HEDGE AND DITCH 173 



and naturalists who skim the surface of a pond or lake with 

 a tow-net sometimes take them in great numbers. The 

 winter-eggs always produce females only, generally after a 

 long interval, and these females are never capable of lay- 

 ing more winter-eggs, but only unfertilised summer-eggs. 

 The male water-flea is only found at the seasons when 

 winter-eggs are due. He is much smaller than the female, 

 has no brood-pouch, and differs in many small details. 



Weismann, who has closely studied the reproduction of 

 many species of Daphnids, finds that the succession of 

 generations is not the same in all. Sometimes many 

 generations of summer-eggs succeed one another, until at 

 last the cycle is completed by the recurrence of winter- 

 eggs ; sometimes every other generation is capable of pro- 

 ducing fertilised winter- eggs. In our commonest species 

 there are two seasons for sexual individuals, viz. early 

 summer and late autumn. There is a relation between 

 the frequency with which fertilised winter-eggs are pro- 

 duced and the conditions of life. Where drying up of the 

 pools which a particular species frequents may be expected 

 to happen many times in the year, the sexual stage recurs 

 more frequently ; in a species which is not liable to be 

 dried up, and where winter- cold is the only hardship to be 

 faced, the sexual stage may come round but once in the 

 year. Indeed, it is found that in deep lakes, where neither 

 drought nor winter-cold can seriously affect the Daphnias, 

 they propagate all round the year by unfertilised or summer- 

 eggs, and winter-eggs are perhaps never formed at all. 



Where there is a well-marked dry season in every year, 

 the winter-eggs may lie dormant for months, or even 

 for years without losing their power of hatching. Dried 

 mud, collected from pools in Jerusalem or Khartoum, 

 regularly produces large and handsome Daphnias, besides 

 other crustaceans, whenever a spoonful is dropped into 

 a tank in an English parlour. A little dry mud from the 

 pool of Gihon, which contains water only for two months 

 of the year, was placed in a globe of water, and the 



