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 
and heart, with its 
Fic. 46.— The same 
in transverse section organs, 
through the ovaries; prolongations into the main 
lettering as in the 
me blood-vessels of the organism. 
preceding Fig. 
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 asa fish, and swims 
by means of a late:al 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 
