Biological Papers. 129 



unknown way, the character of their reaction with their en- 

 vironment. When the instincts harmonize they supplement 

 and strengthen one another, as is usually the case when the 

 sperm and egg come from individuals of the same species. 

 When the body-building and bodj^-using instincts cannot har- 

 monize, as is the case when the sperm and egg come from in- 

 dividuals of widely different species, the result is zero, for 

 very obvious and necessary reasons. 



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