CRUSTACEA AND MOLLUSCS 253 



parthenogenetically : that is to say, they lay eggs which 

 develop without being fertilised by a male. From 

 these eggs females only are produced, and they in turn 

 lay eggs that do not need to be fertilised. Hence the 

 water-fleas that appear in the spring are followed by 

 several generations of the same sex ; towards the end 

 of the summer their number is enormous, and then at 

 last males are developed from a few of the eggs. 

 These fertilise the eggs of the last generation of females, 

 which are only laid in small numbers, generally only 

 one or two. They are larger than the summer-eggs, 

 and have plenty of yolk, because they have not the 

 benefit of the fertilising water of the mother. They 

 have a thick shell, and fall to the bottom, where they 

 may be frozen in, or may lie uninjured without water. 

 In the following spring they produce the first generation 

 of females. 



Parthenogenesis brings up the number of daphnidae 

 in the course of the summer to a huge figure. We saw 

 in the first chapter that a couple of foxes will, if they 

 have three male and three female young, and these 

 three pairs give birth to the same number and so 

 on, increase in ten years to 118,098 individuals. But 

 if the first pair of foxes only gave birth to females, and 

 these could multiply parthenogenetically, the number 

 would rise in ten years to 60,466,176, or incomparably 

 more. With larger figures the effect is still more 

 stupendous, as there is question of a geometrical pro- 

 gression. Hence the increase due to parthenogenesis is 

 so great that though the numbers of winter eggs in 

 each case is small, the total is very large on account of 



