Conditions Inducing Conjugation 165 



food, while the others are subjected to a scarcity, the latter 

 conjugate, while the former do not. This is true even when 

 all the individuals are derived from the same single original 

 parent. It is clear therefore that there is no imperious ne- 

 cessity for conjugation at a particular period in the life 

 history ; and that a period of scarcity following a period of 

 abundance will induce conjugation when it would otherwise 

 not occur. In some of the writer's experiments the offspring 

 of a single individual were divided into two sets ; one set was 

 caused in this way to go through conjugation four times 

 in succession (the mates at any conjugation being the off- 

 spring of the mates at the preceding conjugation) ; while the 

 other set during the entire period did not conjugate at all. 

 In some races of Paramecium aurelia, after a pair had mated 

 their descendants in the fourth generation were thus caused 

 to mate again ; while in other members of the same stock 

 hundreds of generations passed without conjugation. 



Clearly therefore the occurrence of conjugation is in large 

 measure the result of special external conditions. This mat- 

 ter has been much studied of late by Enriques and by his 

 pupil Zweibaum. They have found that conjugation is 

 favored by special conditions in particular species of in- 

 fusoria; thus in Colpoda steinii conjugation occurs when the 

 layer of water in which they are has become a thin film 

 (Enriques 1907), as happens just before a pool is dried 

 up by evaporation. In Cryptochilum Enriques (1910) dis- 

 covered that certain salts greatly favor conjugation. In 

 Paramecium caudatum Zweibaum (1912) has determined 

 with great precision the conditions that induce mating. He 

 finds that after the infusoria have been subjected to a period 

 of scarcity of food for five to six weeks, if the nutritive 

 conditions are suddenly changed for the worse, and at the 

 same time certain salts are present in proper concentration, 



