42 SCIENCE PROGRESS 



females at least depends largely on the nutrition. Somewhat 

 similar statements have also been made about Hydatina, 1 but 

 the latest work 2 on it indicates that they are mistaken, so 

 perhaps judgment must be suspended about the Daphniidae also. 



In the Aphididae the conditions resemble those in the 

 Daphniidae, but are more complicated. The bisexual generation 

 generally differs considerably from the parthenogenetic, and 

 not infrequently there are two or more parthenogenetic forms 

 which differ widely among themselves. Here again the number 

 of generations which intervene between one fertilised egg and 

 the next varies in different species, and while in some it is 

 said that the production of a bisexual generation depends on 

 the conditions, in others 3 it is maintained that always the 

 same number of parthenogenetic generations occurs, whatever 

 are the conditions under which the animals live. Again, in 

 other species, the same individual female may produce both 

 parthenogenetic and fertilisable eggs, and among the former 

 there may be both males and females, or only one or the 

 other. In Aphis and its allies, therefore, we get a very great 

 variety of conditions, ranging from simplicity of life-history 

 very like that of Daphnia to complexity hardly surpassed in 

 the life-cycles of the trematodes. 



As an example of these complicated life-cycles, one may 

 take the genus Chermes} C. abietis lays fertilised eggs on the 

 fir Picea {Abies), which develop into wingless parthenogenetic 

 females. These hibernate, and lay eggs producing winged 

 parthenogenetic individuals, which migrate to the larch, and 

 give rise to a wingless parthenogenetic generation. These, 

 after hibernation, lay eggs, some of which develop into wing- 

 less forms, which continue to reproduce children like them- 

 selves ; the rest develop wings, return to the Picea, and their 

 parthenogenetic eggs develop into sexual males and females 

 like those with which the cycle started two years before. 

 There are, therefore, in the life-history of this species five 

 different kinds of individuals, some existing only on the fir 



1 Maupas, C.R. cxi. 1890, pp. 310, 505. 



2 Punnett, Proc. Roy. Soc. B, 78, 1906, p. 223. 



3 Tannreuther, Zool. Jahrb. xxiv. p. 609. 



4 Cholodkovski, Biol. Centralbl. xx. 1900, p. 265. Borner {Zoo. Anz. xxxii. 1907, 

 p. 413) finds that the life history is more complex than Cholodovski's description, 

 and re-establishes the "parallel series" as originally described by Dreyfus 

 (cf. Camb. Nat. Hist. vol. vi. p. 585). 



