OTHER THEORIES OF SPECIES-FORMING. 203 



forming bony plates will take up more food than the un- 

 stimulated ones, and thus will be developed at 



The struggle the expense o f these others. Similarly with 

 13 for food, J 



all those other marvellously delicate inner adap- 

 tations of fine and minute and oft-repeated structures to the 

 special functions of the organs containing these structures. 

 The stimulus of the function excites a trophic demand on 

 the part of the struggle and an actual capacity for satisfying 

 the demand, that soon leads to the extra-development of 

 the stimulated parts at the expense of adjacent similar parts 

 deriving food from the supply common to all. Thus Roux 

 would explain the exquisite adaptation of the arrange- 

 ment of the muscle-fibres in the walls of the blood-vessels, 

 the tsenidia or spiral threads in the tracheae of insects, the 

 little barbs on the feathers of birds which hold these feathers 

 together in almost air-tight continuity, the numerous protect- 

 ive hairs covering the spiracles of many insects, etc., etc. 



It will be noted that the competition of the parts is really 

 twofold ; thus, while for successful development it is neces- 

 sary for parts to be successful in food-getting 1 , 



The struggle 



of the parts is this success in food-getting seems to depend 

 twofold, upon the prerequisite of receiving a needed 



functional stimulus. Thus there may be said to exist a com- 

 petition for functional stimuli. But obviously success in 

 this competition depends chiefly on the hazard of position. 

 Those plates in the forming spongy tissue of a long bone 

 which happen to lie where the stress comes, and in a special 

 direction to be affected by it, are the winners in the compe- 

 tition for stimuli. 



Roux's theory has appealed strongly to many biologists, 

 but others have rejected it wholly, or at least as an explana- 

 tion of fine inner adaptations. Plate takes this 

 Plate's criti- 

 cism of Roux's latter position, but finds a great service in 'the 



theory. theory in that "Roux has given in it a profound 



analysis of the well-known fact that use strengthens and 



