130 evolution: the ages and tomorrow 



phasis of the sense of smell in favor of a later development 

 of sight and hearing is a turning point in the evolution of 

 the brain. The objective world gradually takes on greater 

 sharpness and solidity. The cerebral hemispheres, which 

 hold almost the whole promise and expectation of high-level 

 conscious mind, are just back of these lobes, but are hardly 

 more than hinted at in the earliest vertebrate brain. They 

 are on the top side of the forebrain, and under and back of 

 them is the thalamus, the "brain bed," which makes the con- 

 nection with the rest of the brain. The paired eyes attach 

 to the side of the forebrain. Originally there was a third eye 

 on the top of the brain (still present in an archaic type of 

 lizard), but it is now reduced to a vestige, the pineal, v/hich 

 is found even in man. Beginning in the fishes and continu- 

 ing up through the amphibia and reptiles to the birds and 

 mammals, the forebrain gradually takes on functions which 

 free the individual from the rigid control of automatic be- 

 havior. 



The cerebral hemispheres assume the role of freeing be- 

 havior and introduce a truly higher type of brain organiza- 

 tion into animal evolution. In the higher vertebrates increas- 

 ingly indirect and intricate channels are evolved for the 

 control and sorting of sensations and motor stimuli. The 

 hemispheres become larger and larger, until in the apes and 

 man they are far more massive than all the rest of the brain 

 put together. Cerebral areas are developed in which are 

 seated memory, consciousness, imagination, and will; new 

 sensations and activities are influenced by the remembered 

 record of the past. The animal finds it easier and easier to 

 learn new ways of life. Phyletically it is probable that the 

 cerebral bodies took on these functions because they were 

 originally the seat of the sense of smell, which in lower 

 forms is the main source for knowledge of the outside 

 world. The cerebrum is entirely associated with smell in 

 fishes and amphibia, but in the reptiles it begins to show dif- 

 ferentiation into a new area which is to be devoted to the 



