Page:Popular Science Monthly Volume 18.djvu/809

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organs. The neck is narrowed to a mere covering to the œsophagus, the vertebræ, and the muscles of head-support. Its necessary flexibility is one reason for this, but the cutting off of extra weight wherever it can be spared is another.

Thus, as a general result of these evolutionary principles, we obtain a body of horizontal shape, supported upon four limbs, and reduced in weight and in elongation so as to give it the best possible control of its motions, and the greatest agility consistent with its necessary gravity.

The necessity of quick knowledge of danger and of ready escape from it have had other equally important results. The organs of sight and hearing has been placed at an elevated point, so as best to perceive distant danger, while the limbs of the herbivora have also elongated so as to lift the body to a wider outlook. This elongation of the limbs is also an important adjunct to rapid flight, so that there are two forces at work upon its development.

In the carnivora, on the contrary, the crouching, springing habits have tended to shorten the limbs, and to adapt them to vigorous leaps instead of to rapid running movements; to a life in ambush instead of to a life in action.

But the particular features of the body of the land vertebrate are as closely a result of natural requisites as are its general features. It forms, in a large sense, a digesting and assimilating machine, the force derived from assimilated food being largely applied to the muscular and nervous functions needful to obtain new food, or to avoid danger. Another portion of this force is applied to excretory and reproductive functions. That is all. There is none of the human employment of force in abstract mental conception. All mentality in undomesticated animals is employed in the art of self-preservation.

The organs arise as direct consequences of these necessities. We may view them as partly governed in position and character by their descent from the fish type; yet such a controlling agency hardly appears, so exactly are the various organs adapted to the life-needs of land-animals.

The digestive function is alike in all animals, and its evolution has ever been in one line, namely, a division of labor so as to secure more perfected results. In the mammalia this separation seems complete. From the masticating teeth to the salivary solvents, the stomach and intestinal digestions, and the intestinal absorption, every distinct portion of the process has gained its separate organ, adapted only to that one duty. The ensuing circulation is similarly specialized, being divided into the blood and the lymphatic circulations, the latter taking up normally the products of digestion and the nutritive products of the waste of the tissues; the former applying these to the nutrition of the tissues.

A third organic requisite is that of oxidation. This has been vari-