92 ORGANIC EVOLUTION — THE FACTORS 



limbs) developed as enable the organism to place itself 

 in harmony with a complex environment, but to further 

 increase this harmony the structures themselves are 

 rendered highly variable; whereby extreme develop- 

 ment of this or that structure or organ occurs in stress 

 of need, but useless developments which would only 

 encumber the organism do not occur. For this reason 

 the muscles in a clerk's arm do not develop to the same 

 extent as the muscles in a blacksmith's arm. 



This evolution of corporal variability corresponds, as 

 will be seen later, to an equal or greater evolution of 

 mental variability, so that physically as well as mentally 

 the action of Natural Selection has been to produce 

 great individual powers of varying ; but the variations 

 produced under this power are not transmitted, only 

 the power to vary and to acquire under appropriate 

 stimulation similar variations is transmitted ; thus, to 

 take an example, no infant is bom of fully developed 

 parents whose limbs or mental powers can attain a 

 similar full development without stimulation from the 

 environment. 



As regards physical variability, with which alone we 

 are at present concerned, in general each individual of a 

 species in his " normal " development nearly attains its 

 limits {e. g. beyond the normal development of muscle^ 

 but a relatively small further development is possible) ; 

 the limit being fixed by the struggle between Natural 

 Selection and Atavism, the one operating to increase 

 variability, the other to diminish it, to bring it to a 

 condition of ancestral non-variability. 



But all the structures do not " normally " approach 

 the limit to an equal degree, horns, hair (in animals in 

 which hair is merely ornamental, and in those living in 

 an equable climate, when variability under stimulation 

 in this respect is not essential to survival), teeth, nails, 

 &c., if we except the effects of disease, which cannot 



