200 MORPHOLOGICAL DEVELOPMENT. 



tebrate animals. Let us see whether the theory of mechani- 

 cal genesis afford us a deductive interpretation of the in- 

 ductive generalizations. 



Before proceeding, we must note a process of functional 

 adaptation which here co-operates with natural selection. I 

 refer to the habitual formation of denser tissues at those 

 parts of an organism which are exposed to the greatest 

 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 

 habitually thickest ; and that in each person sj>ecial points 

 exposed to special pressures become specially dense — often 

 as dense as horn. Further, w^e 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, habitually 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 



