128 evolution: the ages and tomorrow 



instance, react strongly to light but can be trained to remain 

 in the lighted part of a box when given a mild electric shock 

 in the darkened part of the box. The instinctive behavior of 

 these animals will be taken up in the next chapter. 



All vertebrates possess a hollow, tubular nerve trunk, 

 the spinal cord, running along the dorsal side of the body. 

 At the anterior end of this spinal cord is the brain, which in 

 the vertebrates gradually evolves a very high-order differ- 

 entiation and complexity, far beyond any brain found 

 among invertebrates. In general, the vertebrates show 

 among themselves similar special sense organs, nostrils, eyes, 

 and ears, although these have had a varied evolutionary his- 

 tory within the group. From the central nervous system, 

 brain and spinal cord, lateral nerves at every segment are 

 sent off to the various parts of the body. These nerves carry 

 impulses inward from the sensory structures and outward 

 to muscles, glands, and so forth. The spinal cord in the ver- 

 tebrates is a very special structure with a much more defi- 

 nite organization than is found elsewhere. Some minor 

 changes have occurred in the history of the ner\^e trunk in 

 the vertebrates, but it is in the brain that this group shows a 

 tremendous degree of evolutionary differentiation and ad- 

 dition. It would seem that nowhere else in the whole history 

 of life on this earth has any other structure undergone such 

 elaborate and decisive advances as the organ which finally 

 culminates in the brain of man. Nature, here, has been most 

 generous in the living relics left for our study. In fact the 

 racial series of the vertebrate brain from lower chordate 

 animals like Amphioxus to man is the most convincingly 

 complete and satisfactory, and at the same time important, 

 record in the whole evolutionary process. 



Amphioxus, a small fish-like animal not far removed from 

 the ancestral line leading to the true vertebrates, has an ex- 

 tremely primitive brain, merely a slightly enlarged anterior 

 end of the chordate dorsal, tubular nerve cord. As the true 



