CHAPTER XX 



ALTERNATION OF SEXUAL AND PARTHENOGENETIC FORMS 



Parthenogenesis plays a double role in the animal kingdom ; 

 in some species parthenogenetic generations alternate with sexual 

 ones, and in such cases both the male and the female individuals 

 develop from unf ertihzed eggs ; in other species, where no such 

 alternation of generations exists, the fertihzed eggs produce indi- 

 viduals of one sex, the female as a rule, and the unfertilized eggs 

 produce the other sex. It may be well to keep these two cate- 

 gories apart, although intermediate conditions are not un- 

 known, as where unfertihzed eggs produce both males and 

 females. The cases in which external conditions influence the 

 kind of reproduction belong, for the most part, to the first cate- 

 gor}% where parthenogenetic and sexual generations alternate. 



Alternation oj Generations in the Aphids and Phylloxerans 



It was pointed out in the last chapter that in the group of 

 Aphids an alternation of parthenogenetic and sexual generations 

 occurs. In some species the change from the one to the other 

 method depends on a change in external conditions ; but in other 

 species a definite succession of generations exists, and the change 

 is less obviously connected with the effects of the environment. 



The discovery that many of the aphids give rise during the 

 summer to young without the presence of males appears to have 

 been first made known by Leeunwenhoek in 1695, ^^^^ later 

 by Reaumur in 1737, but the series of experiments carried out 

 by Bonnet in 1745 attracted more widespread interest in this 

 mode of development, because of his philosophical discussion of 

 the phenomenon. 



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