MENDELISM AND SELECTION 



tion, for in Daphnia the reproduction was ex- 

 clusively by unreduced and unfertilized eggs. 



The rudimentary eye of Daphnia is an organ 

 the development of which, so far as observed, 

 is wholly independent of environmental influ- 

 ence; but the case is different with another 

 structure of Daphnia, upon which also Wol- 

 tereck made observations, namely, a projection 

 or spine borne on the head of the animal. 

 This is not a constant structure, but is some- 

 times present, sometimes wanting altogether, in 

 the same pure line. In extreme cases it forms 

 a great angular extension of the head forward. 

 To a considerable extent its development is 

 subject to control through the temperature of 

 the surrounding water, but independently of 

 such influence the degree of its development 

 varies and is heritable. Although in general, 

 just as in the experiments of Johannsen and 

 Jennings, selection of animals with the best- 

 developed spine did not increase the degree of 

 development of the organ or the frequency of 

 its occurrence, yet in individual cases such 

 increase was observed, so that the structure 

 occurred in over 50 % of the offspring. In 



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