112 THE POPULAR SCIENCE MONTHLY. 



the only part wLich can be regarded as a cerebral hemisphere lies 

 laterad of the olfactory lobe. In Dipnoi he finds that the cerebral 

 outgrowth is ventrad. In another paper * he says : " In either of 

 these directions in which what may be regarded as the special organ 

 of the mind is projected among these low or generalized forms, there 

 would seem to be mechanical obstacles to any considerable expansion ; 

 but dorsally there is oj^portunity for comparatively unlimited exten- 

 sion, and it is in this direction that the hemispheres begin to develop 

 in the Amphibia and attain such enormous growth in birds and mam- 

 mals." How far the small brain and presumably stolid intellects 

 brought about the extinction of the huge tertiary mammals may be 

 better understood by the suggestions offered by Professor A. E. Ver- 

 rill f in a lecture at Yale College, entitled " Facts Illustrative of the 

 Darwinian Theory," He shows what an important factor parental 

 instinct is in the evolution of species. He regards the lack of parental 

 care " as one of the probable causes, though usually overlooked, of the 

 extinction of many of the large and powerful reptiles of the Mesozoic 

 age and of the large mammals of the Tertiary." He says : " The very 

 small size of the brain and its low organization in these early animals 

 are now well known, and we are justified in believing that their intel- 

 ligence or sagacity was correspondingly low. They were doubtless 

 stupid and sluggish in their habits, but probably had great powers of 

 active and passive resistance against correspondingly stupid carnivo- 

 rous species. But unless the helpless young were protected by their 

 parents, they would quickly have been destroyed ; and such species 

 might, in this way, have been rapidly exterminated whenever they 

 came in contact with new forms of carnivorous animals, having the 

 instinct to destroy the new-born young of mammals, and the eggs 

 and young of oviparous reptiles. Thus it w^ould have come about 

 that the more intelligent forms, by the development of the parental 

 instinct for the active protection of their young against their enemies, 

 would have survived longest, and therefore would have transmitted 

 this instinct, with other correlated cerebral developments, to their de- 

 scendants." 



Professor John Fiske, in his " Cosmic Philosophy," arrived at a 

 similar conclusion in regard to early man. He showed that, when 

 variations in intelligence became more important than variations in 

 physical structure, then they were seized upon, to the relative exclu- 

 sion of the latter. 



The derivative theory has not only clearly revealed the fact that 

 animals have been derived from pre-existing forms, but it shows even 

 more clearly that organs have been evolved as well. It is diflicult, in a 

 general review of this nature, to separate clearly the two classes of facts. 



Professor Cope J has traced the genesis of the quadritubercular 



* " American Naturalist," vol. xxi, p. 546. \ " Science," vol. i, p. 303. 



X " American Naturalist," vol. xvii, p. 407. 



