SECT, i] PHYSICO-CHEMICAL SYSTEM 315 



takes short cuts in its development, and jumps from branch to branch 

 of its genealogical tree instead of climbing steadily upwards. Thus 

 the little West Indian frog, Hylodes, produces eggs which contain a 

 larger amount of yolk than those of the ordinary English frog. The 

 young Hylodes is consequently enabled to pass through the tadpole 

 stage before hatching, and to attain the form of the frog before leaving 

 the c:gg\ the tadpole stage is, in fact, only imperfectly recapitulated, 

 the formation of gills, for instance, being entirely omitted." 



The more yolk, then, the longer the embryo can remain an embryo 

 before having to face the external world, and the more preparations 

 it can make for that event. It is probable that this question is 

 intimately bound up with the penetration of fresh-water surroundings 

 by the originally marine forms. "It has long been noticed", said 

 Milnes-Marshall, following the classical exposition of Sollas, " that 

 marine animals lay small eggs whereas their fresh-water allies 

 lay eggs of much larger size. The eggs of the salmon or trout are 

 much larger than those of the cod or the herring, and the crayfish, 

 though only a quarter the length of the lobster, lays eggs of 

 actually larger size. The larger size of the eggs of the fresh-water 

 forms appears to be dependent on the nature of the environment 

 to which they are exposed. Considering the geological instability 

 of the land as compared with the ocean, there can be no doubt that 

 the fresh-water fauna is, speaking generally, derived from the 

 marine fauna, and the great problem with regard to fresh-water life 

 is to explain why it is that so many groups of animals which flourish 

 abundantly in the sea should have failed to establish themselves in 

 fresh water. Sponges and Coelenterates abound in the sea, but their 

 fresh- water representatives are extremely few in number; Echino- 

 derms are exclusively marine ; there are no fresh-water Cephalopods, 

 no Ascidians, and of the smaller groups of Worms, Molluscs, and 

 Crustacea, there are many that do not occur in fresh water. Direct 

 experiment has shown that in many cases this distribution is not due 

 to the inability of the adult animals to live in fresh water, and the 

 real explanation appears to be that the early larval stages are unable 

 to establish themselves under such conditions. To establish itself in 

 fresh water permanently an animal must either be fixed, or else 

 be strong enough to withstand and make headway against the cur- 

 rents of the streams or rivers it inhabits, for otherwise it will in the 

 long run be swept out to sea, and this condition applies to larval 



