82 THE WHENCE AND THE WHITHER OF MAN 



The important changes from fish upward affect the 

 following organs : 1. The skeleton. A light, solid 

 framework must be developed for the body. 2. The 

 appendages start as fins, and end as the legs and arms 

 of man. 3. The circulatory and respiratory systems 

 developed so as to carry with the utmost rapidity and 

 certainty fuel and oxygen to the muscular and ner- 

 vous high-pressure engines. Or, to change the figure, 

 they are the roads along which supplies and muni- 

 tions can be carried to the army suddenly mobilized 

 at any point on the frontier. 4. Above all, the brain, 

 especially the cerebrum, the crown and goal of verte- 

 brate structure. The improvement is now practically 

 altogether in the animal organs of locomotion and 

 thought. Still, among these animal organs, the lower 

 systems will lead in point of time. The brain must 

 to a certain extent wait for the skeleton. 



1. The skeleton. The axial skeleton consists, in the 

 lowest fish, of the notochord, a cylindrical unseg- 

 mented rod of cartilage running nearly the length of 

 the body. This is surrounded by a sheath of connec- 

 tive tissue, at first merely membranous, later becoming 

 cartilaginous or gristly. Pieces of cartilage extend 

 upward over the spinal marrow, and downward around 

 the great aortic artery, forming the neural and haemal 

 arches. These unite with the masses of cartilage sur- 

 rounding the notochord to form cartilaginous verte- 

 brae, which may be stiffened by an infiltration of car- 

 bonate of lime. The vertebral column of sharks has 

 reached this stage. Then the cartilaginous vertebrae 

 ossify and form a true backbone. I have described 

 the process as if it were very simple. But only the 

 student of comparative osteology can have any con- 



