232 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



with ease. Even a child, when the proper time comes — 

 i. e., when the nervous and muscular systems are suffi- 

 ciently developed — will learn to walk in a week. Even 

 in the child a large portion of this capacity of co-ordina- 

 tion is inherited, though far less than in animals. The 

 whole sum of capacity in all animals is partly inherited 

 and partly individually acquired. In animals, and al- 

 most in proportion as they are lower in the scale, the 

 inherited part is large in proportion to the acquired 

 part. In man the reverse is true. 



SECTION III. 



Comparative Morphology and Physiology of Muscle and 

 Skeleton. 



VERTEBRATES. 



So far as vertebrates are concerned, the function of 

 muscle and skeleton is almost identical with that already 

 explained in man, although there is great variation in 

 the structure by means of which function is carried out. 

 But this will be brought out in a separate chapter on 

 the laws of animal structure in relation to the origin of 

 organic forms by evolution. We pass, therefore, directly 

 on to the invertebrates. 



INVERTEBRATES. 



The function of motion in invertebrates is so infi- 

 nitely various that all that is possible in this work is to 

 give some characteristic examples of widely different 

 modes. The most interesting of these is that of ar- 

 thropods. 



ARTHROPODS. 



We all know the intense muscular activity of ar- 

 thropods, especially insects — the arrowy swiftness of 



