13i EVOLUTION AND GENETICS 



discovery of the occurrence of such modifying ge- 

 netic factors has gone a long way in making clear 

 some of the effects of selection — effects that have at 

 times led some of the neo-Darwinians to assume that 

 the selection process coidd hring about a change that 

 causes the organisms to transcend its original type. 

 Castle stated in 1916: "JNIany students of genetics 

 at present regard unit characters as unchangeable. 

 . . . For several years I have been investigating 

 this question, and the general conclusion at which I 

 have arrived is this, that unit characters are modifi- 

 able as well as recombinable. JNIany Mendelians 

 think otherwise but this is, I believe, because they 

 have not studied the question closely enough. The 

 fact is unmistakable that imit characters are sub- 

 ject to quantitative variation." That Castle was not 

 carelessly playing fast and loose with the term factor 

 (gene) and character is shown by the whole con- 

 text of the entire chapter in which tliis sentence 

 occurs. It is intended to be understood to mean that 

 unit characters may not only be altered by the re- 

 combination of modifying characters, for, Castle 

 has always recognized this possibility, but also 

 that the gene (factor) varies quantitatively and that 

 selection not only produces its results by selecting 

 larger or smaller genes but in doing so it brings 

 about progressive and further advances in the direc- 

 tion of selection. This interpretation is attributed by 

 Castle to Darwin himself, as another quotation from 



