252 DARWINISM AND THE PROBLEMS OF LIFE 



and so to maintain the species ; because during that 

 period it is exposed to all sorts of dangers. The craw- 

 fish, which is closely related to the marine crab, issues fully 

 formed from the egg. This, again, is an adaptation, 

 because the animal lives in running water, and cannot 

 use a larva-form which from its structure is intended 

 to float in the water. The stones afford shelter to the 

 young animal, and it can take refuge under them. It 

 is already a miniature of the adult 



Among the lower crustaceans, also, the copepoda 

 and water-fleas, there are differences of development. 

 The copepods, which swim about in countless numbers 

 in most ponds, lay a large number of eggs, and carry 

 them about for a long time in a little sac. These 

 numerous eggs have to be small, on account of the 

 lack of space and the slight quantity of food available 

 for each; and as they contain little yolk, the animal 

 can only develop in them up to the nauplius-stage, and 

 must then issue forth. It is otherwise with the water- 

 fleas or daphnidse. In their case only a few eggs are 

 laid, and they have consequently more yolk. They also 

 lie in a special breeding-chamber under the mother's 

 shell, and are washed with a fertilising fluid. Thus we 

 can understand how, in the case of the daphnid, the 

 whole nauplius-stage can be passed in the egg, and 

 from it issues a completely developed water-flea, only 

 differing from the adult in size. 



The number of the enemies of the daphnidae is 

 enormous, and as the animals lay only a few eggs, they 

 could not survive if they had not a special adaptation 

 in connection with their fertility. They multiply 



