148 SECRETS OF ANIMAL LIFE 



of the several parts can be altered in adaptation to 

 particular conditions. This is part of the tactics 

 of evolution, and it is interesting to observe the 

 diversity of the problems that alterations in the 

 tempo of life are made to solve. The open-sea 

 larval period in crabs and rock-lobster, in sea- 

 urchins and starfishes, secures diffusion and saves 

 the delicate young life from the intolerable rough- 

 and-tumble conditions of the shore. The swimming- 

 bell or medusoid period (sexed) in the life-history 

 of many a sedentary asexual zoophyte or hydroid 

 colony probably secures the advantage of cross- 

 fertilization. The very general suppression of the 

 free-swimming larval stages in river animals (ex- 

 cepting cases such as insect larvae, where gripping 

 organs are well developed) is evidently an adapta- 

 tion against the risks of being washed down to the 

 sea or being borne into an equally fatal stagnant 

 backwater, A telescoping of not only larval periods 

 but of youth itself into a prolonged embryonic 

 development may mean that circumstances are too 

 tyrannous for delicate young lives, but it may also 

 mean, as in many mammals, that time is given in 

 the long antenatal life for the perfecting of a fine 

 organization, able from birth, in many cases, to 

 cope with the exigencies of life. Robert Chambers, 

 the author of the once famous Vestiges of Creation, 

 was surely right in insisting that the embryo's 

 biding its time within the womb was as precious to 

 it as it was costly to the mother. It meant bigger 

 and better brains. In the prolonged youth, again, 



