THE RECAPITULATION THEORY 313 



eggs of the salmon or trout are much larger than 

 those of the cod or herring ; and the crayfish, 

 though only a quarter the length of a lobster, lays 

 eggs of actually larger size. This larger size of the 

 eggs of fresh-water forms appears to be dependent 

 on the nature of the environment to which they 

 are exposed. Considering the geological instability 

 of the land as compared with the ocean, there can 

 be no doubt that fresh-water fauna are, speaking 

 generally, derived from the marine fauna ; and the 

 great problem with regard to fresh-water life is to 

 explain why it is that so many groups of animals 

 which flourish abundantly in the sea should have 

 failed to establish themselves in fresh water. 

 Sponges and Coelenterates abound in the sea, but 

 their fresh-water representatives are extremely few 

 in number ; Echinoderms are exclusively marine ; 

 there are no fresh-water Cephalopods, and no 

 Ascidians ; and of the smaller groups of Worms, 

 Molluscs, and Crustaceans, there are many that do 

 not occur in fresh water. 



Direct experiment has shown that in many cases 

 this distribution is not due to inability of the adult 

 animals to live in fresh water ; and the real ex- 

 planation appears to be that the early larval stages 

 are unable to establish themselves under such 

 conditions. This interesting suggestion, which has 

 been worked out in detail by Professor Sollas,* 

 undoubtedly affords an important clue. To 



* W. J. Sollas, "On the Origin of Freshwater Fauna," 

 Scientific Transactions of the Royal Dublin Society, vol. iii. Ser. n., 

 1886. 



