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 larve, 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, 
