Page:Popular Science Monthly Volume 18.djvu/813

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its form and organic functions are the best of all adaptations attained by the animal world up to that height.

The mammalia have another advantage which we will but glance at here. The young of the mammal has a better start in life than that of any other type of animal. In this direction, also, there has been a constant development in the process of evolution, the young of lower animals being born in an embryo state, and needing to consume force in passing through various degrees of metamorphosis. The young of the mammal is fed by the mother through all the embryological stages, this being most fully performed in the highest mammals, so that their young commence an independent life at a stage to reach which the young of lower animals consume a considerable portion of their vital energy.

Yet, with all these advantages, the mammalian quadruped has not attained the highest position in animal development. It will be very easy to point out several defects in its organization, which detract from its powers as a living body. In the first place it only imperfectly overcomes gravity. Shortened as the body is, a considerable part of its weight is not supported by the limbs, and needs muscular support. This increases weight and uses up force. The head also is not directly supported by the limbs and needs powerful muscular support in the neck. The need of using the teeth as food-grasping instruments requires a forward extension of the head, instead of a vertical position over the fore-limbs.

In division of labor it is likewise defective. Thus its limbs have a double duty to perform—they are used both as organs of motion and as weapons. The herbivora use their hind-limbs for defense, the carnivora use all the limbs as offensive organs. The same may be said of the teeth. The carnivora use them both as weapons of attack and as organs of mastication. The herbivora are frequently supplied with heavy horns as defensive weapons, thus adding to the weight of the head.

Obviously, then, there are a variety of requirements to be supplied ere the most completely developed animal form can be attained. In what direction shall this further development proceed? How shall the above-named disadvantages be obviated? The first steps toward it are made when mammalian animals begin to differentiate the functions of their fore and hind limbs. We have noticed some forms of this differentiation. A more significant form displays itself in those animals which live in trees. Many of these, it is true, use all the limbs alike in climbing, and remain true quadrupeds upon the land-surface; others, as the monkeys, use the fore-limbs as grasping, the hind-limbs as supporting organs, and thus begin to separate them in function.

This separation proceeds with extreme slowness. It is only imperfectly attained in all existing monkeys and apes. For its complete attainment the differentiation must proceed to that degree that the