XII.] CONJUGATION AND SEXUAL REPRODUCTION, 21 9 



actually produce embryoes within their own bodies before they 

 are born. But here we have to do, not so much with the sudden 

 termination of a limited and changeful developmental period, 

 as with the greatest possible use of the opportunities afforded 

 by an extremely rich nutriment of vegetable juices. The exces- 

 sively rapid multiplication ensures the colony, and therefore the 

 species, from destruction at the hands of its numerous foes, 

 which, just on account of the abundance of food provided by 

 the vast increase of their prey, become themselves still more 

 numerous, so that the multiplication of these plant parasites 

 must be carried on at the highest possible rate. Hence we 

 find that many purely parthenogenetic generations succeed 

 each other, while amphimixis is ensured by a single generation 

 of males and females, appearing towards the close of the period 

 in which the richest nutriment is supplied. 



On the other hand, we find that in many Cynipidae a partheno- 

 genetic alternates with a sexual generation, and it generally 

 happens that the latter appears in the summer, and the former 

 in spring or even winter. The often considerable structural 

 divergence between these two generations depends upon the 

 very divergent conditions of life to which they are respectively 

 exposed, and above all upon the fact that the eggs are laid in 

 various, differently formed parts of plants, necessitating there- 

 fore a corresponding difference in the ovipositing apparatus. 

 But such considerations need not detain us here. The benefits 

 conferred by the absence of amphimixis from the winter genera- 

 tion appear to me to follow from the exceptionally unfavourable 

 conditions of life by which it is beset. Many of these small 

 Hymenoptera, e. g. Biorhiza aptera, emerge in the very middle 

 of winter, on warm days in December or January, and creep 

 upon the oak-trees, laying their eggs in the heart of the 

 winter buds, having laboriously bored through the hard pro- 

 tective scales with the ovipositor. Without taking food, and 

 frequently interrupted by cold and the long nights, they carry on 

 this work until all their eggs are safely deposited or until death 

 from snow or cold puts an end to their labours. It is clear that 

 such hard conditions must prove fatal to many of these insects 

 before they have fulfilled their task, and it must conduce greatly 

 towards the maintenance of the species, not only for all the 

 time occupied in selection by the sexes and in fertilization 



