174 HOUSE, GARDEN, AND FIELD 



crustaceans duly appeared. When they had enjoyed a 

 few weeks of active life, and laid their eggs, the water 

 was drawn off, and the mud left to dry. Next spring 

 the same mud was again placed in water, and the process 

 was regularly repeated for many years. A sample of the 

 mud which had been kept dry for ten years still contained 

 live eggs. 1 



The small size of the eggs of a Daphnia and their power 

 of withstanding long-continued drought, help to explain 

 their wide dispersal. The wind that scatters clouds of 

 summer dust often scatters crustacean eggs as well, and 

 makes it easier to understand how pools of insignificant size, 

 separated from all others by miles of desert, sea or snow, 

 should now and then be found to swarm with live Daphnias. 



The biological questions which the history of Daphnia 

 suggests are of the deepest interest, but many of them 

 are at present insoluble, and nearly all of them are diffi- 

 cult too difficult for the young observer at least. Keep 

 the facts in mind, and you will come across other instances 

 of the same thing as your experience widens. Certain 

 insects, polyzoa, rotifers, and other animals show in their 

 life-histories an alternation of sexual and asexual genera- 

 tions, very similar to the alternation of summer and 

 winter-eggs in Daphnia. In all there is rapid multiplica- 

 tion during the season when food abounds ; resting-stages, 

 mating, and all that takes up time are then left out. When 

 the season of difficulty comes round, the season of drought 

 or the season of cold, loss of time signifies nothing, and 

 safety is all in all. No one, I believe, can explain why 

 only the fertilised egg can outlast the season of cold or 

 drought, nor can we make it clear to our own minds what 

 lasting benefit to the species can result from the exuberance 

 of new individuals in one particular season, if that season 

 is immediately followed by another in which the numbers 

 are cut down to a minimum again. The advantage may 

 be merely this, that if only one per cent, of the individuals 



1 Atkinson, Ann. and Mag. of Nat. Hist., Nov., 1898. 



