By the fauna of a country is meant the system of animals that inhabit it. The flora is its system of plants. Now, inasmuch as both the fauna and the flora of America were so essentially different from those of the old world, that, so far as could be judged from all that was known of the propagation of plants and animals, and of the changes which they may undergo from the influence of climate and soil, and other conditions, the one system, in the opinion of naturalists, could not have been produced from the other, it seemed to be wholly useless to attempt to contrive means by which the progenitors of the present races in America could have been wafted across the ocean, or could have migrated by means of countries and territories which once existed, but are now submerged.
Man Admitted to be an Exception
This reasoning, however, applied only to plants and to inferior animals, but not to man; for the races of men found upon this continent were deemed by naturalists to be of the same species with all the other races now existing in the world: that is, too difference between the different races of men were judged to be not specific differences, that is, not such as to preclude the possibility of their all being deduced from one original pair. This has always been the general opinion among naturalists, and in their systems of classification all the various races of men are classed as one species. Man, therefore, it has always been admitted, may have been brought to America over the ice at Behring's Straits, or by boats blown off from the coast of Africa, or from the islands in the Pacific; but the general stocking of the country with its countless thousands of species, both of animals and vegetable life, it was thought could not be thus explained.
What is a Species?
The degree of probability that the present plants and animals of America could not have been derived, within a modern period, and by direct descent, from those of the old world, depends, of course, upon the degree of difference there is between them, because there is a certain degree of difference, and that not small, which changes of climate and soil, and of other conditions of that kind will account for; but the difference in question was found to be very great indeed. It is a specific difference, that is, a difference in the species. A species of plants or animals, as the term has been generally used by naturalists, comprises all such individuals as are so similar to each other that we may suppose them all to have proceeded from one common parentage, and so dissimilar from all others that they could not have been reproduced from the others, nor the others produced from them, by ordinary generation.
Whether there be or not some extraordinary mode by which at rare and distant intervals, and under conditions seldom occurring, and which have not occurred under the observation of men, by which a new species can arise, having its origin, in some way or other, in a former species, in the same sense as now a new individual, of the same species, has it origin in a former individual of the same species, by the production of a seed or an egg, for example; or whether it may not be possible that in an exceedingly great length of time, and by means of a very long-continued accumulation of minute and almost imperceptible changes, one species should be transformed into another, or, by branching, give origin to several others, adapted to new and peculiar circumstances arising in the world's history, are questions which are now greatly agitated among the learned, and may not soon be settled. 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.
The Distinction of Species Very Permanent
It may, perhaps, be said that although in the comparatively short periods of time that have been covered by the experiments and observations which have been made by man, the transformation of one species into another may have been impossible, still such changes may have been effected in longer periods, and that the various forms of animal and vegetable life which now exist upon the earth may have proceeded from some common origin, or at least from some moderate number of original types existing in former ages. And, indeed, this may possibly be so. But there seems to be quite satisfactory evidence to prove that the distinction of species is as permanent in respect to the past and the future, at least for very long periods, as it is decisive at the present time.
Evidence of Ancient Records
In the first place, we have in Egyptian and Assyrian monuments, which go back with their records several thousand years - much more than half the time, according to the usually received opinion, since the earth was stocked with the present races of animals - many drawings and other representations of plants and animals as they existed then, and even seeds, in some cases, found in the wrappings of Egyptian mummies, all of which show that these plants and animals, and even the races of men, were specifically the same then as now. There have been no changes whatever that encroach at all upon the limits and bounds by which the different species are separated from each other at the present day, or confuse the lines of demarcation in any degree. There is no approach of one type toward another, nor any tendency to such an approach. Now, is a change could be effected in the specific character of a plant or of an animal, in any limited series of generations, we should be very likely to find evidences of it in a period of three or four thousand years, especially in the case of such animals as arrive at maturity in a short time, and thus in any given period reckon as many generations as years. Between the bird carved upon an Egyptian or Assyrian slab, and its representative at the present day, probably three thousand generations may have intervened, and yet the present living specimen is specifically identical with the delineation of its ancestor. The great comparative anatomist Cuvier examined the mummy of an ibis, from three to four thousand years old, comparing it minutely with a living bird of the present day, and found the two specimens in all respects identically the same.