284 CAUSES OF VARIATION 



long as each of the others, while in Branchipus it is divided. It 

 is extremely significant that this division should be produced in 

 Artemia by culture in comparatively fresh water, but the fact is 

 no warrant for the assertion that one genus can be converted 

 into another by altering the environment. It rather casts doubt 

 upon the wisdom of a classification which establishes generic dis- 

 tinctions upon differences so slight and so easily brought about. 



The same experimenter studied species of the genus Daphnia, 

 and found " in their case also considerable structural and physi- 

 ological changes, the fresh- and salt-water forms differing, in his 

 opinion, by characters usually held to be specific." 1 



Bateson studied the common cockle, a mollusk, everywhere 

 present in the Aral Sea and its outlying waters of different 

 degrees of salinity. One of the lakes (Shumish Kul) on its 

 western shore exhibited no less than seven distinct terraces, 

 held to represent successive stages of the water levels during 

 its long period of drying up, with corresponding increase in 

 salinity. The most noticeable differences in the shells taken 

 from these successive terraces, and presumably due to increas- 

 ing salinity, are outlined as follows : 2 



1. A diminution in the thickness of the shells, first apparent 

 in the third terrace. Iri the seventh terrace this change was so 

 marked that the shells were almost horny, and their weight was 

 not a third of that of the shells from the first two terraces. 



2. Diminution of the size of the beak [with the lowering of 

 the level]. 



3. High coloration. [The author does not state which way 

 the changes ran, whether up or down the terraces, but he re- 

 marks that all the shells of a given terrace were " very nearly 

 alike in texture, thickness, and degree of coloration." ] 



4. Grooves between the ribs appearing on the inside of the 

 shell as ridges with rectangular faces. 



5. A great diminution in absolute size of the shells on the 

 lowest terrace. 



6. Alteration in proportion of length to breadth, ranging 

 from i to 0.80 in the shells of the first terrace to i to 0.725 in 

 those of the seventh and i to 0.66 on the shores of neighboring 



1 Vernon, Variation in Animals and Plants, p. 275. 2 Ibid. pp. 275, 276. 



