202 DARWINISM TO-DAY. 



adaptations on the one hand, attributing these to the influ- 

 ence of natural selection, and the adaptations of internal 

 parts on the other, which he would attribute to the influ- 

 ence of his functional stimuli and of his struggle among 

 the inner parts of the body. This struggle, like that among 

 Weismann's hypothetical determinants, is one chiefly for 

 food, but in Roux's theory there is no assumption of hypo- 

 thetical life units, nor any lack of clearness concerning the 

 initiation of the actual struggle. The competing parts in 

 Roux's theory are the chemical molecules composing the 

 The competing ce ^' ^ ne cells themselves, groups or tissues of 

 parts in Konx's cells, and even whole organs. The spurs to the 



theory are actn- ...... ,. 



ally recognised competition for food are functional stimuli, 

 structures. whose result is to set up a special demand and 

 necessity for more food. Roux's classic example will make 

 this clear. It is a matter of fact that the fine plates and 

 layers of bone in the "spongy tissue" of the long bones of 

 the body, are so disposed as actually best to withstand the 

 stresses most usually brought to bear on the bones. Thus 

 they show a fine adaptation of arrangement, which one meets 

 difficulties in trying to explain as due to natural selection. 

 For, if we imagine the thin plates of the spongy tissue 

 purely miscellaneously arranged, the possible slight varia- 

 tions whereby a few plates at a time might fortuitously 

 occur in a position or direction better fit to strengthen the 

 whole bone, are so insignificant in proportion to the condi- 

 tion throughout all the rest of the bone that we cannot possi- 

 bly attribute to them a life-and-death value in the individual's 

 struggle for existence. Roux assumes that the stresses 

 brought to bear on the bone during its development act as 

 functional stimuli to all those plates in the forming spongy 

 tissue, which lie in such places or at such an angle to the 

 stress as to be affected by them, and in response to these 

 stimuli, which in Roux's belief are necessary to the normal 

 structural development and maintenance of any part, these 



