WORMS TO VERTEBRATES Tl 



In the turbellaria the dominant functions were di- 

 gestion and reproduction, and their organs composed 

 almost the whole body. Here only eleven segments at 

 most are devoted to these functions, and nine in head 

 and thorax to locomotion and brain. Head and thorax 

 have increased steadily in importance, while the abdo- 

 men has decreased as steadily in number of segments. 

 And the brain is increasing thus rapidly because there 

 are now muscles and sense-organs of sufficient power 

 to make such a brain of value. And this brain per- 

 ceives not only objects and qualities, but invisible rela- 

 tions between these, and this is an advance amounting 

 to a revolution. It remembers, and uses its recollec- 

 tions. It is capable of learning a little by experience 

 and observation. The A, B, C of thinking was prob- 

 ably learned long before the insect's time, and the bee 

 shows a fair amount of intelligence. 



The line of development which the insect followed 

 was comparatively easy and its course probably rapid. 

 Certain Crustacea, aquatic arthropoda, are among the 

 oldest fossils, and it is possible that insects lived on 

 the land before the first fish swam in the sea. They 

 had fine structure and powers ; and yet during the 

 later geologic periods they have scarcely advanced a 

 step, and are now apparently at a standstill. They ran 

 splendidly for a time, and then fell out of the race. 

 What hindered and stopped them ? 



One vital defect in their whole plan of organization 

 is evident. The external skeleton is admirably suited 

 to animals of small size, but only to these. In larger 

 animals living on land it would have to be made so 

 heavy as to be unwieldy and no longer economical. 

 Their mode of breathing also is fitted only for animals 



