INTRODUCTION. 15 



A nearly parallel series to that exhibiting alternations of 

 sexual generations with generations which produce by budding 

 is supplied by the cases where sexual generations alternate with 

 parthenogenetic ones, or in some instances even with larvae 

 which reproduce sexually or else parthenogenetically. 



The best known examples of this form of alternations of 

 generations are found amongst the Insecta 1 . A simple case 

 is that of the Aphides. The ova deposited by impregnated 

 females give rise to forms differently organised to the parents 

 but provided with an ovary 2 . The eggs from the ovary develope 

 parthenogenetically within the oviduct, and so long as there 

 is plenty of food and warmth the generations produced are 

 always parthenogenetic forms. The failure of warmth and 

 nutriment causes the production of true males and females, and 

 so the cycle is completed. We must suppose that the capacity 

 possessed by so many female insects of producing eggs capable 

 of developing without the influence of the male element, has 

 been, so to speak, taken hold of by natural selection, and has led 

 to the production of viviparous parthenogenetic forms, by which, 

 so long as food is abundant, a clear economy in reproduction is 

 effected. The continuance of the species during winter is secured 

 by the production of males and females, the females laying eggs 

 in autumn which are hatched in the spring. 



In Chermes there is less modification of the primitive condi- 

 tion in that the parthenogenetic generations lay their eggs like 

 the impregnated females. In the gall-flies (Cynipidae), there is 

 frequently an alternation of generations of the same kind as in 

 Chermes ; there being no viviparous forms. The individuals of 

 the different generations differ from each other to some extent 

 in all these cases. 



A second type of alternations of parthenogenetic and sexual 

 generations is exemplified by the cases of Chironomus and 

 Cecidomyia, where the larvce which develope from the eggs of 

 the fertilized female produce parthenogenetically, by means of 

 true ova, forms which eventually after several generations (Ceci- 

 domyia) of larval reproduction give rise to sexual forms. The 



1 For details vide Chapter on Insecta. 



3 The distinction drawn by Huxley between ova and pseudova does not appear to 

 me a convenient one in practice. 



