LEE BARKER WALTON 139 



material units or enzyme, reactions. The prepared poten- 

 tialities exist in either case. 



As examples of extreme types of characters which may 

 arise from the combinations of existing genes and which 

 might have been considered "mutations" at an earlier 

 period when the facts as to their origin were not fully 

 known, one need only mention the "blue" of the Andalu- 

 sian Fowl exhibited by the hybrid between the black and 

 white parental stock, or the "pink" presented by the 

 cross between the red and white "four-o-clocks" of Cor- 

 rens. A type of characters more in line with mutations 

 which have been described and to which there is every 

 reason for believing that many of them may be referred, 

 rests upon multiple gene effects combined with sterility, 

 in accordance with evidence presented by Davis, and oth- 

 ers. Of decided interest in this connection is a recent 

 paper by Muller (1917) calling attention to "An CEno- 

 fbera-l'ike case in Drosophila" where a result quite com- 

 parable to certain mutations of CEnothera is explained 

 through the action of balanced lethal genes. There are 

 other varying degrees of combinations from which "mu- 

 tant" characters may arise and which depend on the 

 behavior of the genetic material in connection with reces- 

 sives, modifiers, lethals, crossovers, non-disjunction, etc. 



There is really nothing extraordinary in the appear- 

 ance and disappearance of the characters thus formed, 

 beyond their interpretation, and this has furnished false 

 premises for many erroneous conclusions, chief of which, 

 in the opinion of the writer, is the mutation theory as 

 outlined by DeVries in so far as it may account for 

 progressive evolution. 



Inasmuch as it seems probable that the results obtained 

 by Castle are to be explained upon the same basis as those 

 of DeVries, it will be well to consider them in this con- 

 nection. Here it is assumed that a continuously variable 

 heritable gene is involved, and that progressive results are 

 obtained through the selection of the "unit characters" 

 produced by such a gene. Castle, however, stands almost 

 alone in vigorous support of such a variation, while op- 

 posed to him are the Hagedoorns, Morgan, Pearl, Punnett, 



