CHAPTER XX 
ALTERNATION OF SEXUAL AND PARTHENOGENETIC FORMS 
PARTHENOGENES!S plays a double réle 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 unfertilized eggs; in other species, where no such 
alternation of generations exists, the fertilized 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 unfertilized 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- 
gory, where parthenogenetic and sexual generations alternate. 
Alternation of 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, and later 
by Réaumur 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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