For a more complete satisfaction of his thought, the reader is referred to Mr. Swainson's volume "On the Natural History and Classification of Quadrupeds," p. 274, where he has given us an incoherent abstract of Colonel Smith's article on the Bovinæ, without, however, making the least attempt to verify the statements there recorded. The descriptions and characteristics are avowedly Colonel Smith's; but, in justice to the latter gentleman, it must be added, that the disquisitions on the circular succession of forms, and the analogical relations, are entirely Mr. Swainson's.
ON SPECIES AND VARIETY.
What constitutes a species? And how far do the limits of varieties extend? Cuvier, who is, perhaps, the best authority we can have upon this subject, in defining a species, says:—A species comprehends all the individuals which descend from each other or from a common parentage, and those which resemble them as much as they do each other. Thus, the different races which they have generated from them are considered as varieties but of one species. Our observations, therefore, respecting the differences between the ancestors and the descendants, are the only rules by which we can judge on this subject; all other considerations being merely hypothetical, and destitute of proof. Taking the word variety in this limited sense, we observe that the differences which constitute this variety depend upon determinate circumstances, and that their extent increases in proportion to the intensity of the circumstances which occasion them.
Upon these principles it is obvious, that the most superficial characters are the most variable. Thus colour depends much upon light; thickness of hair upon heat; size upon abundance of food, &c. In wild animals, however, these varieties are greatly limited by the natural habits of the animal, which does not willingly migrate from the places where it finds, in sufficient quantity, what is necessary for the support of its species, and does not even extend its haunts to any great distances, 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 differences among them, through the whole of that vast space, than a little more or less beauty in their furs. The more savage animals, especially the carnivorous, being confined within narrower limits, vary still less; and the only difference between the Hyæna of Persia and that of Morocco, consists in a thicker or a thinner mane.
Wild animals which subsist upon herbage, feel the influence of climate a little more extensively, because there is added to it the influence of food, both in regard to its abundance and its quality. Thus the Elephants of one forest are larger than those of another; their tusks also grow somewhat longer in places where their food may happen to be more favorable for the production of the substance of ivory. The same may take place in regard to the horns of Stags and Rein-deer. Besides, the species of herbivorous animals, in their wild state, seem more restrained from migrating and dispersing than the carnivorous species, being influenced both by climate, and by the kind of nourishment which they need.
We never see, in a wild state, intermediate productions between the Hare and the Rabbit, between the Stag and the Doe, or between the Martin and the Weasel. Human artifice contrives to produce all these intermixtures of which the various species are susceptible, but which they would never produce if left to themselves.
The degrees of these variations are proportional to the intensity of the causes that produce them, namely, the slavery or subjection under which these animals are to man. They do not proceed far in half-domesticated species.
In the domesticated herbivorous quadrupeds, which man transports into all kinds of climates, and subjects to various kinds of management, both in regard to labour and nourishment, he procures certainly more considerable 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 chief differences among particular races of the Bos Taurus, or domestic Black Cattle; and these differences continue long in such breeds as have been transported to great distances from the countries in which they were originally produced, when proper care is taken to prevent crossing.
Nature appears also to have guarded against the alterations of species which might proceed from mixture of breeds, by influencing the various species of animals with mutual aversion. Hence all the cunning and all the force that man is able to exert is necessary to accomplish such unions, even between species that have the nearest resemblance. And when the mule-breeds that are thus produced by these forced conjunctions happen to be fruitful, which is seldom the case, this fecundity never continues beyond a few generations, and would not probably proceed so far, without a continuance of the same causes which excited it at first.
This being the case, it is quite clear that the fact of two animals producing an intermediate race is no proof whatever of their specific identity; for it is well known, and has been already alluded to, that several animals. Birds as well as Mammalia, produce offspring, and are nevertheless distinct, both as it regards anatomical structure and external form.