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favour of monogenism. To confirm this conclusion, however, we must turn our attention to other facts which correspond to the idea of filiation, and consider the teachings of physiology concerning the phenomena of generation.

CHAPTER VIL

CROSSING OF RACES AND SPECIES IN THE ANIMAL AND

VEGETABLE KINGDOMS.-MONGRELS AND HYBRIDS.

I. SEXUAL unions in plants, as in animals, can take place between individuals of the same species and the same race ; further, between different races of the same species, and, finally, between different species. In the two latter cases we have what is called a cross. This crossing itself is differently named according to whether it takes place between different races or different species. In the first case it produces a mongrel, in the second a hybrid. When the cross unions are fertile the product of the union of mongrels is called a mongrel, the product of the union of hybrids & hybrid.

If the difference of the relations existing between the race and the species has been properly understood, we ought to be inclined to admit that mongrels and hybrids would not present the same phenomena; experience and observation confirm this presentiment.

We have, therefore, in this crossing a means of judging whether the human groups are only races of a single species, or rather distinct species. For this purpose it will be sufficient to study the phenomena which, in other organised and living beings, accompany the production of mongrels and hybrids, and then to compare with both the phenomena which characterise the crosses effected between human groups. If, in the latter case, the phenomena are those which characterise hybridism, we must conclude that the groups are specifically distinct, and admit the multiplicity of human species. If, however, crosses between human 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 a vegetable 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 a spontaneous return to one or the other of the parent types, or to both. M. Naudin crossed the large-leaved primrose with the primula officinalis, and obtained an intermediate hybrid between the two species, having seven fertile seeds. When these were sowo they produced three primroses of the male species, three of the female, and a single bybrid 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 the Linaria comununis and the Linaria purpurea he 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 produces varieties incapable 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 propagates in 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 the female mule. There is not a single known example in the male. 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 often clear. 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 fertile

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