule would not apply here because the whole group of bats possesses wings it could apply only if someone's species had wings developed in a remarkable manner in comparison with the other species of the same genus the rule applies very strongly in the case of secondary sexual characters when displayed in any unusual manner the term secondary sexual characters used by hunter relates to characters which are attached to one sex but are not directly connected with the act of reproduction the rule applies to males and females but more rarely to females they seldom offer remarkable secondary sexual characters the rule being so plainly applicable in the case of secondary sexual characters may be due to the great variability of these characters whether or not displayed in any unusual manner a witch fact I think there can be little doubt but that our rule is not confined to secondary sexual characters is clearly shown in the case of hermaphrodite syripetis I particularly attended to Mr. Waterhouse's remark whilst investigating this order and I am fully convinced that the rule almost always holds good I shall in a future work give a list of all the more remarkable cases I will here give only one as it illustrates the rule in its largest application the opercular vulvas of the syripetis rock barnacles are in every sense of the word very important structures and they differ extremely little even in a distinct genera but in the several species of vongines pyrogoma these vulvas present a marvelous amount of classification the homologous vulvas in the different species being sometimes wholly unlike in shape and the amount of variation in the individuals of the same species is so great that it is no exaggeration to state that the varieties of the same species differ more from each other and the characters derived from these important organs then do the species belonging to other distinct genera as with the birds the individuals of the same species inhabiting the same country vary extremely little I have particularly attended to them and the rule certainly seems to hold good in this class I cannot make out that it applies to plants and this would have seriously shaken my belief in its truth had not the great variability in plants made it particularly difficult to compare their relative degree of variability when we see any part or organ developed in a remarkable degree or manner and the species the fair presumption is that it is of high importance nevertheless it is in this case eminently liable to variation why should this be so? on the few that each species has been independently created with all its parts as we now see them I can see no explanation but on the few that groups of species are descendant from some other species modified through natural selection I think we can obtain some light first let me make some preliminary remarks if in our domestic animals any part of the whole animal be neglected and no selection be applied that part for instance the comp in the dorking fowl or the whole breed and the breed may be said to be degenerating in the rudimentary organs and in those which have been but little specialized for any particular purpose and perhaps in polymorphic groups we can see a nearly parallel case for in such cases natural selection either has not or cannot come into full play and thus the organization is left in a fluctuating condition but what here more particularly concerns us is that those points in our domestic animals which at the present time are undergoing rapid change by continued selection are also eminently liable to variation look at the individuals on the same breed of the pigeon and see what a prodigious amount of difference there is in the beak of tumblers in the beak and wattle of carriers in the carriage and tail of phantels etc these being the points now mainly attended to by English fanciers even in the same sub breed as in that of the short-faced tumbler it is notoriously difficult to breed nearly perfect birds many departing widely from the standard there may truly be said to be a constant struggle going on between on the one hand the tendency to reversion to a less perfect state as well as an innate tendency to new variations and on the other hand the power of steady selection to keep the breed true in the long run selection gains the day and we do not expect to fail so completely as to breed a bird as coarse as a common tumbler pigeon from a good short-faced strain but as long as selection is rapidly going on much variability and the parts undergoing modification may always be expected now let us turn to nature when a part has been developed in an extraordinary manner in any one species compared with the other species of the same genus we may conclude that this part has undergone an extraordinary amount of modification since the period when several species branched off from the common progenitor of the genus this period will seldom be remote in any extreme degree as species rarely endure for more than one geological period an extraordinary amount of modification implies an unusually large and long continued amount of variability which has continually been accumulated by natural selection for the benefit of the species but as the variability of the extraordinary developed part or organ has been so great and long continued with the period not excessively remote we might as a general rule still expect to find more variability in such parts than in other parts of the organization which have remained for a much longer period nearly constant and this is the case that the struggle between natural selection on the one hand and the tendency to reversion and variability on the other hand will in the course of time cease and that the most abnormally developed organs may be made constant I see no reason to doubt hence when an organ however abnormal it may be has been transmitted in approximately the same condition to many modified descendants as in the case of the wing of the bat it must have existed according to our theory for an immense period and nearly the same state and thus it has come not to be more variable than any other structure it is only in those cases in which the modification has been comparatively recent and extraordinarily great that we ought to find that generative variability as it may be called still present in a high degree for in this case the variability will seldom as yet have been fixed by the continued selection of the individuals varying in the required manner and degree and by the continued rejection those tending to revert to a former and less modified condition sub-chapter specific characters more variable than generic characters the principle discussed under the last heeding may be applied to our present subject it is notorious that specific characters are more variable than generic to explain by a simple example if in a large genus of plants some species had blue flowers and some had red the color would be only a specific character and no one would be surprised at any one of the blue species varying into red or conversely but if all the species had blue flowers the color would become a generic character and its variation would be a more unusual circumstance I have chosen this example because the explanation which most naturalists would advance is not here applicable namely that specific characters are more variable than generic because they are taken from parts of less physiological importance than those commonly used for classic genera I believe this explanation is partly yet not undirectly true I shall however have to return to this point in the chapter on classification it would be almost to adduce evidence in support of the statement that ordinary specific characters are more variable than generic but with respect to important characters I have repeatedly noticed in works in natural history that when an author remarks of surprise that some important origin or part which is generally very constant throughout a large group of species differs considerably in closely allied species it is often variable in the individuals of the same species and this fact shows that a character generally of generic value when it sinks in value and becomes only of specific value often becomes variable though its physiological importance may remain the same something of the same kind applies to monstrosities at least is Geoffrey Saint-Hilaire apparently entertains no doubt that the more an origin normally differs in the different species of the same group the more subject it is to anomalies in the individuals on the ordinary few of each species having been independently created why should that part of the structure which differs from the same part in other independently created species of the same genus be more variable than those parts which are closely unlike in the several species I do not see that any explanation can be given but on the few the species are only strongly marked and fixed varieties we might expect often to find them still continuing to vary in those parts of the structure which have varied within a moderately recent period and which have thus come to differ or to state the case in another manner the points in which all the species of a genus resemble each other and in which they differ from allied genera we called generic characters and these characters have been attributed to inheritance from a common progenitor for it can rarely have happened that natural selection will have modified several distinct species fitted to more or less same habits in exactly the same manner and as these so called generic characters have been inherited from before the period where the several species first branched off from the common progenitor and subsequently have not varied or come to differ in any degree or only in a slight degree it is not probable that they should vary at the present day on the other hand the points in which species differ from other species of the same genus are called specific characters and that these specific characters have varied and come to differ since the period when the species branched off from a common progenitor it is probable that they should still often be in some degree variable at least more variable than those parts of the organization which have for a very long period remained constant sub-chapter secondary sexual characters variable I think it will be admitted by naturalists without my entering on details that secondary sexual characters are highly variable it will also be admitted that species of the same group differ from each other more widely in their secondary sexual characters than other parts of their organization compare for instance the amount of difference between the males of Galinaceus birds in which secondary sexual characters are strongly displayed with the amount of difference between the females the cause of the original variability of these characters is not manifest but we can see why they should not have been rendered as constant and uniform as others for they are accumulated by sexual selection which is less rigid in its action than ordinary selection as it does not entail death but only gives fewer offspring to the less favored males whatever the cause may be of the variability of secondary sexual characters as they are highly variable sexual selection will have had a wide scope for action and may thus have succeeded in giving to the species of the same group a greater amount of difference in these than in other respects it is a remarkable fact that the secondary differences between the two sexes of the same species are generally displayed in the very same parts of the organization in which the species of the same genus differ from each other of this fact I will give in illustration the first two instances which happened to stand on my list and as the differences in these cases are of a very unusual nature the relation can hardly be accidental the same number of joints in the Tarsi is common to very large groups of beetles but in the Enjide as Westwood has remarked the number varies greatly and the number likewise differs in the two sexes of the same species again in the Phasorial humanoptera the enuration of the wings is a character of the highest importance because common to large groups but in certain genera the relation differs in the different species and likewise in the two sexes of the same species Sir J. Labak has recently remarked that several minute Christassians offer excellent illustration of this law in Pontella for instance the sexual characters are afforded mainly by the interior antenna and by the fifth pair of legs the specific differences also are principally given by these origins this relation has a clear meaning on my view I look at all the species of the same genus as having as certainly descended from the same progenitor as have the two sexes of any one species consequently whatever part of the structure of the common progenitor its early descendants became variable variations of this part would it is hardly probable be taken advantage of by natural and sexual selection in order to fit the several places in the economy of nature and likewise to fit the two sexes of the same species to each other or to fit the males to struggle with other males females finally then I conclude that the greater variability of specific characters or those which distinguish species from species then of generic characters or those which are possessed by all the species that the frequent extreme variability of any part which is developed in a species in an extraordinary manner in comparison with the same part in its congeners and the slight degree of variability in a part however extraordinarily it may be developed if it be common to a whole group of species that the great variability of secondary sexual characters and the great difference in closely allied species that secondary sexual and ordinary specific differences are generally displayed in the same parts of the organization are all principles closely connected together all being mainly due to the species of the same group being the descendants of a common progenitor from whom they have inherited much in common to parts which have recently and largely varied being more likely still to go on varying than parts which have long been inherited and have not varied to natural selection having more or less completely according to the lapse of time over mastered the tendency to reversion and to further variability to sexual selection being less rigid than ordinary selection and to variations in the same parts having been accumulated by natural and sexual selection and thus having been adapted for secondary sexual and for ordinary purposes subchapter distinct species present analogous variations so that a variety of one species often assumes a character proper to an allied species or reverts to some of the characters of an early progenitor these propositions will be most readily understood by looking to our domestic races the most distinct breeds of the pigeon in countries widely apart present sub-varieties with reversed feathers on the head and with feathers on the feet characters not possessed by the original rock pigeon these then are analogous variations in two or more distinct races the frequent presence of 14 or even 16 tail feathers and the powder may be considered as a variation representing the normal structure of an other race the fan tail I presume that no one will doubt that all such analogous variations are due to the several races of the pigeon having inherited from a common parent the same constitution and tendency to variation when acted on by similar unknown influences in the vegetable kingdom we have a case of analogous variation in the enlarged stems or as commonly called roots of the Swedish turnip and rutabaga which several botanists rank as varieties produced by cultivation from a common parent if this be not so the case will then be one of analogous variation in two so called distinct species and to these assert may be added namely the common turnip according to the ordinary view of each species having been independently created we should have to attribute this similarity in the enlarged stems of these three plants not to the veracauser of community of descent and a consequent tendency to vary in a like manner but to three separate yet closely related acts of creation many similar cases of analogous variation have been observed by Norden by the world family and by various authors in our serials similar cases of curing with insects under natural conditions have lightly been discussed with much ability by Mr. Walsh who has grouped them under his law of equable variability with pigeons however we have another case namely the occasional appearance in all the breeds with two black bars on the wings white loins a bar at the end of the tail with the outer feathers externally etched near the bases with white as all these marks are characteristic of the parent rock pigeon I presume that no one will doubt that this is the case of reversion and not of a new yet analogous variation appearing in the several breeds we may I think confidently come to this conclusion because as we have seen these colored marks are eminently liable to appear in the crossed offspring of two distinct and differently colored breeds and in this case there is nothing in the external conditions of life to cause the reappearance of the slightly blue to several marks beyond the influence of the mere act of crossing on the laws of inheritance no doubt it is a very surprising fact that characters should reappear after having been lost for many probably for hundreds of generations but when a breed has been crossed only once by some other breed the offspring occasionally show generations a tendency to revert in character to the foreign breed some say for a dozen or even a score of generations after 12 generations the proportion of blood to use a common expression from one ancestor is only one in 2048 and yet as we see it is generally believed that a tendency to reversion is retained by this remnant of foreign blood in a breed which has not been crossed but in which both parents have lost some character which the progenitor possessed the tendency by the strong or weak to reproduce the lost character might as was formally remarked for all that we can see to the contrary be transmitted for almost any number of generations when a character which has been lost in a breed reappears after a great number of generations the most probable hypothesis is not that one individual suddenly takes after an ancestor removed by some 100 generations but that in each successive generation the character in question is lying latent and at last under unknown favorable conditions is developed with the bar pigeon for instance which very rarely produces a blue bird it is probable that there is a latent tendency in each generation to produce blue plumage the abstract improbability of such a tendency being transmitted by a number of generations is not greater than that of quite useless or rudimentary organs being similarly transmitted a mere tendency to produce a rudiment is indeed sometimes thus inherited as all the species of the same genus are supposed to be descendant from a common progenitor it might be expected that they would occasionally in an analogous manner so that the varieties of two or more species would resemble each other or that a variety of one species would resemble in certain characters and other and distinct species this other species being according to our view only a well marked and permanent variety but characters exclusively due to analogous variation would probably be of an unimportant nature for the preservation of all functionally important characters will have been determined through natural selection in accordance with the different habits of the species it might further be expected that the species of the same genus would occasionally exhibit reversions to long lost characters as however we do not know the common ancestor of any natural group we cannot distinguish between reversionary and analogous characters if for instance we did not know that the parent rock pigeon was not feather footed or turn crowned we could not have told whether such characters in our domestic breeds were reversions or only analogous variations but we might have inferred that the blue color was a case of reversion from the number of the markings which are correlated with this tint and which would not probably have all appeared together from simple variation more especially we might have inferred this from the blue color and the several marks so often appearing when differently colored breeds are crossed hence although under nature it must generally be left doubtful what cases are reversions to formally existing characters and what are new but analogous variations yet we ought in our theory sometimes to find the varying offspring of a species assuming characters which are already present in other members of the same group and this undoubtedly is the case the difficulty in distinguishing variable species is largely due to the variety smocking as it were other species of the same genus a considerable catalog also could be given in forms of intermediate between two other forms which themselves can only doubtfully be ranked a species and this shows mostly allied forms be considered as independently created species that they have in varying assumed some of the characters of the others but the best evidence of analogous variations is afforded by parts or organs which are generally constant in character but which occasionally varies so as to resemble in some degree the same part or organ allied species I have collected a long list of such cases but here as before I lie under the greatest advantage of not being able to give them I can only repeat that such cases certainly occur and seem to me very remarkable I will however give one curious and complex case not indeed as affecting any important character occurring in several species of the same genus partly under domestication and partly under nature it is the case almost certainly of reversion the S sometimes has very distinct transverse bars on its legs like those of the legs of a zebra it has been asserted that these are planes in the fold and from inquiries which I have made I believe this to be true the stripe on the shoulder is sometimes double and is very variable in length and outline a white S but not an albino has been described without either spinal or shoulder stripe and these stripes are sometimes very obscure or actually quite lost in dark colored aces the cold line of palace is said to have been seen with a double shoulder stripe Mr. Blythe has seen a species man of the Hemeanus with a distinct shoulder stripe though it properly has none and I have been informed by a colonel pool that foals of this species are generally striped on the legs and faintly on the shoulder the quagga so though plainly barred like a zebra over the body is without bars on the legs but Dr. Gray has figured one species man with very distinct super-like bars on the hooks with respect to the horse I have collected cases in England of the spinal stripe in horses of the most distinct breeds and of all colors transverse bars on the legs are not rare in dunes mouse dunes and in one instance in a chestnut a faint shoulder stripe may sometimes be seen in dunes and I have seen a trace in a bay horse my son made a careful examination and sketch for me of a Dun-Belgian cart horse with a double stripe on each shoulder and with leg stripes I have myself seen a Dun-Devenshire pony and a small Dun-Velge pony has been carefully described to me both with these three parallel stripes on each shoulder in the north-west part of India the cativar breed of horses is so generally striped that as I hear from Colonel Poole who examined the breeds for the Indian government a horse without stripes is not considered as purely red the spine is always striped the legs are generally barred and the shoulder stripe which is sometimes double and sometimes treble is coming the side of the face, moreover is sometimes striped the stripes are often plainest in the foal and sometimes quite disappear in old horses Colonel Poole has seen both grey and bay cativar horses striped when first fold I have also reason to suspect from information given me by Mr. W. W. Edwards that with the English race horse the spinal stripe is much commoner in the foal than in the full grown animal I have myself recently bred a foal from a bay mare offspring of a Turkoman horse and a fleamish mare by a bay English race horse this foal when a weak old was marked on its hinders quarters and on its forehead there's numerous very narrow dark, seeper like bars and its legs were feebly striped all the stripes soon disappeared completely without here entering on further details I may state that I have collected cases of leg and shoulder stripes in horses of very different breeds in various countries from eastern to eastern China and from Norway in the north to the Malai archipelago in the south in all parts of the world these stripes occur far offness in dunes and mouse dunes by the term dun a large range of color is included from one between brown and black to a close approach to cream color I am aware that Colonel Hamilton Smith who has written on this subject believes that the several breeds of the horse are descendant from several aboriginal species one of which the dun was striped and that the above described appearances are all due to ancient crosses with a dun stock but this few may be safely rejected for it is highly improbable that the heavy Belgian cart horse Welsh ponies Norwegian cops the lanky catty war race inhabiting the most distant parts of the world should have all have been crossed with one supposed aboriginal stock now let us turn to the effects of crossing the several species of the horse genus Roland asserts that the common mule from the ass and horse is particularly apt to have bars on its legs according to Mr. Goss in certain parts of the United States about 9 out of 10 mules have striped legs I once saw a mule with its legs so much striped that anyone might have thought it was a hybrid zebra and Mr. W. C. Martin in his excellent treatise on the horse has given a figure of a similar mule in four colored drawings which I have seen of hybrids between the ass and Cypria the legs were much more plainly part than the rest of the body and in one of them there was a double shoulder stripe in Lord Morton's famous hybrid from a chestnut mare and male quagga the hybrid and even the pure offspring subsequently produced from the same mare black Arabian sire were much more plainly barred across the legs than is even the pure quagga lastly and this is an other most remarkable case a hybrid has been figured by Dr. Gray and he informs me that he knows of a second case from the ass and the harmonious and this hybrid though the ass only occasionally and the harmonious has none and has not even a shoulder stripe nevertheless had all four legs paired and had three short shoulder stripes like those on the Dun Davenshire and the Welsh ponies and even had some Cypria like stripes on the side of its face with respect to this last fact I was so convinced that not even a stripe of color appears from what is commonly called chance that I was led solely from the occurrence of the face stripes on this hybrid from the ass and harmonious to ask Colonel Poole whether such face stripes ever occurred and the eminently striped cativar breed of horses and was as we have seen answered in the affirmative what now to the several facts we see several distinct species of the horse genus becoming by simple variation striped on the legs like a Cypria or striped on the shoulders like an ass in the horse we see this tendency strong whenever a Dun tint appears a tint which approaches to that of the general coloring of the other species of the genus the appearance of the stripes is not accompanied by any chance of form or by any other new character we see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species now observed the case of the several breeds of pigeons they are descendant from a pigeon including two or three subspecies or geographical races of a bluish color with certain bars and other marks and when any breed assumes by simple variation a bluish tint these bars and other marks invariably reappear but without any other change of form or character when the oldest and truest breeds of various colors are crossed we see a strong tendency for the blue tint and bars and marks to reappear in the Mongrels I have stated that the most probable hypothesis to account for the reappearance of very ancient characters is that there is a tendency in the young of each successive generation to produce the long lost character and that this tendency from unknown causes sometime prevails and we have just seen that in several species of the horse genus the stripes are either planar or appear more commonly in the young than in the old call the breeds of pigeons some of which have bred true for centuries species and how exactly parallel is the case with that of the species of the horse genus I venture confidently to look back thousands on thousands of generations and I see an animal striped like a zebra but perhaps otherwise very differently constructed the common parent of our domestic horse whether or not it be descendant from one or more wild stocks of the ass, the harmonious quagga and zebra he who believes that each equine species was independently created will, I presume assert that each species has been created with the tendency to vary both on a nature and on a domestication in this particular manner so as often to become striped like the other species of the genus and that each has been created with a strong tendency when crossed the species inhabiting distant quarters of the world to produce hybrids resembling the stripes not their own parents but other species of the genus to admit this few is as it seems to me to reject a real for an unreal or at least for an unknown cause it makes the works of God a mere mockery and deception I would almost as soon believe with the old and ignorant cosmogonics that fossil shells had never lived but had been created in stone so as to mock the shells now living on the seashore subchapter summary our ignorance of the laws of variations profound not in one case out of a hundred can we pretend to assign any reason why this or that part is a variant but whenever we have the means of instituting a comparison the same laws appear to have acted in producing the lesser differences between varieties of the same species and the greater differences between species of the same genus changed conditions generally induce mere fluctuating variability but sometimes they cause direct and definite effects and these may become strongly marked in the course of time though we have not sufficient evidence on this head a habit in producing constitutional peculiarities and use in strengthening and this use in weakening and diminishing organs appear in many cases to have been potent in their effects homologous parts tend to vary in the same manner and homologous parts tend to cohere modification in hard parts and in external parts sometimes affect softer and internal parts when one part is largely developed perhaps it tends to draw nourishment from the adjoining parts and every part of the structure which can be saved without detriment will be saved changes of structure early age may affect parts subsequently developed and many cases of correlated variation the nature of which we are unable to understand undoubtedly occur multiple parts are variable in number and in structure perhaps arising from such parts not having been closely specialized for any particular function though that the modifications have not been closely checked by natural selection it follows probably from this same cause that organic beings low in the scale are more variable than those standing higher in the scale and which have the whole organization more specialized rudimentary organs from being useless are not regulated by natural selection and hence a variable specific characters that is the characters which have come to differ since several species of the same genus branched off from a common parent are more variable than generic characters or those which have long been inherited and have not differed within the same period in these remarks we have referred to special parts or organs being still variable because they have recently varied and thus come to differ but we have also seen in the second chapter that the same principle applies to the whole individual 4. in a district where many species of a genus are found that is where there has been much former variation and differentiation or where the manufactory of new specific forms has been actively at work in that district and among these species we now find on an average most varieties secondary sexual characters are highly variable and such characters differ much in the species of the same group variability in the same part of the organization has generally been taking advantage of in giving secondary sexual differences to the two sexes of the same species and specific differences to the several species of the same genus any part or organ developed in an extraordinary size or in an extraordinary manner in comparison with the same part or organ in the allied species must have gone through an extraordinary amount of modification since the genus arose and thus we can understand why it should often still be variable in a much higher degree than other parts for variation is a long continued and slow process a natural selection will in such cases not as yet have had time to overcome the tendency to further variability and to reversion to a less modified state but when a species with an extraordinarily developed organ has become the parent of many modified descendants which on our view must be a very slow process requiring a long lapse of time in this case natural selection has succeeded in giving a fixed character to the organ in however extraordinary a manner it may have been developed species inheriting nearly the same constitution from a common parent and exposed to similar influences naturally tend to present analogous variations or these same species may occasionally revert to some of the characters of the ancient progenitors although new and important modifications may not arise from reversion and analogous variation modifications will add to the beautiful and harmonious diversity of nature whatever the cause may be of each slight difference between the offspring and their parents and a cause for each must exist we have reason to believe that it is the steady accumulation of beneficial differences which has given rise to all the more important modifications of structure in relation to the habits of each species end of chapter 5 laws of variation