FRESH-WATER ANIMALS 83 



fresh water. This result is rendered still more 

 remarkable by a check experiment performed at the 

 same time, in which an equal number of individuals 

 of the same species as before were taken, and kept 

 in salt-water tanks, when it was found that 34 per 

 cent, died ; so that the difference in mortality 

 between those living in their natural medium, and 

 those constrained to change from marine to fresh- 

 water habitat, was but 3 per cent. 



A still more remarkable case is that of the two 

 Entomostracan genera, Artemia and Branchipus, 

 of which the former is marine, the latter fresh- 

 water. Of Artemia two species are known, the 

 differences between which are very well marked. 

 Artemia sahna lives in water containing from 4 to 

 6 per cent, of salt, and is found in the brine pans 

 of salt works, and in other similar places. The 

 other species, Artemia Milhausenii, requires water 

 containing not less than 25 per cent, of salt. By 

 direct experiment it has been shown that the 

 differences between the two species depend simply 

 on the percentage of salt in the water in which 

 they live ; and that by gradually adding salt, or 

 adding water, Artemia salina may be converted into 

 Artemia Milhausenii^ or vice versa. This experi- 

 ment has been performed not merely in the labora- 

 tory, but also by Nature herself.* 



The reverse experiment also succeeded, and by 



gradually adding fresh water, and so reducing the 



strength of the solution, Artemia Milhausenii has 



been converted into Artemia salina. The ex- 



* Vide Lecture on " Environment," p. 31. ED. 



