All we know is, that the plants and animals throughout the world exist in species, each one of which stands at present distinct and isolated wholly apart from all the rest, and one cannot be transformed into another by ordinary generation, through changes of soil and climate, or any other causes whatever known to man, within so short a period as six thousand years.

The apple, for instance, is one species, and the pear is another. In many respects they are similar to each other, and each may be changed by cultivation and by the operation of other causes a great deal, but by no possibility can one be derived from the other. By different modes of cultivation, by different selections of seeds, by changes in soil, and by other such means, a horticulturist may vary the character of his apples very much. He may produce large apples and small apples, sweet apples and sour apples, apples with a skin red, green, yellow, or brown, but he can never produce a pear. The apple, under all it modifications, will remain an apple still. It is a species by itself, separated from all other species whatever by a fixed and permanent bound, which it is impossible, as has always been supposed, that it can ever pass.

It is the same with animals. Each one is subject to a great many modifications in respect to its form, its size, its color, and even it faculties, but through all these changes each on remains entirely within its own bounds, as it were. The distinguishing characteristics of the species remain distinguishing characteristics of the species remain unchanged. Take for instance, any species of the dog. We may, perhaps, by means of differences of treatment, of food, of climate, or of immediate parentage, procure big dogs and little dogs, weak dogs and strong dogs, gentle dogs and fierce dogs, all proceeding from the same original stock, but we can have no cats, nor anything that shall bear the least specific resemblance to a cat.