FRESH-WATER ANIMALS 85 



pointed out that though the adult animal might be 



thoroughly well adapted to fresh-water life, yet 



it would by no means follow that the early 



stages of development would be equally well 



suited to the change. Professor Sollas also shows 



that the early larval stages of most of the marine 



animals are forms peculiarly ill-adapted to living in 



fresh- water streams, forms indeed which could not 



hold their own under such conditions. Thus, in 



the Echinodermata, which we have already seen 



are exclusively marine, the young hatch as very 



minute larvae swimming freely in the water by 



means of cilia ; and larvae similarly occur in 



almost all the other groups of marine invertebrates. 



Now such small ciliated larvae are altogether 



unsuited to fresh-water streams, and could never 



hold their own in them. They are quite incapable 



of swimming against even weak currents, and the 



inevitable consequence is that each succeeding 



generation would be carried further and further 



down stream, and ultimately the whole species 



carried out to sea. This is a very important point, 



and as it is one that has not yet received general 



attention I think it will be well that we should 



enquire into it more fully, taking the several groups 



one by one and seeing how far it will serve to 



explain the special characters in the distribution of 



fresh-water animals. 



Before doing so, there is one further point of a 

 preliminary nature that will require attention. It 

 is a very familiar fact that the eggs of different 

 animals vary greatly in size. Thus, the egg of a 



