Essay on the Theory of the Earth

by baron Georges Cuvier

Before entering upon this inquiry, it is proper to define what is meant by a species, so that the definition may serve to regulate the employment of the term. A species, therefore, may be defined, as comprehending the individuals which descend from each other, or from common parents, and those which resemble them as much as they resemble each other. Thus, we consider as varieties of a species, only the races more or less different which may have sprung from it by generation. Our observations, therefore, regarding the differences between the ancestors and descendants, afford us the only certain rule by which we can judge on this subject; all other considerations leading to hypothetical conclusions destitute of proof. Now, considering the varieties in this view, we observe that the differences which constitute it, depend upon determinate circumstances, and that their extent increases in proportion to the intensity of these circumstances.

Thus, the most superficial characters are the most variable: the colour depends much upon the light; the thickness of the fur upon the heat; the size, upon the abundance of food. But in a wild animal, even these varieties are greatly limited by the natural habits of the animal itself, which does not voluntarily remove far from the places where it finds, in the necessary degree, all that is requisite, for the support of its species, and does not even extend its haunts to any great distance, unless it also finds all these circumstances conjoined. Thus, although the wolf and the fox inhabit all the climates from the torrid to the frigid zone, we hardly find any other difference among them, in the whole of that vast space, than a little more or a little less beauty in their fur. I have compared skulls of foxes from the northern countries and from Egypt, with those of the foxes of France, and have found no difference but such as might be expected in different individuals. Such of the wild animals as are confined within narrower limits, vary still less, especially those which are carnivorous. The only difference between the hyena of Persia and that of Morocco, consists in a thicker or a thinner mane.

The wild herbivorous animals feel the influence of climate somewhat more extensively, because there is added to it in their case, the influence of the food, which may happen to differ both as to quantity and quality. Thus, the elephants of one forest are often larger than those of another; and their tusks are somewhat longer in places where their food may happen to be more favourable for the production of the matter of ivory. The same may take place with regard to the horns of rein-deer and stags. But let us compare two elephants the most dissimilar, and we shall not discover the slightest difference in the number and articulations of the bones, the structure of the teeth, &c.

Besides, the herbivorous species, in the wild state, seem more restrained from dispersing than the carnivorous animals, because the sort of food which they require, combines with the temperature to prevent them.

Nature also takes care to guard against the alteration of the species, which might result from their mixture, by the mutual aversion with which it has inspired them. It requires all the ingenuity and all the power of man to accomplish these unions, even between species that have the nearest resemblances. And, when the individuals produced by these forced conjunctions are fruitful, which is very seldom the case, their fecundity does not continue beyond a few generations; and would not probably proceed so far, without a continuance of the same cares which excited it at first. Thus, we never see in our woods individuals intermediate between the hare and the rabbit; between the stag and the doe; or between the martin and the pole-cat.

But the power of man changes this order; it discloses all those variations, of which the type of each species is susceptible; and from them derives productions which the species, if left to themselves, would never have yielded.

Here the degree of the variations is still proportional to the intensity of their cause, which is slavery. It is not very high in the semi-domesticated species, such as the cat. A softer fur; more brilliant colours; greater or less size; these form the whole extent of the variations in this species; for the skeleton of an Angora cat differs in no regular and constant circumstance from that of a wild cat.

In the domesticated herbivorous animals, which we transport into all kinds of climates, and subject to all kinds of management, both with regard to labour and nourishment, we certainly obtain greater variations; but still they are all merely superficial. Greater or less size; longer or shorter horns, or even the want of these entirely; a hump of fat, larger or smaller, on the shoulder; these form the differences between the various races of the common ox or bull; and these differences continue long, even in such breeds as have been transported from the countries in which they were produced, when proper care is taken to prevent crossing.

Of this nature are also the innumerable varieties of the common sheep, which consist chiefly in differences of their fleeces, as the wool which they produce is an important object of attention. These varieties, although not quite so perceptible, are yet sufficiently marked among horses. In general, the forms of the bones vary little; their connections and articulations, and the forms of the large grinding teeth, never vary at all.

The small size of the tusks in the domestic hog, compared with the wild boar’s, and the junction of its cloven hoofs into one in some races, form the extreme point of the differences which we have produced in the domesticated herbivorous quadrupeds.