ASPECTS OF KINETIC EVOLUTION 3 53 



RELATION OF HEREDITY TO IIETERISM. 



The recognition of normal diversity inside the species neces- 

 sitates a modification of the older view of heredity which predi- 

 cated an exact likeness among the members of a species. The 

 uniformity which the older authors had chiefly in mind was that 

 of the members of one species compared with those of another 

 species. This is indeed a wonderful phenomenon, and it is not 

 surprising that mechanical explanations were suggested. It 

 was also quite to be expected that when the idea of internal 

 "mechanisms of heredity" had arisen it should have seemed 

 necessary to predicate a complete uniformity of individuals as 

 the normal result of the workings of such a device. The 

 mechanical inference was carried even to the extent of suggesting 

 that the diagnostic characters like those enumerated in system- 

 atic manuals are each represented by one of the chromosomes 

 or minute masses of infinitesimal granules found in the nuclei 

 of reproductive cells. 



As a matter of fact, natural species do not differ merely by 

 six or seven formally expressed characters. They are different 

 throughout, and the diversity does not end with the distinctions 

 between the species, but extends to the individuals of each of 

 the groups. Appreciating the necessity of greater flexibility for 

 the mechanisms of descent, Mr. Walter T. Swingle suggested 

 several years ago that the expression of characters might not 

 depend directly or entirely upon the chromosomes or granules 

 themselves, but upon their positional relations. This sugges- 

 tion avoids all occasion of resorting to the character-unit hypoth- 

 esis, and may afford a clue to a cytological explanation of the 

 phenomena of heterism. 1 



It is not necessary to think that the granules determine the 

 characters as such ; they need be considered only as representing 

 the characteristics of the ancestral lines of descent. It is then 



1 Mr. Swingle also calls my attention to the very pertinent fact that the nar- 

 rowly mechanical character-unit hypotheses, to which objection is taken in the 

 present paper, have not been proposed or defended by those who have made the 

 truly important contributions to the science of cytology. Indeed, it is exactly 

 these investigators with first-hand knowledge of the anatomy of cells who appre- 

 ciate most keenly the wholly hypothetical nature of the character-unit specula- 

 tions. 



