A RECESSIVE CHARACTER AND SELECTION 171 



3. The males showed greater effects from 'erossing-in' than 

 did the females (table 3). 



4. The size and form of the vestigial wing were affected by 

 high temperature (tables 1, 2 and 3, and text figs. 1 and 2). 



5. The males were more easily affected by high temperature 

 than were the females (tables 1 and 2). 



V. DISCUSSION 



1 . Effect of selection 



It is possible to assume that selection had no effect upon the 

 size of vestigial wing in this experiment, because of the gametic 

 purity of this character. Upon this assumption the variability 

 was due to environmental factors alone and under such conditions 

 one would not expect selection to be effective. Such an hypothe- 

 sis is supported by evidence from the work of Johannsen ('03, 

 '09) with beans, Jennings ('09) with the protozoan Paramecium, 

 Tower ('06) with the potato-beetle, Leptinotarsa decemlineata, 

 Ewing ('14 a, '14 b, '16) with Aphis, and Lashley ('15, '16) 

 with Hydra. 



However, there is also evidence that in some cases unit-char- 

 acters do change under the process of selection. Castle and 

 Phillips ('14) found that the amount of pigment in the hooded 

 rat could be increased or decreased at will by selection. Cuenot 

 ('04) noted the same results in working with spotted mice. The 

 work of Jennings ('16) on Difflugia corona, the reproduction of 

 which is uniparental, showed that selection was effective in 

 changing the number and length of spines and size of body. 

 Similar results were obtained by Middleton ('15) with Stylonychia 

 and Stocking ('15) with Paramecium. All of these results may 

 be explained by assuming that the factors themselves are vari- 

 able. Castle held this view concerning the hooded character in 

 rats. The pure-linist attempts to explain Castle's results upon 

 the assumption that modifiers are present and that selection 

 separates diverse types. Concerning the work of Jennings on 

 Difflugia, Morgan ('16) says: ''If through sexual union (a proc- 

 ess that occurs in Difflugia) the germ plasm (chromatin) of these 



