274 MAN --AN ADAPTIVE MECHANISM 



has shown conclusively that, in some instances, organs 

 transplanted from the body of one animal to the body 

 of another of the same species may undergo histologic 

 changes. Most transplanted kidneys are broken down 

 in time. Lexer's brilliant surgical feat of transplanting 

 entire knee joints from one human being to another 

 showed later that the transplant gradually disappeared 

 and was replaced by a new structure identical in form 

 and bulk with the transplant, but actually the result 

 of cell multiplication by the host organism. Thus we 

 have experimental and clinical evidence that normal 

 individuals of the same species may possess slight 

 chemical differences. 



The beginning of the process of procreation is the 

 transplantation of a unit of tissue from one individual 

 of a species to another individual of the same species. 

 Species are probably only exaggerated varieties ; and 

 varieties are exaggerations of individual differences. 

 The reason why different species do not cross is doubt- 

 less because the chemical reaction of the female kills 

 the spermatozoon, which is a foreign protein. The 

 same reason doubtless explains why varieties cross less 

 well than individuals of the same species. And one 

 may assume that two normal individuals who produce 

 no offspring, but who, when they separate and remate, 

 are fertile, are infertile in the first instance because of 

 chemical incompatability. 



The spermatozoon presumably brings with it the 

 chemical characteristic of the male. Hence, in the 

 growth and development of the placenta and fetus 

 there should be a slight chemical difference between 

 the mother and the fetus, which would increase dur- 



