SELECTIVE FERTILIZATION IN POLLEN MIXTURES. 285 



a way, merely a parasitic organism. It would be difficult to 

 find evidence from true parasites that they are restrained by their 

 hosts in proportion as they are genetically dissimilar although 

 in most cases the differences are so great that probably there is 

 no basis for comparison. 



The only evidence which has any direct bearing on the problem 

 comes from grafting experiments. It is well known with plants 

 that the affinity of stock and cion is directly proportional to their 

 phylogenetic similarity. In animals the same rule holds, and 

 very fine distinctions are manifested. Morgan (1910) cites the 

 results of Schoene in which the skin of the mouse is readily 

 grafted back upon the same individual or member of the same 

 litter but not upon unrelated mice. Such results as these are 

 quite similar to the greater receptiveness of plants to their own 

 pollen. 



For some time there has been current in biological literature 

 the hypothesis that heterogeneity in the structure of organisms 

 favors growth and reproduction and conversely that homogeneity , 

 is unfavorable. This is a heritage from Darwinism and has 

 appeared again and again in theories of rejuvenation, vigor 

 derived from crossing, and selective fertilization. It has been 

 stated in many different ways but in general terms it amounts to 

 the supposition that similarity in protoplasmic structure brought 

 about by a line of similar ancestors is not conducive to physio- 

 logical efficiency and that the differences brought about by the 

 union of unlike elements and the consequent lack of balance 

 stimulates growth. The only basis for this hypothesis is found 

 in the necessity, in most cases, for the union of two differentiated 

 sex cells to start the development in the egg. The attempt to 

 argue by analogy than the union of dissimilar protoplasms is, 

 in itself, an immediate physiological benefit is not supported by 

 the facts and is founded upon fallacious reasoning. 



Rejuvenation in vegetatively reproduced organisms by sexual 

 union is no longer looked upon as beneficial in destroying simi- 

 larity in structure. That the process of forming gametes and 

 their reunion may bring about a reorganization of the proto- 

 plasmic substances and an elimination of waste products so as 

 to result in greater growth seems quite plausible. But the 



