312 Experimental Zoology 



subjected roses with aphids on them to the cold of an ice 

 box without producing any effect. Moreover, the sexual form 

 may appear in the autumn before the cold weather has come. 

 Even under ordinary conditions all of the aphids do not become 

 sexual forms and hundreds of them perish by the frost. I have 

 kept a potted rose out of doors for three months after the sexual 

 forms had appeared, and yet those individuals that remained 

 alive after this time continued to reproduce parthenogenetically. 

 Finally, the individuals that seem to be less affected are those 

 that are found on the growing tips of the branches, where the 

 leaves are still young and succulent. These observations sug- 

 gest that the change is not due to the cold, but to some changes 

 in the food plant that take place in the autumn. Whether the 

 result is due, as seems probable, to a lack of food, or to a reduc- 

 tion in the amount of water, or to both combined, remains to be 

 shown. 



A case apparently similar to that of the aphids is found in the 

 Daphnians. These Crustacea also produce parthenogenetically 

 during the summer, and in the autumn the sexual forms appear. 

 It has recently been shown by Issakowitsch that the change is 

 probably due to a change in the food supply, and that the trans- 

 formation can be quickly induced artificially by altering the 

 external conditions. This and other cases will be given more 

 fully in later chapters. 



In the hfe histories of the cihate infusoria, periods of division 

 are succeeded by periods of conjugation. Maupas thought that 

 after a long succession of divisions this mode of propagation 

 comes slowly to an end, and that unless conjugation occurs 

 the individuals will die. Immediately after conjugation, when 

 division is very active, there is little tendency to conjugate 

 again; but the longer the time elapsing after this process, the 

 more prone are the individuals to unite in pairs, especially those 

 of a different parentage. It would seem that an internal factor 

 is here involved ; but there are some indications that external 

 factors may also enter into the result. It has been shown by 

 Calkins that when paramoecium has undergone many divisions, 



