156 A. FRANKLIN SHULL 



to appear is not the only means by which they may be prevented 

 from appearing. The presence of creatin, the presence of beef 

 extract, the presence of certain ammonium compounds and of 

 urea, all tend to prevent the occurrence of male-producers. These 

 facts should not blind us, however, to the possibility, which Whit- 

 ney points out, that other substances may cause male-producers 

 to appear. As to the internal factors, which Whitney is inclined 

 to discard in accounting for all male-producers, I have shown 

 elsewhere that these internal agents exist; and I believe that we 

 may attribute to internal agents, alone or in combination with 

 external agents, some of the phenomena which have been held to 

 indicate the presence of external agents alone. 



Certain points of theoretical interest may be mentioned in 

 connection with chemical substances. Inasmuch as the question 

 was raised whether the action of temperature in altering the pro- 

 portion of male-producers is direct, and was answered in the 

 negative, it may also be asked whether that of chemical substances 

 is direct. The chief evidence for supposing the influence of tem- 

 perature to be indirect was the fact that it sometimes increased, 

 sometimes diminished, the proportion of male-producers. Among 

 chemical substances, on the other hand, with the exception of 

 the degree of alkalinity, which produced differences too small to be 

 of much value, the effect of each chemical was in every case tried 

 of the same sign. This of itself would lead us to believe that 

 the action of these substances is direct. But this is not necessary. 

 The substances may, for example, induce a physiological state 

 which is directly responsible for determining the mode of repro- 

 duction. 



It may also be pointed out here that nothing in the evidence 

 yet obtained shows whether these substances actually decide 

 how events shall occur in a given cell, or whether, events having 

 happened differently in two classes of cells, the substances in the 

 medium merely decide which class may most rapidly and success- 

 fully develop. 



The conclusion that certain external agents may modify the 

 ratio of sexual to parthenogenetic females has, I believe, been 



