MAN AND THE VERTEBRATE SERIES. 793 



its form and organic functions are the best of all adaj^tations 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 im^^erfectly 

 overcomes gravity. Shortened as the body is, a considerable part of 

 its weight is not supported by the limbs, and needs muscular supj^ort. 

 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 re- 

 quires 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 car- 

 nivora 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 ani- 

 mals 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 imper- 

 fectly attained in all existing monkeys and apes. For its complete 

 attainment the differentiation must proceed to that degree that the 



