period over which this evolution has extended, the extraordinary variety of forms which have appeared, and the strict limitations of the problem by natural influences, chemical, meteorological and vital, it is not easy to perceive how the final result could have deviated widely from that which we have before us. If the process were gone over again upon the earth, the great probability is that it would end once more in the mammalian quadruped. On other planets different chemical and physical conditions might affect the result, though the general principles of vital action could not greatly deviate and the evolution of the organs would doubtless pursue much the same course. As regards external form, the struggle for existence must operate in the same way and probably to the same effect.
Let us, for example, take the head of the quadruped, with its facility of motion, its apparatus for mastication, its sense organs, its nerve center. Can any one suggest an improvement upon the general arrangement of these organs, the ultimate outcome from a myriad of experimental efforts? The nasal openings stand above the mouth, in the best position to give warning of dangerous odors from food. The eyes are placed at the highest altitude and in the frontal position, the best location for their special duty. The ears are situated to catch sounds from the rear and the front, but preferably the latter. The brain is situated in the immediate vicinity of these organs of special sense, as if to favor quickness of sensation. All the organs of the head, indeed, seem remarkably well placed and adapted to their particular duty, and when we consider the varied positions which these organs have occupied in lower forms of life, we may justly look upon those in the quadruped as the final 'posts of vantage' resulting from a multitude of trials. Similar deductions might be made from other sections of the body, internal and external.
But we have not yet reached the evolution of a thinking being, an animal dependent much more upon its mental than upon its physical powers. In each advanced type of animal some mental progress was made; largest of all in the quadruped mammal; yet even in the latter it ended at a low stage. This is evident if we compare the quadruped with man, the former dependent very largely upon its physical, the latter mainly upon his mental powers. Evidently, in any planet, some step of progress beyond the quadruped was necessary for this result. On the earth this step was towards the form of man, the only true biped. May it not have been different in other planets, yielding human beings widely diverse in form and general bodily relations?
The answer to this query depends upon that special characteristic to which man—wherever found—owes his superiority. His physical difference from the lower animals is by no means great. It consists principally in an adoption of the upright attitude, a reduction of the
