620 Causes and Course of Organic Evolution 



brain areas connected with these also constitute the steadily 

 enlarging masses in the above groups, while the tactile, the 

 thermotactic, and the diffuse chemotactic sense receptive cen- 

 ters differentiate later and are mainly centered in the smaller 

 ventral brain lobes of turbellarians and nemerteans, which we 

 have considered (p. 435) to become the medulla in protoverte- 

 brates, and in still higher groups the medulla-cerebellum. 



Now if we reflect on the enormously greater number of sense 

 impressions that are of prime importance to each organism, 

 which are received by the former or dorsal set, as compared 

 with the fewer and less important ones received by the latter 

 or ventral set, further how abundant are the cumulated re- 

 sultant responses developed in the brain from summation of 

 the former set as compared with those of the latter, it is not 

 surprising that the dorsal brain mass of turbellarians and 

 nemerteans has gradually led up to the enlarging fore- and mid- 

 brains of cyclostomes, urodeles, and mammals. For the in- 

 creasingly abundant use of the anterior snout part of the head, 

 specially of the olfactory area and of the eyes, in guiding each 

 animal toward its food, in enabling it to ward off enemies, in 

 aiding it toward suitable depositing of its eggs and caring for 

 its young, have all conjoined in stimulating the brain to steady 

 growth in size, and increase in correlated nervous responses. 



Transition from a fresh-water to a land existence, and after- 

 wards to an arboreal life, must have stimulated further the 

 above centers to increasing complexity and correlated nervous 

 activity. For in all cases along this ascending line of verte- 

 brate evolution when side lines arise that reassume or retain 

 an aquatic life these are nearly always less intelligent than the 

 terrestrial ones. Thus the land as compared with the fresh- 

 water Apoda and Urodela, the land carnivora as compared 

 with the seals and walruses, the ungulates as compared with 

 the sirenians and cetaceans, are examples. 



But again the development and steady perfectioning of four 

 ambulatory limbs during the above transitions to a land and 

 arboreal existence would powerfully cooperate with enlarge- 

 ment of the cerebral and optic centers in stimulating to nervous 



