THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 1 9 



results in the formation of a brain region more like that of the higher 

 vertebrates than exists in Animocoetes. 



In these cases the development is upward — the adult form is of a 

 higher type than that of the larva. It is, however, possible for the 

 reverse to occur, so that the individual development leads to degene- 

 ration, not to a higher type. Instances are seen in the Tunicata, and 

 in various parasitic arthropod forms, such as Lernaea, etc. In these 

 cases, the transformation from the larval to the adult form leads to 

 degradation, and in this degradation the central nervous system is 

 always involved. 



It is perhaps a truism to state that upward progress is necessarily 

 accompanied by increased development of the central nervous system ; 

 but it is necessary to lay special stress upon the importance of the 

 central nervous system in all problems of evolution, because there is, 

 in my opinion, a tendency at the present time to ignore this factor to 

 too great an extent. 



The law of progress is this — The race is not to the swift, nor to 

 the strong, but to the wise. 



This law carries with it the necessary corollary that the imme- 

 diate ancestor of the vertebrate must have had a central nervous 

 system nearly approaching that of the lowest undegenerated verte- 

 brate. Among all the animals living on the earth at the present 

 time, the highest invertebrate group, the Arthropoda, possesses 

 a central nervous system most closely resembling that of the 

 vertebrate. 



The law, then, of the paramount importance of a steady develop- 

 ment of the central nervous system for the upward progress of the 

 animal kingdom, points directly to the arthropod as the most probable 

 ancestor of the vertebrate. 



Evolution of Tissues. 



In the whole scheme of evolution we can recognize, not only an 

 upward progress in the organization of the animal as a whole, but 

 also a distinct advance in the structure of the tissues composing an 

 individual, which accompanies that upward progress. Thus it is 

 possible to speak of an evolution of the supporting tissues from the 

 simplest form of connective tissue up to cartilage and thence to bone; 

 of the contractile tissues, from the simplest contractile protoplasm 



