THE SHAPES OF VERTEBRATE SKELETONS. 217 



strains either compressions or tensions. Instances of hard- 

 ening under compression are made familiar to us by the 

 skin. We have the general contrast between the soft skin 

 covering the body at large, and the indurated skin covering 

 the inner surfaces of the hands and the soles of the feet. 

 We have the fact that even within these areas the parts 

 on which the pressure is habitually greatest have the skin 

 always thickest; and that in each person special points 

 exposed to special pressures become specially dense often 

 as dense as horn. Further, we have the converse fact that 

 the skin of little-used hands becomes abnormally thin even 

 losing, in places, that ribbed structure which distinguishes 

 skin subject to rough usage. Of increased density directly 

 following increased tension, the skeletons, whether of men 

 or animals, furnish abundant evidence. Anatomists easily 

 discriminate between the bones of a strong man and those of 

 a weak man, by the greater development of those ridges and 

 crests to which the muscles are attached ; and naturalists, on 

 comparing the remains of domesticated animals with those 

 of wild animals of the same species, find kindred differences. 

 The first of these facts shows unmistakably the immediate 

 effect of function on structure, and by obvious alliance with 

 it the second may be held to do the same: both implying 

 that the deposit of dense substance capable of great resist- 

 ance, constantly takes place at points where the tension is 

 excessive. 



Taking into account, then, this adaptive process, con- 

 tinually aided by the survival of individuals in which it 

 has taken place most rapidly, we may expect, on tracing up 

 the evolution of the vertebrate axis, to find that as the mus- 

 cular power becomes greater there arise larger and harder 

 masses of tissue, serving the muscles as points d'appui; and 

 that these arise first in those places where the strains are 

 greatest. Now this is just what we do find. The myocom- 

 mata are so placed that their actions are likely to affect first 

 that upper coat of the notochord, where there are found 



