132 Dynamic Theory. 



will be found in the action of water with respect to its saltness or fresh- 

 ness. There are many animals that inhabit salt water only, many that 

 live in fresh water only, while still others are so constituted that they 

 can swim from one into the other with impunity. There are many that 

 inhabit the salt water and would die if transferred to fresh water, that 

 nevertheless have relatives not easily distinguishable from themselves, 

 that live in fresh water and would die in salt water. 



The amount of salt in sea-water is from 3^- to 4 per cent. Its effect 

 on animals is due probably to their different facilities of endosmose, or 

 the passage of fluid through the skin. A Crocodile is not affected by a 

 change as its tissues are protected from the absorption of salt by its im- 

 pervious hide. The Medusa, which is a very soft gelatinous animal, 

 whose tissues are thoroughly impregnated by and differentiated to salt, 

 will die almost instantaneously in fresh water. Experiments with 

 frogs, in which their head and mouth were kept out of the water and 

 their skin only allowed to come in contact with it, showed that they 

 would absorb, by endosmose, enough salt to kill in two and a half hours 

 in a medium holding 5 per cent, of salt; in three hours in water holding 

 3 per cent. ; in seven hours in 2 per cent. , and in twenty-four hours in 

 1 J per cent. , while they could endure with impunity water holding only 

 one per cent. From this it is evident that while any amount of salt is 

 injurious to a frog, and 2 per cent, would kill him, yet the injury of 

 one per cent, would not be vital, and consequently he could become 

 habituated to it, and when he did, that amount of salt' in the water 

 would suit him better than none, and two per cent, would suit as well 

 as none. If he be allowed time, therefore, to become habituated to 

 each step before being compelled to take another, he could, no doubt, 

 be made to flourish in sea-water. 



The injurious effects of a transfer from fresh to sea water, or vice 

 versa, are of various kinds. Some animals are killed, some are checked 

 in the deposit of their eggs, and some are checked in their growth. 

 There are numerous cases going to show that animals have transferred 

 themselves by a gradual migration from salt to fresh water. Since 

 1860 (Semper states) a certain Polyp, the Cordylophora lacustns, 

 which formerly was found only in brackish water near the mouths of 

 rivers, has migrated up the Seine as far as Paris, up the Elbe to Ham- 

 burg, and also into English and Belgian streams. At Hamburg it 

 made itself troublesome by infesting the city water pipes. 



Several fresh water animals, as Bivalve Shell-fish, the Water Louse, 

 &c. , have artificially been gradually adapted to salt water till they were 

 made to live and breed in the sea. Of course it must be held that the 

 intimate molecular structure of an animal that has undergone such a 

 change must have also been altered, even when not discoverable, but 



