144 Darwin, and after Darwin. 



namely, the digestive tract with its glandular ap- 

 pendages, the circulating system and the respiratory 

 system. In transverse section, 

 therefore, the ideal vertebrate 

 consists of a solid axis, with a 

 small tube occupied by the 

 nervous system above, and a 

 large tube, or body -cavity, 

 below. This body-cavity con- 

 tains the viscera, breathing 



FIG. 40. The same . . . . 



in transverse section organs, and heart, with its 



through the ovaries; prolongations into the main 



X V "* blood-vessels of the_ organism. 



Lastly, on either side of the 



central axis are to be found large masses of muscle 



two on the dorsal and two on the ventral. As yet, 



however, there are no limbs, nor even any bony 



skeleton, for the primitive vertebral column is hitherto 



unossified cartilage. This ideal animal, therefore, is to 



all appearance as much like a worm as a fish, and swims 



by means of a lateral undulation of its whole body, 



assisted, perhaps, by a dorsal fin formed out of skin. 



Now I should not have presented this ideal repre- 

 sentation of a primitive vertebrate for I have very 

 little faith in the " scientific use of the imagination " 

 where it aspires to discharge the functions of a Creator 

 in the manufacture of archetypal forms I say I should 

 not have presented this ideal representative of a 

 primitive vertebrate, were it not that the ideal is 

 actually realized in a still existing animal. For there 

 still survives what must be an immensely archaic 

 form of vertebrate, whose anatomy is almost identical 

 with that of the imaginary type which has just been 



