Conditions Inducing Conjugation 167 



again be subjected to five or six weeks' scarcity before they 

 will again conjugate. This is really the essential point; for 

 if this turns out to be the case, then evidently the length of 

 time since a previous conjugation is one of the things that 

 determine whether conjugation shall now occur. 



But independently of this doubtful point, it is clear that 

 mating at a particular period is not required independently 

 of the outer conditions, for Paramecium aurelia will live in- 

 definitely (over 6000 generations) without conjugation 

 (Woodruff), yet may be induced to conjugate if the required 

 outer conditions are supplied (Woodruff, 1914) ; and in some 

 stocks of this species a second conjugation may be induced 

 in the fourth generation after a previous conjugation (Jen- 

 nings, 1910, p. 286). Certainly by far the largest part is 

 played by external conditions (past or present) in producing 

 conjugation. 



The conditions under which mating occurs (sudden 

 scarcity of food and the like) are conditions which are dis- 

 tinctly unfavorable to the life of the organisms. Some 

 species of infusoria respond to such conditions by becoming 

 encysted; they transform into a small sphere, protected by 

 an outer coating; and in this state they can withstand con- 

 ditions that would otherwise destroy them. Some other 

 Protozoa respond by first conjugating, then encysting. In 

 others, such as Paramecium, there is only conjugation, with- 

 out encystment. But as we have seen, conjugation results 

 in the production of many diverse stocks, some of which are 

 more resistant to given conditions than others. It appears 

 that some of the stocks so produced may be able to survive 

 the unfavorable conditions which induced conjugation, al- 

 though (as observation shows) most of them die out if the 

 conditions are not altered for the better. Later generations 

 would therefore all be derived from the most vigorous and 



