"If, in any species, a number of individuals, bearing a ratio not infinitely small to the entire number of births, are in every generation born with a particular variation which is neither beneficial nor injurious, and if it is not counteracted by reversion, then the proportion of the new variety to the original form will increase till it approaches indefinitely near to equality."
It is not impossible that some definite varieties, such as the melanic form of the jaguar and the bridled variety of the guillemot are due to this cause; but from their very nature such varieties are unstable, and are continually reproduced in varying proportions from the parent forms.
They can, therefore, never const.i.tute species unless the variation in question becomes beneficial, when it will be fixed by natural selection.
Darwin, it is true, says--"There can be little doubt that the tendency to vary in the same manner has often been so strong that all the individuals of the same species have been similarly modified without the aid of any form of selection."[46] But no proof whatever is offered of this statement, and it is so entirely opposed to all we know of the facts of variation as given by Darwin himself, that the important word "all" is probably an oversight.
On the whole, then, I submit, not only has it not been proved that an "enormous number of specific peculiarities" are useless, and that, as a logical result, natural selection is "not a theory of the origin of species," but only of the origin of adaptations which are usually common to many species, or, more commonly, to genera and families; but, I urge further, it has not even been proved that any truly "specific"
characters--those which either singly or in combination distinguish each species from its nearest allies--are entirely unadaptive, useless, and meaningless; while a great body of facts on the one hand, and some weighty arguments on the other, alike prove that specific characters have been, and could only have been, developed and fixed by natural selection because of their utility. We may admit, that among the great number of variations and sports which continually arise many are altogether useless without being hurtful; but no cause or influence has been adduced adequate to render such characters fixed and constant throughout the vast number of individuals which const.i.tute any of the more dominant species.[47]
_The Swamping Effects of Intercrossing._
This supposed insuperable difficulty was first advanced in an article in the _North British Review_ in 1867, and much attention has been attracted to it by the acknowledgment of Mr. Darwin that it proved to him that "single variations," or what are usually termed "sports," could very rarely, if ever, be perpetuated in a state of nature, as he had at first thought might occasionally be the case. But he had always considered that the chief part, and latterly the whole, of the materials with which natural selection works, was afforded by individual variations, or that amount of ever fluctuating variability which exists in all organisms and in all their parts. Other writers have urged the same objection, even as against individual variability, apparently in total ignorance of its amount and range; and quite recently Professor G.J. Romanes has adduced it as one of the difficulties which can alone be overcome by his theory of physiological selection. He urges, that the same variation does not occur simultaneously in a number of individuals inhabiting the same area, and that it is mere a.s.sumption to say it does; while he admits that "if the a.s.sumption were granted there would be an end of the present difficulty; for if a sufficient number of individuals were thus simultaneously and similarly modified, there need be no longer any danger of the variety becoming swamped by intercrossing." I must again refer my readers to my third chapter for the proof that such simultaneous variability is not an a.s.sumption but a fact; but, even admitting this to be proved, the problem is not altogether solved, and there is so much misconception regarding variation, and the actual process of the origin of new species is so obscure, that some further discussion and elucidation of the subject are desirable.
In one of the preliminary chapters of Mr. Seebohm"s recent work on the _Charadriidae_, he discusses the differentiation of species; and he expresses a rather widespread view among naturalists when, speaking of the swamping effects of intercrossing, he adds: "This is unquestionably a very grave difficulty, to my mind an absolutely fatal one, to the theory of accidental variation." And in another pa.s.sage he says: "The simultaneous appearance, and its repet.i.tion in successive generations, of a beneficial variation, in a large number of individuals in the same locality, cannot possibly be ascribed to chance." These remarks appear to me to exhibit an entire misconception of the facts of variation as they actually occur, and as they have been utilised by natural selection in the modification of species. I have already shown that every part of the organism, in common species, does vary to a very considerable amount, in a large number of individuals, and in the same locality; the only point that remains to be discussed is, whether any or most of these variations are "beneficial." But every one of these variations consists either in increase or diminution of size or power of the organ or faculty that varies; they can all be divided into a more effective and a less effective group--that is, into one that is more beneficial or less beneficial. If less size of body would be beneficial, then, as half the variations in size are above and half below the mean or existing standard of the species, there would be ample beneficial variations; if a darker colour or a longer beak or wing were required, there are always a considerable number of individuals darker and lighter in colour than the average, with longer or with shorter beaks and wings, and thus the beneficial variation must always be present. And so with every other part, organ, function, or habit; because, as variation, so far as we know, is and always must be in the two directions of excess and defect in relation to the mean amount, whichever kind of variation is wanted is always present in some degree, and thus the difficulty as to "beneficial" variations occurring, as if they were a special and rare cla.s.s, falls to the ground. No doubt some organs may vary in three or perhaps more directions, as in the length, breadth, thickness, or curvature of the bill. But these may be taken as separate variations, each of which again occurs as "more" or "less"; and thus the "right" or "beneficial" or "useful" variation must always be present so long as any variation at all occurs; and it has not yet been proved that in any large or dominant species, or in any part, organ, or faculty of such species, there is no variation. And even were such a case found it would prove nothing, so long as in numerous other species variation was shown to exist; because we know that great numbers of species and groups throughout all geological time have died out, leaving no descendants; and the obvious and sufficient explanation of this fact is, that they did _not_ vary enough at the time when variation was required to bring them into harmony with changed conditions. The objection as to the "right" or "beneficial" variation occurring when required, seems therefore to have no weight in view of the actual facts of variation.
_Isolation to prevent Intercrossing._
Most writers on the subject consider the isolation of a portion of a species a very important factor in the formation of new species, while others maintain it to be absolutely essential. This latter view has arisen from an exaggerated opinion as to the power of intercrossing to keep down any variety or incipient species, and merge it in the parent stock. But it is evident that this can only occur with varieties which are not useful, or which, if useful, occur in very small numbers; and from this kind of variations it is clear that new species do not arise.
Complete isolation, as in an oceanic island, will no doubt enable natural selection to act more rapidly, for several reasons. In the first place, the absence of compet.i.tion will for some time allow the new immigrants to increase rapidly till they reach the limits of subsistence. They will then struggle among themselves, and by survival of the fittest will quickly become adapted to the new conditions of their environment. Organs which they formerly needed, to defend themselves against, or to escape from, enemies, being no longer required, would be enc.u.mbrances to be got rid of, while the power of appropriating and digesting new and varied food would rise in importance. Thus we may explain the origin of so many flightless and rather bulky birds in oceanic islands, as the dodo, the ca.s.sowary, and the extinct moas. Again, while this process was going on, the complete isolation would prevent its being checked by the immigration of new compet.i.tors or enemies, which would be very likely to occur in a continuous area; while, of course, any intercrossing with the original unmodified stock would be absolutely prevented. If, now, before this change has gone very far, the variety spreads into adjacent but rather distant islands, the somewhat different conditions in each may lead to the development of distinct forms const.i.tuting what are termed representative species; and these we find in the separate islands of the Galapagos, the West Indies, and other ancient groups of islands.
But such cases as these will only lead to the production of a few peculiar species, descended from the original settlers which happened to reach the islands; whereas, in wide areas, and in continents, we have variation and adaptation on a much larger scale; and, whenever important physical changes demand them, with even greater rapidity. The far greater complexity of the environment, together with the occurrence of variations in const.i.tution and habits, will often allow of effective isolation, even here, producing all the results of actual physical isolation. As we have already explained, one of the most frequent modes in which natural selection acts is, by adapting some individuals of a species to a somewhat different mode of life, whereby they are able to seize upon unappropriated places in nature, and in so doing they become practically isolated from their parent form. Let us suppose, for example, that one portion of a species usually living in forests ranges into the open plains, and finding abundance of food remains there permanently. So long as the struggle for existence is not exceptionally severe, these two portions of the species may remain almost unchanged; but suppose some fresh enemies are attracted to the plains by the presence of these new immigrants, then variation and natural selection would lead to the preservation of those individuals best able to cope with the difficulty, and thus the open country form would become modified into a marked variety or into a distinct species; and there would evidently be little chance of this modification being checked by intercrossing with the parent form which remained in the forest.
Another mode of isolation is brought about by the variety--either owing to habits, climate, or const.i.tutional change--breeding at a slightly different time from the parent species. This is known to produce complete isolation in the case of many varieties of plants. Yet another mode of isolation is brought about by changes of colour, and by the fact that in a wild state animals of similar colours prefer to keep together and refuse to pair with individuals of another colour. The probable reason and utility of this habit will be explained in another chapter, but the fact is well ill.u.s.trated by the cattle which have run wild in the Falkland Islands. These are of several different colours, but each colour keeps in a separate herd, often restricted to one part of the island; and one of these varieties--the mouse-coloured--is said to breed a month earlier than the others; so that if this variety inhabited a larger area it might very soon be established as a distinct race or species.[48] Of course where the change of habits or of station is still greater, as when a terrestrial animal becomes sub-aquatic, or when aquatic animals come to live in tree-tops, as with the frogs and Crustacea described at p. 118, the danger of intercrossing is reduced to a minimum.
Several writers, however, not content with the indirect effects of isolation here indicated, maintain that it is in itself a cause of modification, and ultimately of the origination of new species. This was the keynote of Mr. Vernon Wollaston"s essay on "Variation of Species," published in 1856, and it is adopted by the Rev. J.G. Gulick in his paper on "Diversity of Evolution under one Set of External Conditions" (_Journ. Linn. Soc. Zool._, vol. xi. p. 496). The idea seems to be that there is an inherent tendency to variation in certain divergent lines, and that when one portion of a species is isolated, even though under identical conditions, that tendency sets up a divergence which carries that portion farther and farther away from the original species. This view is held to be supported by the case of the land sh.e.l.ls of the Sandwich Islands, which certainly present some very remarkable phenomena. In this comparatively small area there are about 300 species of land sh.e.l.ls, almost all of which belong to one family (or sub-family), the Achatinellidae, found nowhere else in the world. The interesting point is the extreme restriction of the species and varieties. The average range of each species is only five or six miles, while some are restricted to but one or two square miles, and only a very few range over a whole island. The forest region that extends over one of the mountain-ranges of the island of Oahu, is about forty miles in length and five or six miles in breadth; and this small territory furnishes about 175 species, represented by 700 or 800 varieties. Mr.
Gulick states, that the vegetation of the different valleys on the same side of this range is much the same, yet each has a molluscan fauna differing in some degree from that of any other. "We frequently find a genus represented in several successive valleys by allied species, sometimes feeding on the same, sometimes on different plants. In every such case the valleys that are nearest to each other furnish the most nearly allied forms; and a full set of the varieties of each species presents a minute gradation of forms between the more divergent types found in the more widely separated localities." He urges, that these constant differences cannot be attributed to natural selection, because they occur in different valleys on the same side of the mountain, where food, climate, and enemies are the same; and also, because there is no greater difference in pa.s.sing from the rainy to the dry side of the mountains than in pa.s.sing from one valley to another on the same side an equal distance apart. In a very lengthy paper, presented to the Linnean Society last year, on "Divergent Evolution through c.u.mulative Segregation," Mr. Gulick endeavours to work out his views into a complete theory, the main point of which may perhaps be indicated by the following pa.s.sage: "No two portions of a species possess exactly the same average character, and the initial differences are for ever reacting on the environment and on each other in such a way as to ensure increasing divergence in each successive generation as long as the individuals of the two groups are kept from intercrossing."[49]
It need hardly be said that the views of Mr. Darwin and myself are inconsistent with the notion that, if the environment were absolutely similar for the two isolated portions of the species, any such necessary and constant divergence would take place. It is an error to a.s.sume that what seem to us identical conditions are really identical to such small and delicate organisms as these land molluscs, of whose needs and difficulties at each successive stage of their existence, from the freshly-laid egg up to the adult animal, we are so profoundly ignorant.
The exact proportions of the various species of plants, the numbers of each kind of insect or of bird, the peculiarities of more or less exposure to sunshine or to wind at certain critical epochs, and other slight differences which to us are absolutely immaterial and unrecognisable, may be of the highest significance to these humble creatures, and be quite sufficient to require some slight adjustments of size, form, or colour, which natural selection will bring about. All we know of the facts of variation leads us to believe that, without this action of natural selection, there would be produced over the whole area a series of inconstant varieties mingled together, not a distinct segregation of forms each confined to its own limited area.
Mr. Darwin has shown that, in the distribution and modification of species, the biological is of more importance than the physical environment, the struggle with other organisms being often more severe than that with the forces of nature. This is particularly evident in the case of plants, many of which, when protected from compet.i.tion, thrive in a soil, climate, and atmosphere widely different from those of their native habitat. Thus, many alpine plants only found near perpetual snow thrive well in our gardens at the level of the sea; as do the tritomas from the sultry plains of South Africa, the yuccas from the arid hills of Texas and Mexico, and the fuchsias from the damp and dreary sh.o.r.es of the Straits of Magellan. It has been well said that plants do not live where they like, but where they can; and the same remark will apply to the animal world. Horses and cattle run wild and thrive both in North and South America; rabbits, once confined to the south of Europe, have established themselves in our own country and in Australia; while the domestic fowl, a native of tropical India, thrives well in every part of the temperate zone.
If, then, we admit that when one portion of a species is separated from the rest, there will necessarily be a slight difference in the average characters of the two portions, it does not follow that this difference has much if any effect upon the characteristics that are developed by a long period of isolation. In the first place, the difference itself will necessarily be very slight unless there is an exceptional amount of variability in the species; and in the next place, if the average characters of the species are the expression of its exact adaptation to its whole environment, then, given a precisely similar environment, and the isolated portion will inevitably be brought back to the same average of characters. But, as a matter of fact, it is impossible that the environment of the isolated portion can be exactly like that of the bulk of the species. It cannot be so physically, since no two separated areas can be absolutely alike in climate and soil; and even if these are the same, the geographical features, size, contour, and relation to winds, seas, and rivers, would certainly differ. Biologically, the differences are sure to be considerable. The isolated portion of a species will almost always be in a much smaller area than that occupied by the species as a whole, hence it is at once in a different position as regards its own kind. The proportions of all the other species of animals and plants are also sure to differ in the two areas, and some species will almost always be absent in the smaller which are present in the larger country. These differences will act and react on the isolated portion of the species. The struggle for existence will differ in its severity and in its incidence from that which affects the bulk of the species. The absence of some one insect or other creature inimical to the young animal or plant may cause a vast difference in its conditions of existence, and may necessitate a modification of its external or internal characters in quite a different direction from that which happened to be present in the average of the individuals which were first isolated.
On the whole, then, we conclude that, while isolation is an important factor in effecting some modification of species, it is so, not on account of any effect produced, or influence exerted by isolation _per se_, but because it is always and necessarily accompanied by a change of environment, both physical and biological. Natural selection will then begin to act in adapting the isolated portion to its new conditions, and will do this the more quickly and the more effectually because of the isolation. We have, however, seen reason to believe that geographical or local isolation is by no means essential to the differentiation of species, because the same result is brought about by the incipient species acquiring different habits or frequenting a different station; and also by the fact that different varieties of the same species are known to prefer to pair with their like, and thus to bring about a physiological isolation of the most effective kind. This part of the subject will be again referred to when the very difficult problems presented by hybridity are discussed.[50]
_Cases in which Isolation is Ineffective._
One objection to the views of those who, like Mr. Gulick, believe isolation itself to be a cause of modification of species deserves attention, namely, the entire absence of change where, if this were a _vera causa_, we should expect to find it. In Ireland we have an excellent test case, for we know that it has been separated from Britain since the end of the glacial epoch, certainly many thousand years. Yet hardly one of its mammals, reptiles, or land molluscs has undergone the slightest change, even although there is certainly a distinct difference in the environment both inorganic and organic. That changes have not occurred through natural selection, is perhaps due to the less severe struggle for existence owing to the smaller number of competing species; but, if isolation itself were an efficient cause, acting continuously and c.u.mulatively, it is incredible that a decided change should not have been produced in thousands of years. That no such change has occurred in this, and many other cases of isolation, seems to prove that it is not in itself a cause of modification.
There yet remain a number of difficulties and objections relating to the question of hybridity, which are so important as to require a separate chapter for their adequate discussion.
FOOTNOTES:
[Footnote 41: See _Origin of Species_, pp. 176-198.]
[Footnote 42: See Kerner"s _Flowers and their Unbidden Guests_ for numerous other structures and peculiarities of plants which are shown to be adaptive and useful.]
[Footnote 43: _Nature_, vol. xx. p. 603.]
[Footnote 44: _Nature_, vol. x.x.xviii. p. 328.]
[Footnote 45: A very remarkable ill.u.s.tration of function in an apparently useless ornament is given by Semper. He says, "It is known that the skin of reptiles encloses the body with scales. These scales are distinguished by very various sculpturings, highly characteristic of the different species. Irrespective of their systematic significance they appear to be of no value in the life of the animal; indeed, they are viewed as ornamental without regard to the fact that they are microscopic and much too delicate to be visible to other animals of their own species. It might, therefore, seem hopeless to show the necessity for their existence on Darwinian principles, and to prove that they are physiologically active organs. Nevertheless, recent investigations on this point have furnished evidence that this is possible.
"It is known that many reptiles, and above all the snakes, cast off the whole skin at once, whereas human beings do so by degrees. If by any accident they are prevented doing so, they infallibly die, because the old skin has grown so tough and hard that it hinders the increase in volume which is inseparable from the growth of the animal. The casting of the skin is induced by the formation on the surface of the inner epidermis, of a layer of very fine and equally distributed hairs, which evidently serve the purpose of mechanically raising the old skin by their rigidity and position. These hairs then may be designated as _casting hairs_. That they are destined and calculated for this end is evident to me from the fact established by Dr. Braun, that the casting of the sh.e.l.ls of the river crayfish is induced in exactly the same manner by the formation of a coating of hairs which mechanically loosens the old skin or sh.e.l.l from the new. Now the researches of Braun and Cartier have shown that these casting hairs--which serve the same purpose in two groups of animals so far apart in the systematic scale--after the casting, are partly transformed into the concentric stripes, sharp spikes, ridges, or warts which ornament the outer edges of the skin-scales of reptiles or the carapace of crabs."[1] Professor Semper adds that this example, with many others that might be quoted, shows that we need not abandon the hope of explaining morphological characters on Darwinian principles, although their nature is often difficult to understand.
During a recent discussion of this question in the pages of _Nature_, Mr. St. George Mivart adduces several examples of what he deems useless specific characters. Among them are the aborted index finger of the lemurine Potto, and the thumbless hands of Colobus and Ateles, the "life-saving action" of either of which he thinks incredible. These cases suggest two remarks. In the first place, they involve _generic_, not _specific_, characters; and the three genera adduced are somewhat isolated, implying considerable antiquity and the extinction of many allied forms. This is important, because it affords ample time for great changes of conditions since the structures in question originated; and without a knowledge of these changes we can never safely a.s.sert that any detail of structure could not have been useful. In the second place, all three are cases of aborted or rudimentary organs; and these are admitted to be explained by non-use, leading to diminution of size, a further reduction being brought about by the action of the principle of economy of growth. But, when so reduced, the rudiment might be inconvenient or even hurtful, and then natural selection would aid in its complete abortion; in other words, the abortion of the part would be _useful_, and would therefore be subject to the law of survival of the fittest.
The genera Ateles and Colobus are two of the most purely arboreal types of monkeys, and it is not difficult to conceive that the constant use of the elongated fingers for climbing from tree to tree, and catching on to branches while making great leaps, might require all the nervous energy and muscular growth to be directed to the fingers, the small thumb remaining useless. The case of the Potto is more difficult, both because it is, presumably, a more ancient type, and its actual life-history and habits are completely unknown. These cases are, therefore, not at all to the point as proving that positive specific characters--not mere rudiments characterising whole genera--are in any case useless.
Mr. Mivart further objects to the alleged rigidity of the action of natural selection, because wounded or malformed animals have been found which had evidently lived a considerable time in their imperfect condition. But this simply proves that they were living under a temporarily favourable environment, and that the real struggle for existence, in their case, had not yet taken place. We must surely admit that, when the pinch came, and when perfectly formed stoats were dying for want of food, the one-footed animal, referred to by Mr. Mivart, would be among the first to succ.u.mb; and the same remark will apply to his abnormally toothed hares and rheumatic monkeys, which might, nevertheless, get on very well under favourable conditions. The struggle for existence, under which all animals and plants have been developed, is intermittent, and exceedingly irregular in its incidence and severity. It is most severe and fatal to the young; but when an animal has once reached maturity, and especially when it has gained experience by several years of an eventful existence, it may be able to maintain itself under conditions which would be fatal to a young and inexperienced creature of the same species. The examples adduced by Mr.
Mivart do not, therefore, in any way impugn the hardness of nature as a taskmaster, or the extreme severity of the recurring struggle for existence. (See _Nature_, vol. x.x.xix. p. 127.)]
[Footnote 46: _Origin of Species,_ p. 72.]
[Footnote 47: Darwin"s latest expression of opinion on this question is interesting, since it shows that he was inclined to return to his earlier view of the general, or universal, utility of specific characters. In a letter to Semper (30th Nov. 1878) he writes: "As our knowledge advances, very slight differences, considered by systematists as of no importance in structure, are continually found to be functionally important; and I have been especially struck with this fact in the case of plants, to which my observations have, of late years, been confined. Therefore it seems to me rather rash to consider slight differences between representative species, for instance, those inhabiting the different islands of the same archipelago, as of no functional importance, and as not in any way due to natural selection"
_(Life of Darwin_, vol. iii. p. 161).]
[Footnote 48: See _Variation of Animals and Plants_, vol. i. p. 86.]
[Footnote 49: _Journal of the Linnean Society, Zoology,_ vol. xx. p.
215.]
[Footnote 50: In Mr. Gulick"s last paper (_Journal of Linn. Soc. Zool._, vol. xx. pp. 189-274) he discusses the various forms of isolation above referred to, under no less than thirty-eight different divisions and subdivisions, with an elaborate terminology, and he argues that these will frequently bring about divergent evolution without any change in the environment or any action of natural selection. The discussion of the problem here given will, I believe, sufficiently expose the fallacy of his contention; but his ill.u.s.tration of the varied and often recondite modes by which practical isolation may be brought about, may help to remove one of the popular difficulties in the way of the action of natural selection in the origination of species.]
CHAPTER VII
ON THE INFERTILITY OF CROSSES BETWEEN DISTINCT SPECIES AND THE USUAL STERILITY OF THEIR HYBRID OFFSPRING
Statement of the problem--Extreme susceptibility of the reproductive functions--Reciprocal crosses--Individual differences in respect to cross-fertilisation--Dimorphism and trimorphism among plants--Cases of the fertility of hybrids and of the infertility of mongrels--The effects of close interbreeding--Mr. Huth"s objections--Fertile hybrids among animals--Fertility of hybrids among plants--Cases of sterility of mongrels--Parallelism between crossing and change of conditions--Remarks on the facts of hybridity--Sterility due to changed conditions and usually correlated with other characters--Correlation of colour with const.i.tutional peculiarities--The isolation of varieties by selective a.s.sociation--The influence of natural selection upon sterility and fertility--Physiological selection--Summary and concluding remarks.
One of the greatest, or perhaps we may say the greatest, of all the difficulties in the way of accepting the theory of natural selection as a complete explanation of the origin of species, has been the remarkable difference between varieties and species in respect of fertility when crossed. Generally speaking, it may be said that the varieties of any one species, however different they may be in external appearance, are perfectly fertile when crossed, and their mongrel offspring are equally fertile when bred among themselves; while distinct species, on the other hand, however closely they may resemble each other externally, are usually infertile when crossed, and their hybrid offspring absolutely sterile. This used to be considered a fixed law of nature, const.i.tuting the absolute test and criterion of a _species_ as distinct from a _variety_; and so long as it was believed that species were separate creations, or at all events had an origin quite distinct from that of varieties, this law could have no exceptions, because, if any two species had been found to be fertile when crossed and their hybrid offspring to be also fertile, this fact would have been held to prove them to be not _species_ but _varieties_. On the other hand, if two varieties had been found to be infertile, or their mongrel offspring to be sterile, then it would have been said: These are not varieties but true species. Thus the old theory led to inevitable reasoning in a circle; and what might be only a rather common fact was elevated into a law which had no exceptions.
The elaborate and careful examination of the whole subject by Mr.
Darwin, who has brought together a vast ma.s.s of evidence from the experience of agriculturists and horticulturists, as well as from scientific experimenters, has demonstrated that there is no such fixed law in nature as was formerly supposed. He shows us that crosses between some varieties are infertile or even sterile, while crosses between some species are quite fertile; and that there are besides a number of curious phenomena connected with the subject which render it impossible to believe that sterility is anything more than an incidental property of species, due to the extreme delicacy and susceptibility of the reproductive powers, and dependent on physiological causes we have not yet been able to trace. Nevertheless, the fact remains that most species which have hitherto been crossed produce sterile hybrids, as in the well-known case of the mule; while almost all domestic varieties, when crossed, produce offspring which are perfectly fertile among themselves.
I will now endeavour to give such a sketch of the subject as may enable the reader to see something of the complexity of the problem, referring him to Mr. Darwin"s works for fuller details.
_Extreme Susceptibility of the Reproductive Functions._