142 THE BOOK OF NATURE STUDY 



shell-less, free-swimming creatures, but are like miniature adults. 

 In other words, there is here no special provision for distribution. 

 As the young have to spend a much longer time in the egg-mass, 

 the special protection of the tough capsule is necessary, while 

 those of Doris, which are thrown upon their own resources very 

 early, have no special protection. Again, the Doris larvse must 

 seek their own food from a very early period. The young whelks 

 within their egg-case cannot do this, and we find that in each 

 capsule a few larvae get the start of the others and proceed to feed 

 upon their aborted brethren. The result is that of the five hundred 

 or so eggs in a capsule, perhaps five, or one per cent., may develop, 

 all the others being sacrificed. On the other hand, the chance 

 of survival of these five at hatching is very many times greater 

 than the chance of survival of any particular Doris larva. The 

 subject has been dwelt upon in a little detail, because these two 

 conditions tend to occur in almost all groups of shore animals. 

 On the one hand, we have forms which produce myriads of minute, 

 prematurely hatched, free-swimming larvae, any particular one 

 of which has a small chance of surviving, but whose object seems 

 to be to ensure the distribution of the species. On the other 

 hand, again, we have forms which produce fewer young, which are in 

 some way protected and fed until they approach the adult in 

 structure, which do not serve to distribute the species, but whose 

 chance of survival is relatively great. The antithesis is found 

 in both simple and complex animals. Thus in fishes the dab 

 and flounder, as mentioned above, produce young different from 

 the parents, which swim about in the open water instead of 

 haunting the bottom like the adults ; while the skate, another 

 bottom-haunting form, lays its eggs in capsules from which the 

 young does not emerge until it has the form and habit of the 

 parent. In this case the little skate is furnished with a supply of 

 food in the form of yolk to feed it within the capsule, and very 

 much fewer eggs can be produced by the parent than in the case 

 of the flounder where much less yolk is necessary ; but the little 

 skate at hatching has a better chance of survival than the flounder. 

 Among the molluscs, in addition to the types named above, 

 the conditions seen in the periwinkles should be noted. The 

 common edible form lays its eggs in little jelly-like masses on 



