336 THE STUDY OF ANIMAL LIFE CHAP. 



continuously elastic thread, with non-extensible intervals 

 here and there, and that the tension of the several parts 

 can be altered in adaption 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 star- 

 fishes, secures diffusion and saves the delicate young life 

 from the intolerable rough-and-tumble conditions of the 

 shore. The medusoid, or swimming-bell period (sexed) 

 in the life-history of many a zoophyte or hydroid colony 

 (see " alternation of generations," p. 305) probably secures 

 the advantage of cross-fertilisation. 



The very general suppression of the free-swimming 

 larval stages in river animals (excepting cases such as 

 insect larvae, where gripping organs are well developed) 

 is evidently an adaption against the risks of being w r ashed 

 down to the sea or being borne into an equally fatal 

 stagnant backwater. In birds nesting in safe places there 

 is often a long fledgling period, which corresponds to 

 prolonged infancy ; in ground birds, where the risks are 

 greater, the young are usually precocious and able to run 

 about soon after hatching. The relatively large eggs of 

 mound-birds are produced at intervals, and incubation 

 is impossible ; the young, hatched in a heap of ferment- 

 ing vegetation, are able to fly right aw r ay. In young as 

 in adult there is a suppression of a chapter. 



A telescoping of not only larval periods, but of youth 

 itself into a prolonged embryonic development in shel- 

 tered conditions may mean that circumstances are too 

 tyrannous for delicate young lives. In many mammals, 

 as we have already hinted, the prolongation of the ante- 

 natal period may have something to do with the perfect- 

 ing of a fine organisation, 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 finer brains. 



Just as there are plants which remain for life like half- 



