W. E. Agar 193 



ending in senility and to the necessary death, through internal causes, 

 of the asexual colony. As better and better methods of cultivation have 

 been evolved the duration of this "cycle" has been gradually lengthened, 

 and artificial stimuli substituted for the "rejuvenescing" conjugation, 

 till at last we have Woodruffe, after breeding Paramecium aurelia for 

 3340 asexual generations without evoking any signs of " degeneration," 

 coming to the conclusion " dass das Protoplasma einer einzigen Zelle 

 unter giinstigen ausseren Umstanden ohne Hilfe von Konjugation oder 

 einer kiinstlicher Reizung imstande ist, sich unbegrenzt fortzuptlanzen 

 und zeigt ferner in klarer Weise, dass das Altern und das Befruchtungs- 

 bediirfiiis nicht Grundeigenschaften der lebendigen Substanz sind " 

 (p. 36). 



Similar conclusions have been reached by Jennings as the result of 

 extensive experiments on individuals of Paramecium which had conju- 

 gated, and others which had been prevented from conjugating though 

 ready to do so. 



Conclusion. 



The following conclusions, though refen-ing particularly to S. vetulus, 

 may probably be safely extended to a wide range of the Cladocera. 



1. Certain not yet fully elucidated factors in the environment 

 influence the onset of sexuality. 



2. Certain factors likewise bring about " degeneration " or high 

 rate of mortality. 



3. Certain factors of the environment may act cumulatively over 

 a number of generations. 



4. Therefore the increasing sexuality and " degeneration " (or high 

 mortalit}') observed inidei' certain supposedly constant experimental 

 conditions receive a ready explanation in the supposition that the 

 environment is one favourable to the development of these phenomena. 



5. This explanation is made much more probable when we find 

 that under other experimental conditions there is no tendency to 

 increasing sexuality or degeneration. 



6. Many species exhibit the phenomenon of specially labile periods, 

 when sexuality is easily influenced by certain factors of the environment. 

 This labile condition is usually asci'ibed to the fact that the line is in 

 about the middle of the reproductive cycle, the diminishing tendency 

 to parthenogenesis being about equally balanced by the increasing 

 tendency to sexual reproduction. Such a balanced condition must 

 however be passed through equally whether the tendency to sexuality 



