THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM 57 



matter of the brain as completely as the central canal of the spinal 

 cord of the vertebrate is surrounded by the white and grey nervous 

 material. 



Truly, at the time when vertebrates first appeared, the direction 

 and progress of variation in the Arthropoda was leading, owing to 

 the manner in which the brain was pierced by the oesophagus, to a 

 terrible dilemma — either the capacity for taking in food without 

 sufficient intelligence to capture it, or intelligence sufficient to capture 

 food and no power to consume it. 



Something had to be done — some way had to be found out of this 

 difficulty. The atrophy of the brain meant degeneration and the 

 reduction to a lower stage of organization, as is seen in the Tunicata. 

 The further development of the brain necessitated the establish- 

 ment of a new method of alimentation and the closure of the old 

 oesophagus, its vestiges still remaining as the infundibular canal of 

 the vertebrate, meant the enormous upward stride of the formation 

 of the vertebrate. 



At first sight it might appear too great an assumption even to 

 imagine the possibility of the formation of a new gut in an animal so 

 highly organized as an arthropod, but a little consideration will, I 

 think, show that such is not the case. 



In the higher animals we are accustomed to speak of certain 

 organs as vital and necessary for the further existence of the animal ; 

 these are essentially the central nervous system, the respiratory 

 system, the circulatory system, and the digestive system. Of these 

 four vital systems the first cannot be touched without the chance 

 of degeneration ; but that is not the case with the second. The 

 passage from the fish to the amphibian, from the water-breathing 

 to the air-breathing animal, has actually taken place, and was effected 

 by the modification of the swim-bladder to form new respiratory 

 organs — the lungs ; the old respiratory organs — the gills — becoming 

 functionless, but still persisting in the embryo as vestiges. The 

 necessity arose in consequence of the passage of the animal from 

 water to land, and with this necessity nature found a means of over- 

 coming the difficulty ; air-breathing vertebrates arose, and from the 

 very fact of their being able to extend over the land-surfaces, 

 increased in numbers and developed in complexity in the manner 

 already sketched out. 



For a respiratory system all that is required is an arrangement 



