THE STRUCTURE AND HABITS OF ANIMALS 33 



the water was reduced to about 6 per cent, all the 

 Artemice were converted to typical specimens of 

 Artemia salina. Having thus satisfied himself of 

 the causal relations between the two species of 

 Artemia he determined to push his experiments 

 still further. He therefore went on adding fresh 

 water after he had obtained Artemia salma, and by 

 this means converted Artemia salma into a well- 

 known fresh-water form that had previously been 

 considered not only a separate species but even a 

 distinct genus Branchipus. 



From these very striking experiments it would 

 appear that the relation between the structure of 

 the animal and the degree of saltness of the water 

 it inhabits is a perfectly definite one, so much so, 

 that if the percentage of salt be told us, we are 

 able to foretell with perfect confidence and certainty, 

 not only what will be the general character of the 

 animal inhabiting it, but such minutiae as the size 

 and shape of its tail-lobes, and even the number 

 of hairs borne by these lobes. 



Here then is a very striking proof of the direct 

 action of environment on structure ; a phenomenon 

 as definite as a chemical reaction. Nor must the 

 case be made light of or explained away by saying 

 that it simply shows that zoologists were at fault 

 in describing these forms as distinct genera and 

 species, and that the experiment showed them to 

 be mere varieties ; for it must be noticed that the 

 characters by which the three forms differ are 

 characters of the very kind that are invariably 

 employed by zoologists to distinguish the genera 



