92 LIFE IN NATURE. 



and becomes relatively smaller afterwards, and con- 

 stitutes respectively the optic and auditory nerve. 



Or, let us look at the fully developed brain of 

 any of the higher mammalia. Fig. 18 is a repre- 

 sentation of that of man. The surface is wrinkled 

 up in all directions, constituting quite a maze of 

 elevated ridges, called convolutions. Do not these 

 recall the "dorsal plates" (Fig. 12)? Are they not 

 evidently formed in the same way? The external 

 layer of the brain, expanding beneath the dense 

 resisting skull, is folded into these " convolutions " 

 for lack of space. 



Surely, we have thus discovered one of the causes 

 of the forms of living things, in the mechanical 

 conditions under which they are developed. The 

 chemical forces, as we have seen, are used to pro- 

 duce the living substance ; mechanical force, in the 

 resistance of the structures which surround the 

 growing organism, is used to shape it into the 

 necessary forms. This is nature's division of labour. 

 These are the simple means employed by the 

 Creator for bringing into being the marvels of the 

 organic world. Chemical force stores up the power, 



