30 THE INFLUENCE OF ENVIRONMENT ON 



more rigid proof being necessary before we can 

 accept it. I wish first to ask your attention to one 

 or two cases in which this proof appears to be 

 present, and in which we are able to establish not 

 only a general but a direct causal relationship 

 between environment and structure ; as unquestion- 

 able indeed as that which holds in the case of oxide 

 of mercury. 



Now we know of at least one well authenticated 

 case in which the structure of an animal responds 

 in as precise a manner to simple and definite 

 changes in one of the elements of the environment 

 as certain chemical reactions do to alterations of 

 temperature. 



Artemia salina is a small aquatic crustacean 

 about half an inch in length, and somewhat like a 

 small shrimp in appearance ; its distribution is 

 peculiar, inasmuch as it is only found in water 

 containing from 4 to 8 per cent, of salt, a much 

 higher proportion than is found in the sea. It 

 consequently can only occur in salt lakes and in 

 such places as the salt pans of Lymington and 

 other similar places where the sea-water is allowed 

 to gradually concentrate by evaporation. Another 

 form, Artemia Milhausenii, is known, whose habits 

 are still more peculiar, for it is only found in water 

 containing at least 25 per cent, of salt ; its dis- 

 tribution is consequently a very limited one, since 

 it can only occur in certain inland lakes in which, 

 owing to special circumstances, the water attains 

 this very high percentage of saltness. 



Artemia Milhausenii differs from Artemia salina 



