LIFE HISTOEY STUDIES OF ANIMALS. 489 



at the surface of the ocean conquering forms have a free course; in 

 lakes and rivers they are soon checked by physical barriers. 



A large proportion of animals are armor clad and move about with 

 some difficulty when they have attained their full size. The dispersal 

 of the species is therefore in these cases effected by small and active 

 Lirvpe,. Marine animals (whether littoral or pelagic) commonly produce 

 vast numbers of locomotive larvse, which easily travel to a distance. 

 Floating is easy and swimming not very difficult. A verj^ slightly 

 built and immature larva can move about by cilia or take advantage 

 of currents, and a numerous brood may be dispersed far and wide 

 while they are mere hollow sacs, without mouth, nerves, or sense 

 organs. Afterwards they will settle down and begin to feed. In fresh 

 waters armor is as common, for all that I know, as in the sea, but 

 locomotive larvse are rare.' There is no space for effective migration. 

 Even a heavy-armored and slow-moving crustacean or pond snail can 

 cross a river or lake, and to save days or hours is unimportant. In 

 rivers, as Sollas has pointed out, free swimming larvae would be subject 

 to a special risk — that of being swept out to sea. This circumstance 

 may have been influential, but the diminished motive for migration is 

 probably more important. At least an occasional transport to a new 

 area is indispensable to most fresh- water organisms, and very unex- 

 pected modes of dispersal are sometimes employed, not regularly in 

 each generation, but at long intervals, as opportunity offers. 



Early migration by land is nearly always out of the question. Walk- 

 ing, aud still more flying, are difficult exercises, which call for muscles 

 of complex arrangement and a hard skeleton. A very small animal, 

 turned out to shift for itself on land, would in most cases ])erish with- 

 out a struggle. There might be just a chance for it if it could resist 

 superficial drying and were small enough to be blown about by the 

 wind (infusoria, rotifera, and certain minute Crustacea) or if it were 

 born in a wet pasture, like some parasitic worms. 



We can define two policies between which a species can make its 

 choice. It may produce a vast number of eggs, which will then be 

 pretty sure to be small and ill furnished with yolk. The young will 

 hatch out early, long before their development is complete, and must 

 migrate at once in search of food. They will, especially if the adult is 

 slow moving or sedentary, be furnished with simple and temporary 

 organs of locomotion, and will generally be utterly unlike the parent. 

 The majority will perish early, but one here and there will survive to 

 carry on the race. 



Or the parent may produce a few eggs at a time, stock them well 

 with yolk, and perhaps watch over them, or even hatch them within 



' Dreyssensia and Cordylophora are examples of animals which seem to have quite 

 recently become adapted to fresh-water life and have not yet lost their locomotive 

 larvae. Many instances could be quoted of marine forms which have become Huvia- 

 tile. The converse is, I believe, comparatively rare. 



