I. From the very first, in the union of two individuals belonging to different stocks, the race and the species display very distinct and characteristic phenomena. We shall now see this opposition as strongly marked in the product of these unions inmongrelsandhybrids.
Several questions are raised by the mixed nature of these beings. I shall confine myself to those which refer to filiation, and which have therefore a special interest for us. They may be stated generally as follows:—aremongrel races, that is, those derived fromtwodistinctraces, andhybrid races, that is those which are derived from the crossing oftwo species, formed naturally, or can they be obtained artificially? In other words, do mongrels and hybrids retain, during an indefinite number of generations, the faculty of reproducing and transmitting to their descendants the mixed character they inherited from the first parents which effected the cross?
II. In regard to mongrels there is not a shadow of doubt. Facts which frequently occur, often without our intervention, and sometimes in spite of our precautions, prove again and again that the mongrels of the first generation are as fertile as the parents, and transmit equal fertility to their offspring. Our gardeners and breeders always take advantage of this property of mongrels in order to vary, modify or ameliorate from their point of view the plants and animals in which they are interested; the careful experiments of Buffon, of Geoffroy St.-Hilaire, father and son, andthe testimony of Darwin, on this point very significant, prove beyond a doubt that unions between different races remain fertile, whatever morphological differences there may be between them. I shall confine myself to quoting one example from Darwin. The niata will unite indifferently in both senses with the ordinary ox, and the offspring is fertile.
If several races of a single species are in habitual contact and left to themselves, they will intermix in every degree. This results in bastard offspring, devoid of definite characters, but which, when methodically studied, would lead throughinsensible shadesto the different primitive types. In this manner our street dogs and cats have come into existence, which remain perfectly fertile in spite of innumerable crossings of every kind.
With human intervention it is possible, when care is taken, to regulate the crossing between two races, and to obtain a mongrel race. After a few oscillations between the paternal and maternal types it becomes consolidated and settled. But whatever constancy it may have acquired as a whole, it almost always happens that some individuals reproduce, to a varying extent, the characters of one of the types originally crossed.
This phenomenon is designated by the name ofAtavism. It sometimes occurs in the midst of a race considered to be perfectly pure, and is the result of a single crossing several generations back. Darwin quotes the case of a breeder, who having crossed his fowls with the Malay race, wished afterwards to free them from the strange blood. After spending forty years in the attempt, he is still unsuccessful, the Malay blood always reappearing in some of his fowls.
In animals as in plants, universal, free and indefinite fertility, whether between themselves or between all the races of the same species, is one of the characters of mongrels. Atavism attests the physiological bond which unites all mongrels.
III. In hybrids we shall meet with some very different phenomena.
Let us first, with M. Godron, establish the fact that in the vegetable hybrid the physiological equilibrium is destroyed in favour of the organs conducive to the life of the individual, and at the expense of those conducive to the life of the species. The stalk and leaves are always developed in an exaggerated manner relatively to the flowers. The most common animal hybrid, the mule, is an entirely similar case, being invariably stronger, more robust, more hardy than its parents, but sterile.
This sterility is not absolute, however, among all hybrids of the first generation. It generally affects the male organs in an entirely special manner. Koelreuter, to whom we should always refer when treating of plants, states that the anthers scarcely ever enclose veritable pollen, but merely irregular granulations. It was not quite so unusual to find ovules in good condition in the ovary. Guided by these observations, Koelreuter artificially fertilised hybrid flowers with pollen from the male species, and thus obtained avegetable quadroon. By continuing this process he soon brought back again to the original male type the descendants of the first hybrid, which regained all their generative faculties, but at the same time lost all trace of the female type. These experiments have been repeated and varied, but always with the same result.
In a small number of hybrids of the first generation the elements which characterise the two sexes remained capable of reproduction. Nevertheless the fertility is always immensely reduced. From his hybrids of the datura, M. Naudin only obtained five or six fertile seeds from each plant. All the others had completely failed, or were without an embryo. The capsules themselves were only half the normal size.
If two of these first hybrids are united they produce hybrids of the second generation. In most cases, however, the latter are either sterile, or present the phenomenon of aspontaneous returnto one or the other of the parent types, or to both. M. Naudin crossed thelarge-leaved primrosewith theprimula officinalis, and obtained an intermediate hybrid between the two species, having seven fertile seeds. When these were sown they produced three primroses of the male species, three of the female, and a single hybrid plant which was perfectly barren.
In some still rarer cases fertility continues during several generations. Then, however, a curious phenomenon is exhibited, called by M. Naudin, who discovered it,Disordered variation.With theLinaria communisand theLinaria purpureahe produced a hybrid, the descendants of which he was able to follow through seven generations, in each of which several individuals reverted to the characters either of the original male or female. The others neither resembled the primitive types nor the hybrid resulting from their crossing, nor the plants of which they were the immediate offspring, nor was there any resemblance between the plants themselves.
Thus the crossing does not produce a race, even in cases where it allows a certain amount of fertility; it only producesvarietiesincapable of transmitting their individual characters. In order to establish a series of generations presenting a certain amount of uniformity, the hybrid must lose some of its mixed characters, and resume the normal livery of the species, as M. Naudin says; in other words, it must return to one of the parent types.
IV. The same facts which we have just noticed among plants, occur also among animals. We must observe in the first place, that the only two species, the crossing of which displays anything approaching to regular fertility, the horse and the ass, merely produce a hybrid almost entirely devoid of fertility. It is more than 2000 years since Herodotus regarded the fertility of mules as a prodigy, and almost 1800 years since Pliny expressed the same opinion.
And yet in some works we read that the fertility of the mule is displayed in the present day; that it often propagatesin hot countries, especially in Algeria. The true value of these singular assertions will be recognised if we recall the effect which was produced in 1828 upon the whole Mussulman population of Algeria by the announcement that a mule had conceived near Biskra. The astonishment was general; the Arabs gave themselves up to long fasts to conciliate the wrath of heaven, thinking the end of the world had come. Fortunately the mule miscarried; but long afterwards the Arabs still spoke with terror of this event.
If this fact were occasionally repeated in Algeria it would never have produced such an impression upon a people so curious about everything connected with the horse. The impression itself proves that the facts are in our days similar to what they were in the time of Herodotus.
Examples of fertility in the hybrids of the ass and the horse have never been observed except in thefemale mule. There is not a single known example in themale. We meet with something analogous to this in birds, where the sterility of certain hybrids is less absolute. Thus vertebrata are similarly affected with plants; and in their case also the inequality between the two sexes can be explained by anatomical and microscopic examination. The male organs are generally but slightly developed, even the essential elements of the fertilising liquid undergo alteration. The female organs and elements, though modified, are relatively unaffected.
There are some hybrids among animals, as among plants, which are not subject to the general law. Among birds in particular, a certain number, always however very limited, of more or less fertile hybrids have been obtained. But, with the males the faculty of reproduction is constantly weakened, and habitually disappears before the usual age; the female lays more rarely, and the eggs are fewer in number, and very oftenclear. This is an exact repetition of what took place in M. Naudin’s datura seeds, which he observed to become abortive or devoid of embryo.
We must, moreover, exclude from the number of fertilehybrids a certain number of examples quoted by some authors, and which statements are proved by facts, now either better known or better appreciated, to have an erroneous foundation. Thus Hellenius thought he had crossed the Finnish ram with the Sardinian doe, but he had confounded the then little known moufflon with the roebuck. He thus obtained a mongrel, which having been crossed for two generations with the male parent, returned to the type of the latter. We have here evidently only a companion experiment to those of Koelreuter, which resulted in a reversion of the hybrid to the male type under a similar series of crossings.
There are, however, some examples among birds and among mammalia of hybrids which have propagatedinter sefor several generations, four or five at the most. The celebrated experiment of Buffon upon the crossing of the dog with the wolf in particular, belongs to this order of facts. It was unfortunately interrupted by the death of the great naturalist at the fourth generation. It is clear that there is nothing here which does not perfectly agree with our observations upon hybrid plants, which, although exceeding this number of generations, have never produced hybrid races.
Fertility, and the number of succeeding generations is increased, when a superiority is given to one of the crossed species over the other. This fact has been recognised in plants, and we meet with it again in animals. By crossing and recrossing in a fixed manner the goat and the sheep, hybrids calledchabinsare obtained which possess three-eighths of the paternal and five-eighths of the maternal blood. These animals produce a fleece much valued in South America, and are the source of real industry. They can be maintained for several generations, but at length all the crossings to which they owe their existence must be recommenced, they having returned to the parental types, ‘like plants,’ as M. Gay said.
This proportion—three-eighths to five-eighths—appears to be very favourable to the maintenance of hybrid races;it is the proportion which characterises the famous leporides, the result of the crossing of the hare and the rabbit. But can these hybrids, of which so much has been said, maintain themselves without reverting to the parental types? M. Roux evidently believed it, and it is still asserted by M. Gayot. But the testimony of those who have established and impugned their assertions leaves scarcely any room for doubt. Isidore Geoffroy, who had at first believed in their fixity, and had spoken of it as a conquest, did not hesitate afterwards to admit the reversion. The fact has been established in the Jardin d’Acclimatation, and M. Roux himself, upon the assertion of M. Faivre, appears to have abandoned his previous assertions. The observations and experiments made by the Agricultural Society of Paris clearly show that theleporides, sent or presented by the breeders themselves, had entirely reverted to the rabbit type. Lastly, M. Sanson, discussing the anatomical side of the question, has arrived at the same conclusions. Moreover, whoever will credit the observations made by M. Naudin upon the Linariæ, will easily recognise thereversionand thedisordered variationexhibited by the leporides of the Abbé Cagliari, who was the first to obtain a fertile crossing between the hare and the rabbit.
These phenomena appear in an equally well marked manner in the result of the cross between the silkmoth (Bombyx cynthia) and the castor-oil silkmoth (Bombyx arrindia), obtained by M. Guérin Méneville. The hybrids of the first generation were almost exactly intermediate between the two species, and resembled each other. In the second this uniformity disappeared, in the third the dissimilarity had increased, some of the insects having reassumed all the characters of the paternal or maternal types. In the seventh generation this curious experiment was destroyed by ichneumons. But, as M. Valée, their intelligent breeder, told me, nearly all the moths had returned to the type of the Bombyx arrindia. The resemblance to what took place in the case of M. Naudin’s Linariæ is here complete.
V. The phenomenon of thereversionof the descendants of a hybrid to the paternal or maternal type, ordisordered variation, has given rise to some interpretations which it will be well to rectify, and has also raised important questions.
The attempt has been made to assimilate the latter to theoscillationspresented by mongrels for some generations. But daily experience should suffice to refute this opinion. Breeders are crossingracesevery day for some purpose or other, and they would never do so if the crossing were to result in the production of a disorder which would exhibit the smallest resemblance to that displayed by the Linariæ of M. Naudin, and the silkmoths of Guérin Méneville. They expect, however, a few irregularities more or less marked, in the first generations, but they know that the race will soonsettlewhile the disorder would only increase if the crossing had taken place betweenspecies.
Again, an attempt has been made to consider the facts ofatavismandreversionas identical. There is, however, a fundamental difference between them, for the mongrel which by atavism reassumes the characters of one of its paternal ancestors, for example, still preserves its mixed nature. This is proved by the possibility of its offspring of the first or second generation reproducing, on the contrary, the essential traits of its own maternal ancestors. Darwin gives many examples of facts of this nature from the agricultural history of his country. One of the best to quote is that furnished by the genealogy of a family of dogs observed by Girou de Buzareingues. These animals were crosses between the setter and spaniel. Now one male, a setter to all appearances, united with a female of pure setter breed, produced spaniels, which makes it evident that the latter blood was by no means annihilated, and that the return to the setter type was only apparent.
It is different in the cases ofreversiondisplayed by hybrids, for one of the two bloods is irrevocably expelled. We are justified in making this assertion in the case of mammalia,by experience extending as far back as the Roman period, or at least as far as the seventeenth century. Titires and musmons have never since those times hadoffspring affected by atavism. A ram and sheep have never been known to produce a kid, nor a male and female goat to produce a lamb. It is the same with plants, according to statements with which M. Naudin has kindly furnished me.
Far from being similar, the phenomena ofatavismandreversionare absolutely different and characteristic, the one of crossing betweenraces, the other of crossing betweenspecies. The first proclaims the persistency of the physiological connections between all the representatives, more or less modified, of one species; the second proves the complete rupture of the same connections between the descendants of two species accidentally brought into contact by the promoter of the hybridism.
VI. In none of the preceding cases has hybridism, no matter in what degree, given rise to a series of individuals descended the one from the other; and preserving the same characters. An exception is, however, known to this general fact. It is unique, and is produced in the vegetable kingdom from the crossing of wheat withÆgilops ovata.
The hybrid of the first generation from these two species is sometimes produced naturally, and was regarded by Requien as a species. Fabre, who frequently met with it in the fields, considered it to be the commencement of the transmutation of the Ægilops into wheat. Afterwards a quadroon hybrid, accidentally obtained and cultivated during several years, gave him descendants resembling thebeardless wheatof the South. It was the result ofreversion. Fabre, however, who did not recognise the hybrid, thought it was a transmutation, and flattered himself that he had discovered wild wheat in the Ægilops.
M. Godron, on the contrary, understood the nature of the phenomenon, and demonstrated it experimentally. He crossed the Ægilops and the wheat, and obtained the first plant of Requien, theÆgilops triticoïdesof Fabre. Heagain crossed this hybrid with the true wheat, and reproduced the pretended artificial wheat of the Montpellier botanist. He gave to it the name ofÆgilops speltæformis.
It is this latter form, having as we see three-fourths of the true wheat, and a fourth of the Ægilops, that M. Godron has cultivated at Nancy since 1857. The clever naturalist who has produced it, believes that he has not had one case of reversion like those at Montpellier and those of Fabre. But at the same time he informs us that the most minute and special precautions alone can preserve this artificial plant. The ground must be prepared with the greatest care, and each seed placed by hand in the desired position. If put into the ground carelessly, or scattered over the bed, the seeds never germinate. M. Godron considered that the Ægilops speltæformis would entirely disappear, perhaps in a single year, if left to itself.
VII. Finally, the characters of hybrids are: infertility, as a general rule, and, in the exceptions, a very limited fertility; series suddenly cut short either by infertility, by disordered variation, or by reversion without atavism.
The Ægilops triticoïdes alone seems to stand in opposition to all other known facts. This exception is undoubtedly remarkable, but does not in any way impair our general conclusions. A product of human industry, this hybrid plant only exists by virtue of the same industry, and cannot, from any point of view, be compared to the succession of mongrel individuals which are unceasingly propagated without our aid, and in spite of our precautions, in the midst of our animal and vegetable races.
“But,” say those writers who deny the reality of a distinction between species and race, “what man has done nature must be able to do also, for she governs space and time, and is therefore more powerful than man.” This form of argument rests upon a confusion of ideas and a strange neglect of the most ordinary facts.
Most true, nature is more powerful than man in certain cases and for certain ends, but man also has his domain, inwhich he is much superior to nature. Natural forces act in virtue of blind and necessary laws, the result of which is constant. Now man has acquired the knowledge of these laws, he has made use of them to constrain and master the natural forces one after another, he now knows how to exaggerate some and to weaken others. In this manner he changes their resultants, and obtains products which nature herself could not realise. Give to the latter all the time and space that you will, she would never be able either to produce or preserve potassium or sodium in a metallic form; in spite of the physico-chemical forces, or rather by directing them, man has obtained and preserved these two metals, as he has obtained and preserved the Ægilops triticoïdes, which is destroyed by the inflexibility of natural forces as soon as it is exposed to their action.
VIII. The infertility, or, if you will, the restricted and rapidly limited fertility between species, and the impossibility of natural forces, when left to themselves, producing series of intermediary beings between two given specific types, is one of those general facts which we call alaw. This fact has an importance in the organic world equal to that rightly attributed to attraction in the sidereal world. It is by virtue of the latter that the celestial bodies preserve their respective distances, and complete their orbits in the admirable order revealed by astronomy. Thelaw of the sterility of speciesproduces the same result, and maintains between species and between different groups in animals and plants all those relations, which, in the palæontological ages, as well as in our own, form the marvellous whole of theOrganic Empire.
Imagine the suppression of the laws which govern attraction in the heavens, and what chaos would immediately be the result. Suppress upon earth the law of crossing, and the confusion would be immense. It is scarcely possible to say where it would stop. After a few generations the groups which we call genera, families, orders, and classes would most certainly have disappeared, and the branches also would rapidly have become affected. It is clear that only a fewcenturies would elapse before the animal and vegetable kingdoms fell into the most complete disorder. Now order has existed in both kingdoms since the epoch when organised beings first peopled the solitudes of our globe, and it could only have been established and preserved by virtue of the impossibility of a fusion of species with each other through indifferently and indefinitely fertile crossings.
IX. There are some writers, very often entirely unacquainted with the natural sciences, who, labouring under the most varied prejudices, especially that of exaggerating the transmutation doctrines which I shall presently discuss, have denied thereality of species; they affirm that there are no serious barriers between the groups designated by this term, and have compared it in a more or less formal manner to the groups always somewhat arbitrarily called genera, tribes, families, orders, etc. Though only a brief recapitulation, the preceding facts would be sufficient to answer them. It is, however, necessary to mention the principal objections which are brought forward against such ideas, and to shew how they may be refuted.
1st. It is useless to take any notice o£ the good humoured or malicious banter, of the raillery and sarcasm too often made use of by some writers against those who admit the reality of species. It is evident that those who employ such weapons do not address themselves to men of science, but appeal directly to the passions. We cannot sufficiently express our regret at seeing men of undoubted merit resorting to such means.
2nd. At the present time, perhaps more than ever, those who believe in species are reproached with beingorthodox: I could never myself understand why there should be this mixture of scientific discussions and dogmatic and anti-dogmatic polemics.
3rd. I shall, moreover, refuse to dispute with those who, rejecting on their own authority a whole century of work accomplished by the greatest naturalists, and by a number of men distinguished in botany and zoology, declare that itis useless to try and discover what species and race are, and laugh at those who take the trouble to do so. I say the same to those who regard species and race as more or less arbitrary groups which may be compared to the genus, family and order. It will be enough to remark that they themselves incessantly employ the wordspeciesandrace, and we must not be surprised if they take one thing for the other.
4th. After what we have said, discussion is useless with those naturalists who only base the distinction of species upon external characters. They forget all the experiments made from Buffon to the two Geoffroys, from Koelreuter to M. Naudin; they forget the innumerable observations made in our orchards, gardens and stables. To refuse to abandon morphological considerations, and to neglect the data of physiology and the lessons of filiation, is clearly going further back than Ray and Tournefort, and all discussion becomes impossible.
5th. Some of our opponents allow that things are now what we think them to be. “But,” say they, “it is possible that at some other time it was different.” What answer can be given to those who base their arguments uponpossibilities? Is modern science composed of possibilities?
6th. Naturalists have often been reproached with multiplying the definitions of species. From the variety of terms employed by them in expressing ideas, it has been inferred that they were not agreed as to the ideas themselves. We may easily convince ourselves of their mistake, if we give these definitions a careful reconsideration. We shall see that their several authors have only endeavoured to express with greater clearness and precision, the double idea resulting from the facts of resemblance and filiation. In reality, divergencies only begin where experiment and observation cease. It is this which caused Isidore Geoffroy, however interested he might be in discussions of this nature, to remark—“Such are Species and Races, not only for one of the schools into which naturalists are divided, but for all.”
7th. It has been asserted that the distinction of species and race rests upon a syllogistic circle; that naturalists decidedà prioriupon calling all those groups incapable of intercrossing,species, and all those amongst which crossing was possible,races. To appeal to the difference of the phenomena presented by the hybrids and mongrels is therefore only solving the question by the question.—This is an historical error. Naturalists came into contact with species, races and varieties, before they gave names to them. It was by experiment and observation that they learnt to distinguish them.Knowledge of factsprecededterminology.
8th. Again, it has been said, that the discussions which are always arising between naturalists as to whether a species should be preserved or regarded as a race, as to the genus, family, order, and sometimes the class in which it should be placed, betray a want of precision in general ideas.—Those who talk in this manner forget the immense number of species and races accepted and classified without discussion. They shut their eyes to all cases except those in which divergences of opinion occur. If, however, facts of this nature prove anything against a science and its fundamental data, then even mathematical theorems must be considered as wanting in precision, for there are disputes among mathematicians.
9th. I have already replied to the arguments drawn from the fertility of certain hybrids by showing to what it is reduced. Writers who insist upon this point invariably forget the lesson taught us by disordered variation and reversion without atavism. I regret being obliged to place among them Darwin, who, in his later writings, has shewn much less reserve than in his earlier publications. In the last edition of his book, he quotes what I have said of the cross between the Bombyx cynthia and the Bombyx arrindia; he speaks of the number of generations obtained, but he forgets to mention that disordered variation appeared in the second generation, and that reversion to one of the parentaltypes was almost complete at the termination of the experiment.
X. Species is then a reality.
Let us take a group of individuals more or less similar, but always capable of contracting fertile unions, and let us, with M. Chevreul, trace it in imagination to its origin. We shall see it divided intofamilies, each of which will have risen either mediately or immediately from one pair of parents. We shall see that the number of these families decrease ateach generation, and rising still higher we shall at length find the initial term of asingle primitive pair.
Has this really been the case? Has each species indeed arisen from one single pair, or have several pairs, resembling each other perfectly both morphologically and physiologically, appeared simultaneously or successively? These arequestions of factwhich science neither can nor ought to approach, for neither experiment nor observation is able to furnish us with the smallest data requisite for the solution.
But what science may affirm is thatfrom all appearanceseach species has had, as point of departure, a single primitive pair.