158 THE INFLUENCE OF INANIMATE SUBEOUNDINGS. 



water — wMch in one case determines- the character of Artemia 

 Milhausenii and in anotlier that of Artemia salina. But 

 Schmankewitsch was so fortunate as to be able to carry the 

 experiment still further. He kept Artemia salina in salt water, 

 which he constantly diluted by adding fresh water, till at last 

 it was perfectly fi-esh; the Crustaceans had meanwhile gone 

 through several generations, and had gradually so completely 

 changed their character that finally they had acquired those of 

 the genus Branchipus. 



These discoveries are certainly of. the greatest interest ; for 

 they afford a proof we can scarcely doubt, that a change in the 

 amount of salt contained in the water can produce a regularly 

 recurring and very conspicuous modification of the specific and 

 even of the generic characters of certain animals. Darwin's 

 opponents will probably say that in this case those zoologists 

 were in error who attributed to the difierences batween Artemia 

 Milhausenii, A. salina, and Branchipus stagnalis, a specific and 

 even a generic value, and that all these forms must now be 

 regarded merely as varieties of one single species, since proof has 

 been given that they pass into each other. It is no part of my 

 purpose here to oppose such a view of the case ; it will suffice to 

 observe, on the other hand, that, logically speaking, writers 

 on Crustaceans must then cease to have any justification for 

 separating or describing species at all, since those difierences 

 between Branchipus and ArtemAa which, according to this view, 

 have neither specific nor generic value are precisely those of 

 which they constantly avail themselves for distinguishing the 

 species and genera when describing other Crustaceans. 



Thus evidence has been given in this chapter that changes in 

 the degree of saltness of the water exert not merely a selective 

 influence on the animals exposed to them, but also sometimes 

 efiect a remarkable modification of them ; and it is probable 

 that other soluble elements in the water besides simply sodic 

 chloride may be able to exert a similar influence. We are only 

 at the beginning of our knowledge on this point. A careful 

 repetition of the experiments here briefly described, with as 

 great a variety as possible of animals and with as much 

 thoroughness as SchmankesFitsch exercised, would, beyond 



