304 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



acter which has been gradually developing during Ce- 

 nozoic time ; since the earliest Cetacea had consider- 

 ably longer necks than the later ones, and had their 

 vertebral centra distinct. It appears to me probable 

 that the shortening was the result of disuse. This 

 disuse would arise from gradually increasing powers 

 of locomotion through the water, a progress, which, 

 judging from the character of the limbs of the Zeuglo- 

 don, was evidently made after the time of the Eocene. 

 The increase of speed would enable the animal to over- 

 take and capture its prey, without the necessity of 

 using a long prehensile neck in seizing it in the pur- 

 suit. 



The ball-and-socket articulation of the vertebrae is 

 well known to be the predominant condition in the 

 Reptilia, and the fact that it is necessarily associated 

 with the flexibility of the column is equally well under- 

 stood. The flexibility is directly as the weakness of 

 the limbs, for in the large and long-limbed terrestrial 

 Reptilia of the order Dinosauria, the vertebral articu- 

 lations of the dorsal region, at least, are plane. That 

 it is chiefly confined to, and best developed in, the 

 most flexible regions, i. e., the cervical and lumbar, of 

 the column of the Mammalia, also shows this neces- 

 sary connection. There can be no doubt but that the 

 ball-and-socket vertebral articulation has been pro- 

 duced by constant flexures of the column in all direc- 

 tions, as has been suggested by Marsh. 



iii. INCREASE OF SIZE THROUGH USE. 



Under this head I enumerate examples where the 

 mechanical causes in operation are less self-evident 

 than those included under the preceding section. They 



