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.