290 THE THEORY OF THE GENE 



the presence of a sex-chromosomal mechanism that is sex- 

 determining. Such a mechanism is an agent that tips the 

 scale one way or the other under a given set of environ- 

 mental conditions. The mechanism has never been under- 

 stood in any other way. It may be overborne by other 

 agents that turn the scale without thereby losing its 

 power to act in its usual way when the conditions return 

 under which it is accustomed to work. No better example 

 of this relation could we hope to find, if the tentative con- 

 clusions stated above are confirmed, namely, the change 

 of a homogametic female into a homogametic male in a 

 species in which the normal male is heterogametic. This, 

 in fact, would furnish another convincing proof of the 

 genetic explanation of sex-determination, and one that 

 would be especially instructive for those who fail to 

 understand the interpretation that geneticists place on 

 this mechanism and on Mendelian phenomena in general. 



Another plant, Mercurialis annua, has separate sexes 

 but rarely a pistillate flower appears on a male plant, 

 and, conversely, a starainate flower on a female plant. A 

 male plant may have 25,000 male flowers and only from 

 1 to 47 pistillate flowers, while the staminate flowers on a 

 female may be as 1 to 32. 



Yampolsky has reported the sex of offspring produced 

 from both these kinds of plants after self-fertilization. 

 Offspring from selfed female plants are female or pre- 

 dominantly female. Offspring of selfed male plants are 

 male or predominantly male. 



It is not possible at present to give a satisfactory ex- 

 planation of these results on the XX-XY formula unless 

 rather arbitrary assumptions are made. For instance, if 

 the female plant is XX, then all the pollen grains she 

 produces should carry one X, hence all the offspring 

 should be females, as was the case. But if the male plant 

 is XY, half the mature eggs should be X and half Y. Simi- 



