164 Life and Death, Heredity and Evolution 



others are weak. In the former rejuvenescence appears; in 

 the latter it does not. 



What conditions bring about mating in these organisms? 

 It is of interest to bring the facts observed on this point 

 into relation with those we have above set forth as to the 

 results of mating. Many generations pass in which the in- 

 dividuals do not mate; then at a certain time some or many 

 or all of them mate. Is this the result of the internal 

 changes that have been in slow progress, so that conjugation 

 occurs when a condition of ripeness or of need for it has 

 been reached ? This was the idea held by many who believed 

 the life of these creatures to go in cycles of youth and age; 

 mating occurs, it was held, in a certain period of the cycle. 

 Or, on the other hand, is mating rather brought on by certain 

 external conditions? Much study has been devoted to these 

 questions. 



In Paramecium and many other infusoria it has been ob- 

 served that an epidemic of mating usually occurs when a 

 period of high nutrition, resulting in rapid multiplication, 

 is followed by a period of scarcity of food. Artificial cul- 

 tures of hay or other vegetation in the laboratory often go 

 through such a cycle; at first bacteria are abundant and 

 the infusoria flourish on them ; then fermentative changes go 

 so far that the appropriate bacteria are scarce; the in- 

 fusoria become thin, and begin to mate. It is easy to fur- 

 nish these conditions if from a flourishing hay culture we 

 remove a watch glass of the water with many of the infusoria 

 and allow it to stand, without any of the vegetable material, 

 for 24 hours. As the bacteria become scarce the infusoria 

 conjugate. This method has been used practically by many 

 investigators in order to obtain matings for study. 



If part of the animals are kept supplied with abundant 



