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 cesophagus, 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 
cesophagus, 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 
