rHE LOGIC OF EVOLUTION 73 



The internal, almost purely locomotive skeleton of verte- 

 brates was exceedingly difficult and slow of development. 

 It required a long series of none too successful experiments 

 before it was completely attained. It resulted in a marvel- 

 lously adapted support for the well molded, powerful muscles 

 of large animals, giving swiftness, agility, and tireless locomo- 

 tion. Jaws and teeth furnish unequaled weapons. The sense 

 organs are large and keen, adequate to great speed over a 



wide range. The nerve-cells of many segments are drawn 

 into the capacious brain. They have fulfilled the promises of 

 annehds. 



The large size of the vertebrate is correlated with long life, 

 with varied but oft-repeated experiences of the individual. 

 He forms instincts; he has at least the promise of intelligence. 

 The lower vertebrate is mainly a locomotive engine of offensive 

 warfare, his world is an arena of fighting, devouring brutes, 

 a not altogether pleasant spectacle. But we find plenty of 

 movement and numberless experiments. 



The amphibian has crawled out on land; a poor, clumsy 

 crawling animal, whose short feeble legs can scarcely lift it 

 from the ground. But it walks, and every step is a series 

 of muscular movements demanding close and accurate control 

 in special nerve-centers, while the direction of locomotion is 

 determined by the brain in response to the reports of the 

 higher sense-organs. There is some variety in the use of the 

 two pair of appendages. Somehow the brain, the great cen- 

 tral switchboard of the nervous system, manages to distribute 

 these stimuli to the proper muscles in adequate intensity. Its 

 problems and responsibihties are steadily increasing. The ap- 

 pendages lengthen and strengthen, become adapted to locomo- 

 tion on land or in the air; the internal locomotive skeleton has 

 been practically completed, its possibilities pretty well ex- 

 hausted; reptiles, birds and mammals have appeared. 



Reptiles tried a vast series of experiments in framing a 

 vertebrate body. Some are of huge bulk, others small. The 

 snakes gave up the appendages, returned to the primitive writh- 

 ing method of locomotion and carried it practically to perfec- 



