A PHYSIOLOGICAL CHARACTER. 155 



not really genetic and some of which are clearly cytoplasmic are well 

 known. If Sudan III is fed to fowls some of the dye is deposited in 

 the egg-yolk (Riddle, 1908) and reappears in certain of the tissues 

 of the chick (Gage and Gage, 1908). This is merely the carrying 

 over of a cytoplasmic inclusion. Other cytoplasmic inclusions, the 

 plastids of plants, parasitic or symbiotic bacteria and protozoa, are 

 carried over in the cytoplasm of the egg; but these are mere inclusions 

 and this is not true inheritance. Starving occasionally causes the 

 production of young of small size. The effect would seem probably 

 due to reduced quantity of food material (or to deleterious sub- 

 stances) in the egg cytoplasm. Later generations are not affected. 

 The effects of immunization are transmitted by the mother and not 

 by the father. The effects are not permanently transmitted and 

 are presumably cytoplasmic. Agar (1913) was able to produce 

 young Cladocera with carapace gaping open by keeping the mothers 

 in a particular food medium. This peculiarity was transmitted for 

 one generation by affected stock when reared in the usual culture 

 medium, but the abnormality rapidly decreased and ceased to appear 

 after one generation. This transmission may readily have been 

 cytoplasmic, but it is clearly not a case of real inheritance. The 

 substances carried over in the egg cytoplasm (if there were such) 

 were rapidly dissipated and lost. 



These are typical cases of cytoplasmic transmission, and one 

 may conclude with Conklin (1920) that "in the present state of 

 our knowledge there is not sufficient evidence to conclude that 

 modifications of the cytoplasm of germ-cells are ever really inherited 

 or that they are ever the initial steps in evolution." 



To assume that the changes in phototropism in the present case 

 were not really genetic would seem to place them in the class of 

 cytoplasmic transmissions and to assume a permanent cytoplasmic 

 or extra-genetic transmission which in our present state of knowledge 

 would seem unwarranted. 



Aside from its theoretical difficulty, there does not seem to be 

 any evidence favoring the assumption of general and purely physio- 

 logical changes as accounting for the divergence in reactiveness of 

 the two strains of Line 757, since these two strains are indistinguish- 

 able, except in their reactiveness to light. 



However, it is of course possible to assume that a non-chromo- 

 somal inheritance occurs and to apply this interpretation to the 

 present results, though the writer can see no reason for favoring such 

 an interpretation. If, however, this should prove to be the correct 

 interpretation a practically new phase of inheritance is opened up, 

 and in any case the importance of the present results as bearing 

 upon selection within the pure line is not lessened. 



If, on the other hand, physiological changes having a genetic 

 basis be assumed to explain the present results of selection, the nature 

 of the factorial changes involved is still the pertinent question. 



