Control of Heredity: Eugenics 257 



als than in others, as is the case for example in "blending" inheri- 

 tance (pp- 108-111), and selection merely sorted out some individ- 

 uals with a larger or smaller number of factors, and consequently 

 with a larger or smaller development of the character in question, 

 but it did not in the least modify any individual factor. This 

 view finds support in the work of McDowell, of Zeleny and 

 Mattoon and of Morgan and his associates on the effects of se- 

 lection on certain characters of Drosophila. In a recent paper 

 Castle (1918) himself has abandoned his former position and has 

 adopted this explanation of his results, and thus this controversy 

 comes to an end. 



The crux of this whole controversy lay in the question as to 

 whether inheritance factors fluctuate or not; Castle maintained 

 that they do, Johannsen that they do not. If inheritance factors 

 fluctuate they may be changed gradually in one direction or an- 

 other by selection; if they do not fluctuate, but mutate only, 

 changing only rarely and not in all directions, selection can act 

 only by sorting out mutations, but can do nothing to produce 

 them. In general fluctuations are due to environment and are 

 not inherited, therefore they concern development rather than 

 heredity, developed characters rather than inheritance factors. 

 There is much evidence that inheritance factors are relatively 

 stable and that when they change, they undergo a complete change 

 or mutation comparable to what occurs in a chemical reaction. As 

 we have seen it is possible to explain almost all phenomena of 

 inheritance on this basis even including Castle's results. 



In another paper Jennings has shown that continued selection 

 in one direction does, apparently, shift the mode of certain char- 

 acters in a pure line of asexually reproducing Difflugia, and Mid- 

 dleton has found the same to be true of Stylonychia. These re- 

 sults differ totally from Jennings' earlier work on Paramecium, 

 which has been repeated and confirmed by Ackert. It is a 

 hard thing to believe that different organisms differ irrecon- 

 cilably in so fundamental a matter and it seems much more prob- 

 able that these discrepancies are due to an incomplete analysis 



