ORIGIN AND SIGNIFICANCE OF SEX. 229 



stimuli of the environment, for each developmental 

 reaction is caused only by certain definite stimuli 

 and cannot occur without them. The facts of in- 

 heritance show that the stimuli of an environment 

 may produce developmental reactions, some of which 

 are inherited from one parent, and some from the 

 other parent. If the stimuli acting on a developing 

 individual resemble more closely the stimuli that 

 have acted for a long time upon the protoplasm de- 

 rived from one parent, and differ slightly from the 

 stimuli that have acted upon the other parent, then 

 the individual will develop more after the fashion 

 of the parent whose environment was like its own 

 present environment. Mr. Herbert Spencer, after 

 discussing some of the phenomena of breeding 

 domestic animals, seems also to arrive at this same 

 conclusion.^ When the conditions of stimuli have 

 long been the same for both parents, or both 

 parental races, then we might expect the union to 

 intensify the hereditary impulse, and, under the same 

 conditions of stimuli, to produce a more perfect 

 development along the same line — as in our most 

 approved cases of artificial breeding. On the other 

 hand, when the stimuli acting on the developing 

 organism are a selected combination of two different 



1 " Inadequacy of Natural Selection," by Herbert Spencer, Con- 

 temporary Keview, March, 1893. 



