no ORGANIC EVOLUTION — THE FACTORS 



lesser developiaciit of tlicm all. But, of a thousand 

 structures, some must vary in one direction and some 

 in the other; the general result must therefore be 

 according as the most vary, or as the most important 

 vary ; but whatever the general result, there is ever 

 preserved a considerable degree of harmony and pro- 

 portion between the structures in normal animals ; in 

 abnormal animals (^. c. in monsters, in deformed persons, 

 and persons with tumours) the structures have not the 

 power to vary proportionately ; under stimulation either 

 some develop more in proportion, or others less in 

 proportion than is normal ; but these animals being 

 usually among the unfit, perish Avithout leaving 

 descendants. 



The general effect of Natural Selection, at least as 

 regards higher animals, therefore, is to produce in the 

 individual organism the power of varying proportionately 

 and harmoniously in all its parts along certain lines in 

 response to stimulation, direct or indirect, from the 

 environment ; for instance, owing to direct stimulation 

 the skin of the sole thickens and hardens, owino- to 

 indirect stimulation {i. e. in response to functional 

 activity caused indirectly by events or objects in the 

 environment), the muscles of a limb grow and strengthen. 

 From Avhich it follows, that the size any individual of 

 a species attains, depends firstly on the power to vary 

 under stimulation inborn in his structures, and secondly, 

 on the amount of appropriate stimulation to which the 

 structures are subjected, whereby this power to vary is 

 called into activity ; the structures in the limbs of an 

 infant, for example, grow into an adult limb under 

 appropriate stimulation, that of use ; but if the stimu- 

 lation be withheld, the limbs remain infantile, nay, if 

 stimulation be withheld absolutely, even the infantile 

 standard of development is not maintained, and the 

 more active tissues, those most largely composed of 



