MAN AND THE VERTEBRATE SERIES. 789 



organs. The neck is narrowed to a mere covering to the oesophagus, 

 the vertebrae, 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 per- 

 ceive distant danger, while the limbs of the herbivora have also elon- 

 gated 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 undoraesticated 

 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 in- 

 testinal 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- 



