134 Dynamic Theory. 



iting the gill cavities of marine fish. So the Palemon, in migrating 

 from salt to fresh water, took along his faithful parasite and both were 

 modified to suit their new surroundings. Cowries, a marine univalve 

 mollusk, have migrated up the Niger as far as Timbuctoo. In Min- 

 dinao, one of the Phillipine Islands, Semper found oysters in water 

 which at low tide is quite fresh, and never very salty. Likewise the 

 American Manatus or Sea-cow, a marine animal, has gone up some of 

 the South American rivers, and a species of the Dolphin is found 600 

 miles up the Irawady river, but considerably modified from its relatives 

 in the Indian Ocean. According to Cuvier there is another Dolphin, 

 the Soosoo, living in the Ganges. There are numerous other cases of 

 migrations with such modifications as their new surroundings have im- 

 posed of snakes, worms, fish, and mollusks. 



Another curious fact, developed also by Semper, is that the volume of 

 water in which animals grow has an effect, within certain limits, on 

 their size. The pond snail Lymnea stagnates was experimented upon. 

 As fast as the animals were hatched from a certain mass of ova, they 

 were separated into two lots, one of which was divided equally and 

 placed in unequal bodies of water, and the other divided unequally and 

 placed in equal bodies of water. After a period of sixty-five days it 

 was ascertained that those animals that had an average of 100 cubic 

 centimetres of water, had attained a length of six millimetres, those 

 having 250 parts of water were nine mm., those having 600 were 12 

 mm. and those having 2,000 were 18 mm. By working out these 

 lengths into cubical bulks, I find that the relative sizes of the animals 

 were as 1, 3-J-, 8 and 27. These numbers divided into the quantity of 

 water each had, shows that while 100 cu. centimetres gave its bulk to 

 the first one, 74 or 75 cu. centimetres was sufficient to give the same bulk 

 to the larger ones. In 5, 000 cu. centimetres the snail would, in about a 

 year, attain its greatest growth of 24 mm., or 95-100 of an inch. Any 

 more water would make no difference. As the snail breathes air, and 

 abundant food was supplied to all, the result shows that the water must 

 supply in proportion to its bulk between the limits of 100 and 5,000 cu. 

 centimetres, something else besides mere wetness and a potential some- 

 thing, too. A parallel effect to the above is seen in nature in the fact 

 that Salmon, Trout, and other kinds of fish, are always larger in larger 

 streams, and it is not improbable that the mere volume of water may 

 supply the cause, as in the case of the snail. 



When such causes as the saltness or volume of water can effect the 

 anatomy of animals, many changes in the natural history of the past 

 are accounted for. A change in the size of a pond or lake, a diminu- 

 tion in the rainfall, by which a lake may cease to have an outlet as in 

 the case of the Utah Salt Lake, and the Devil's Lake in Dakota a cir- 



