The human species

by A. de Quatrefages

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 communis 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 hybrids 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 propagated inter se for 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 called chabins are 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.