158 THE HAUNTS OF LIFE 



parts of which are elastic, so that they can be 

 stretched out further or shortened down. Thus 

 some animals have a very long embryo period 

 (like Peripatus), others a very long larval pe- 

 riod (like May-flies), others a long childhood 

 (like kittens), others a long maturity (like 

 horses) . When it is necessary, a part of the life- 

 curve can be, as it were, stretched out in the 

 course of generations man is stretching out 

 his youthful period and another part can be 

 shortened down. Many fresh-water animals 

 have shortened down the riskful juvenile 

 period. 



A clear example of what we mean may be 

 found in the fresh-water crayfish. It is as high 

 up the genealogical tree as the lobster, and 

 almost as high up as the crab. But while the 

 shore-crab has a long life-history, sketched in 

 our study of the seashore, with one larval stage 

 after another, the young of the fresh-water cray- 

 fish is hatched as a miniature of its parent. It 

 is practically identical with its parents, except 

 that the tips of its claws are bent in, the better 

 for gripping the empty egg-shells which are 

 glued to the swimmerets of the mother. It has 

 no larval stages to pass through; it remains in 

 shelter under its mother's tail until it is able to 



