78 McCLUNG. 



of the fact that it reverses the ordinary relations of the elements 

 and removes the power of choice from the female. 



We have in the case of the spermatozoa, however, the observed 

 fact that there are two essentially different forms and that they 

 are present in equal proportions. No other feature, save sex, 

 separates the resulting offspring into two approximately equal 

 groups. By exclusion then, it would seem that the determina- 

 tion of this difference is reposed in the male element. 



There are, I am aware, certain observations upon the determi- 

 nation of sex with which my hypothesis does not seem to agree. 

 Some of these I should like to mention in order to suggest pos- 

 sible explanations or reasons for regarding them within the limit 

 of error set by our present knowledge of the subject. These 

 objections may be suggested by the following questions : 



Is sex potentiality by which I mean the tendency of the spe- 

 cies to perpetuate itself in individuals of two sexes of approxi- 

 mately equal numbers a constant and uniform factor prevailing 

 throughout all classes of animals ? Is sex determined at the 

 time of fertilization ; if so, is such determination absolute, or 

 may it be changed by varying conditions ? Under the unusual 

 circumstance of parthenogenesis, will it be possible to reconcile 

 a theory which postulates the presence of a determinant in the 

 male element with the fact of the entire absence of this element 

 in unisexual reproduction ? 



An answer to the first question is not difficult. It is a matter 

 of common observation that all animals are not alike in their 

 methods of reproduction. In the insects, for instance, it is 

 known that certain forms invariably produce young after the 

 sexual method and that parthenogenesis never occurs ; in others, 

 parthenogenesis is the common method and sexual union of male 

 and female only an infrequent occurrence ; while in yet others, 

 one sex is produced by fertilized eggs and the other sex from 

 those unfertilized. 



The logical conclusion to be drawn from these facts is that sex, 

 per se, is not an unchangeable attribute of organisms but is an 

 adaptation of the species to secure the most favorable conditions for 

 its perpetuation. Given favorable conditions of environment and 

 aphides will reproduce indefinitely with only one sex as a repre- 



