No. 591] SHOBTER ARTICLES AND DISCUSSION 179 



are unfavorable for further continuance of the organism either 

 by vegetative reproduction or even by parthenogenesis. 1 But 

 whether or not Walton's own observations are considered per- 

 tinent, the question which it leads him to consider is one of pro- 

 found evolutionary significance — does cross fertilization (as com- 

 pared with close fertilization) tend to produce greater or less 

 variability. 



In parts of his discussion "Walton fails to keep clearly in view 

 distinctions, which he nevertheless recognizes, between the re- 

 spective variabilities of F lf F 2 and mixed populations. The 

 fact has been known since the days of the early plant hybridizers, 

 and is expressed clearly in one of Focke's laws of hybridization 

 that the first generation (F x ) offspring of a hybrid cross are 

 not, as a rule, more variable than the more variable parent race. 

 In other words the generalization which Walton attacks, that 

 crossing produces variability, is not commonly, if at all, held by 

 biologists to apply to F : populations but only to the conditions 

 obtaining in subsequent generations. But Walton's own obser- 

 vations are made exclusively upon F t zygotes. Supposing his 

 two classes of zygotes to be morphological and physiological 

 equivalents of each other (which, however, may reasonably be 

 questioned) there was no ground for expecting one sort to be 

 more variable than the other, so far as existing knowledge of the 

 effects of inbreeding and cross-breeding is concerned. 



Walton cites two experimental investigations, in support of 

 his own observations on Spirogyra, to show that close fertiliza- 

 tion produces greater variability than cross fertilization, viz., 

 that of Jennings on Paramecium and that of Barrows and my- 

 self on Drosophila. But neither of these investigations deals 

 with the same sort of cases as Walton's. Jennings is compar- 

 ing the variability of conjugants with that of non-con jugants. 

 This is a case where sexual is contrasted with asexual reproduc- 

 tion and is in no way comparable with a case in which the ef- 

 fects of cross and close fertilization are compared with each 

 other. I quite agree with Walton's conclusion that the results 

 are statistically considered far from conclusive, and would add 

 that they are quite aside from the question which Walton is con- 

 sidering. Barrow's comparisons were made between single lines 

 of Drosophila inbred (brother with sister) for from 30 to 61 

 generations and a culture derived from two original pairs of 



1 8ee Coulter, 1914, "The Evolution of Sex in Plants." 



