In considering the question as to how long man has inhabited this continent, his influence upon nature cannot be overlooked. In the animal kingdom, certain animals were domesticated by the aborigines from so remote a period that scarcely any of their species, as in the case of the lama of Peru, were to be found in a state of unrestrained freedom at the advent of the Spaniards. In the vegetable kingdom more abundant testimony of the same nature is presented. A plant must be subjected to the transforming influences of cultivation for a long time before it becomes so changed as no longer to be identified with the wild species, and infinitely longer before it becomes entirely dependent upon cultivation for propagation. Yet we find that both of these facts have been accomplished with reference to the maize, tobacco, cotton, quinoa and mandico plants; and the only species of palm cultivated by the South American Indians, that known as the _Gulielma speciosa_, has lost through that culture its original nut-like seed, and is dependent upon the hands of its cultivators for its life.[103] Alluding to the above-named plants, Dr. Brinton remarks: “Several are sure to perish unless fostered by human care. What numberless ages does this suggest? How many centuries elapsed ere man thought of cultivating Indian corn? How many more ere it had spread over nearly a hundred degrees of latitude and lost all resemblance to its original form?”[104] Certainly this class of evidence, though furnishing no chronometric scale, points us to an antiquity for man on this continent more venerable than that suggested either by tumuli or architectural remains. The peculiar value of this argument rests in the fact that with the exception of cotton, none of the plants indicated have ever been cultivated by any other people than the aborigines of America, and could not have matured their characteristics of dependence in the old world, and been brought hither through the channel of immigration.