Page:Popular Science Monthly Volume 27.djvu/813

Rh physical impress of the environment, as temperature, light, humidity, movements of the medium, etc., which are only influenced by the animal as it places itself, by its movements, within or without the range of their influence. In spite of these facts, I believe the movements and habits of animals to lie at the foundation of their principal characters, and that the superstructure, be it due to whatever cause it may, rests upon that foundation. It will now be well to take a look at the evidence in favor of or against these theories, as presented by the science of vertebrate paleontology. A few examples will suffice.

In the first place, I will select an illustration of the effects of use on the articulations of the limbs and feet of the mammalia. I take first the ankle-and wrist-joints. In the ruminating animals (ox, deer, camel, etc.) and in the horse, among other living species, the ankle-joint is a very strong one, and yet admits of an extensive bending of the foot on the leg. It is a treble tongue-and-groove-joint; that is, two keels of the first bone of the foot, the astragalus, fit into two grooves of the lower bone of the leg, the tibia, while between these grooves a keel of the tibia descends to fill a corresponding groove of the astragalus. Such a joint as this can be broken by force, but it cannot be dislocated. Now, in all bones the external walls are composed of dense material, while the centers are spongy and comparatively soft. The first bone of the foot (astragalus) is narrower, from side to side, than the tibia which rests upon it. Hence the edges of the dense side-walls of the astragalus fall within the edges of the dense side-walls of the tibia, and they appear to have pressed into the more yielding material that forms the end of the bone, and pushed it upward, thus allowing the side-walls of the tibia to embrace the side-walls of the astragalus. Now, this is exactly what would happen if two pieces of similar dead material, similarly placed, should be subjected to a continual pounding in the direction of their length tor a long period of time. And we can not ascribe any other immediate origin to it in the living material; but the probability of such origin is more probable in such substance, because of the perpetual waste and repair which are going on, and because of