166 Life and Death, Heredity and Evolution 



the animals will always conjugate. The salts found to favor 

 conjugation were the compounds of sodium and other metals 

 with chlorine, bromine and other halogens. Aluminium 

 chloride was found to be the most favorable of those studied. 



Thus Zweibaum and Enriques hold that the environmental 

 conditions, past or present, fully determine whether conjuga- 

 tion shall occur. It is true that of two stocks side by side 

 under the same present conditions, one may conjugate, the 

 other not ; but this in their opinion is due to the fact that 

 one has been subjected to a long period of scarcity of food, 

 while the other has not. That is, while the two stocks may 

 indeed at a given time differ in their internal conditions, this 

 difference is not a matter of diversity in the life cycle, com- 

 parable to youth, maturity and age, but is merely a result of 

 the different external conditions under which they have been 

 living. The occurrence of conjugation is, they hold, in last 

 analysis, determined by external conditions. 



There is certainly a large measure of truth in this conclu- 

 sion, though it is perhaps not yet completely established in 

 its absolute form. The question may be asked why it is 

 necessary that the period of scarcity of food should last so 

 long as five to six weeks before it induces conjugation? Does 

 this perhaps indicate that a certain number of generations 

 after a foregoing conjugation are necessary before a new 

 mating can occur? * Zweibaum's experiments need to be 

 repeated in such a way that after one conjugation a new 

 culture is produced from an ex-con jugant, and the period of 

 time determined (or if possible the number of generations) 

 that must necessarily elapse before a new conjugation can 

 be induced. Zweibaum did not determine whether an inter- 

 vening conjugation does away with the accumulated effects 

 of continued scarcity of food, so that the organisms must 



*This question has already been raised by Erdmann (1913). 



