MENDELISM AND SELECTION 



forming a large part of the food supply of 

 fresh-water fishes. It multiplies chiefly by the 

 production of unfertilized eggs, — those which 

 undergo no reduction and which develop with- 

 out fertilization into an individual like the 

 parent. The germinal composition, therefore, 

 of all descendants produced in this way by the 

 same mother should be identical, unless germi- 

 nal composition can be modified in other ways 

 than by reduction and recombination of unit- 

 characters. Now the German zoologist, Wol- 

 tereck, has shown that, among the offspring 

 developed from the unfertilized eggs of the 

 same mother Daphnia, variations do occur 

 which are heritable, so that if one selects ex- 

 treme variants he obtains a modified race. 

 Systematic zoologists recognize as a generic 

 distinction between Daphnia and Hyalodaphnia 

 absence from the latter of the rudimentary eye 

 found in Daphnia. Woltereck observed that 

 in a pure line of Hyalodaphnia the rudi- 

 mentary eye, usually wanting, may occur in 

 indi^ddual cases. He found further that it 

 occurred in varying degrees of development, 

 which ranged all the way from a group of 

 9 117 



