DIMINISHING ENVIRONMENTAL INFLUENCE 319 



scribed conditions are never anything like three to five fold in a linear 

 dimension. 



Other classes of modification experiments on mollusks are not so 

 exact, but are perhaps worth mentioning. Vernon writes that " the 

 permanent effects of temperature on size are probably very considerable 

 among many of the mollusks," and quotes Cooke as stating that " a 

 deficiency of lime, in the composition of the soil of any particular 

 locality, produces very marked effects upon the mollusca which inhabit 

 it; they become small and very thin, occasionally almost transparent." 

 Unfortunately there is no statement that changes can be brought 

 about in one generation, though very likely this is the case. Vernon 

 says, in quoting Costa, page 314: 



On transferring young oysters from the English shores to the Mediter- 

 ranean it was found that their manner of growth at once altered, and prominent 

 diverging rays were formed, like those on the shell of the native Mediterranean 

 oyster. 



Eegeneration experiments are numerous among the mollusks and 

 indicate, as Przibram points out, 13 that these animals belong, with re- 

 gard to their relation to the question of regeneration, in the middle rank 

 between the lowest invertebrates and the highest vertebrates. He places 

 mollusks in the fourth of six classes with respect to the power of re- 

 generation, which can regenerate besides the tail, also limbs and organs 

 of sense as long as the connection with the central nervous system is 

 intact. 



Crustaceans 



There have not been many modifications experiments performed 

 upon crustaceans except such as come within the more limited field of 

 regeneration. There is evidence, however, that either different quan- 

 tities of oxygen or differences in amount of the products of metabolism 

 cause marked variation in the rate of reproduction of Daphnia magna. xi 

 There is also much evidence that among the Daphnias important 

 changes in the life cycle may be artificially induced. These creatures 

 may be made to continue parthenogenetic reproduction into the winter 

 if kept in a warm place. 



According to Irvine and Woodhead crabs can not produce their 

 shells if they are allowed to grow in sea water from which chloride 

 of calcium has been excluded, even if sulphate of lime and chloride of 

 sodium are present. Chloride of calcium is absolutely essential for 



13 " Eegeneration," Leipzig und Wein, 1909, Tafel XVI. Przibram here, in 

 a chart, shows the general decrease in regenerative power with increase in 

 phylogenetie and ontogenetic stages, but does not treat of other aspects of 

 modification. 



"Warren, Q. J. Nicrosc. Society, Vol. 43, p. 212, 1900. 



13 Morgan, "Exp. Zool.," pp. 312, 336. 



