Perhaps the expression "a different plane" is scarcely correct, for man can exist on many different planes--a fact which has produced some confusion in the minds of naturalists not versed in psychological questions, though, when rightly considered, it marks very strongly the distinction between the man and the mere animal.
The lower animals are tied up by invariable instincts to certain lines of action which keep all the individuals of any species on nearly the same level, except where some little disturbance may be caused by man in his processes of domestication. But with man it is quite different. He is emancipated from the bond of instinct, and left free to follow the guidance of his own will, determined by his own reason. It follows that the habits and the actions of a man depend on what he knows and believes, and on the deductions of his reason from these premises. Without knowledge, culture, and training, man is more helpless than any brute. With the noblest and highest capacities, he may devise and follow habits of life more base than those of any mere animal. Thus there is an almost immeasurable difference between the Godlike height to which man can attain by the right use of his powers and the depth to which ignorance and depravity may degrade him. It follows that the degradation of the lower races of men is as strong a proof of the difference between man and the lower animals as is the elevation of the higher races. Both are characteristic of a being emancipated from the control of instinct, knowing good and evil, free to choose, and differing in these respects from every other creature on earth. Such is man as we find him; and we may well ask by what process animal instinct could ever spontaneously develop human freedom and human reason.
But we might have evidence of such a process, however strange and improbable it might at first sight appear. We might be able to trace man back in history or by prehistoric remains to greater and greater approximation to the lower animals, and might thus bridge over the great chasm now existing between man and beast. It may be instructive, therefore, to glance at what geology discloses as to the origin of man and his first appearance on the earth.