ABSORPTION OF SALT. 151 



of different animals under the effects of the same degree of con- 

 centration in the salt solution is by no means identical ; the 

 maximum of strength which is perfectly innocuous to the 

 frog is about one per cent., while the stickleback can bear from 

 two to two and a half per cent. ; migratory fish, as the shad, 

 salmon, eel, &c., have still greater powers of resistance, as they 

 can bear as much as from three and a half to four per cent, of 

 salt in the water. 59 



It results from this, that the difference in the osmotic action 

 of the skin in different animals, and the various degrees of re- 

 sistance to the amount of salt absorbed into their tissues, con- 

 nected with such a difference, do, in a certain sense, cause and 

 maintain the distinction which prevails between the fauna of the 

 ocean on the one hand, and that of rivers and fresh-water lakes on 

 the other. We may assume that the absorption of salt is most 

 rapid in animals with a soft skin ; we are not surprised when 

 we find that the soft gelatinous Medusa is almost instantane- 

 ously killed on coming into contact with fresh water, while 

 crocodiles with their strong and horny scaly covering, through 

 which salt, as it would seem, cannot penetrate, can live equally 

 well in the sea and in fresh water. Between these two extremes 

 the gradations are infinite. Every variation in the amount of 

 salt in fresh or salt water must therefore influence the animals 

 living in it in a different manner ; some will be killed, others 

 checked in depositing their eggs or hindered in their growth, 

 while others will bear the change without any injury. It would 

 be a very interesting problem to determine by exact experi- 

 ments a curve showing the resistance of different species to the 

 absorption of salt by the osmotic -action of the skin. 



Unfortunately hardly any such experiments are on record ; 

 but the few that are before xis offer so much that is of interest, 

 ven under the scarcely exhaustive treatment they have met 

 ,with, that we must here go into them somewhat more closely. 



In the first place an experiment must be mentioned which 



' Nature herself has made on a certain Polyp. It is, so far as I 



know, the only example of an animal that can be proved to 



have oiiginally lived in the sea, or in brackish water, and 



which, within our own time, has gradually accustomed itself to 



