CAUSES OF GENETIC VARIATION 223 



In the results of these experiments, as described, there is one 

 feature which I regard as quite unaccountable. Tower makes no 

 comment upon it. Indeed, from the general tenour of the paper, 

 I infer, not only that he does not perceive that he is recounting 

 anything contrary to usual experience, but rather that he regards 

 the result as conforming to expectations previously formed. 

 The point in question is the genetic behaviour of the dominant 

 normals produced under the abnormal conditions. These 

 normals were the result of the breeding of parents declared to be 

 at the same time giving off many recessive gametes. Some of 

 these normals must be expected therefore to be heterozygous 

 unless some selective fertilisation occurs. Nevertheless in every 

 case they and their offspring are reported to have continually 

 bred true. I allude especially to the tables given on pp. 288, 289, 

 292, and 293. Tower does not mention any misgiving about 

 this result, and I think he regards himself as recounting phe- 

 nomena in general harmony with the ideas of mutation expressed 

 by De Vries. This they may be; but to anyone familiar with 

 analytical breeding the course of these experiments must seem so 

 surprising as to call for most careful, independent confirmation. 



In 19 io 10 Tower published an account of further experiments 

 with Leptinotarsa. The work described related to two subjects. 

 Crosses were made between three forms, undecimlineata Stal, 

 signaticollis Stal and " diver sa" named by Tower as a new 

 species. The distinctions between these three depend partly on 

 characters of the adults and partly on those of the larvae. The 

 adults of undecimlineata and diver sa have the elytra striped, but 

 the elytra of signaticollis are unstriped. The larvae of sig- 

 naticollis and of diver sa are yellow, but those of undecimlineata 

 are white. 11 Moreover, in signaticollis and diversa the black in- 

 creases in the third stage of the larvae to form transverse bands 

 which are absent in undecimlineata. The general course of the 

 experiments shows that these differences may be approximately 



™Biol. Bull., XVIII, 1910, p. 285. 



11 This description does not quite agree with the representation of the larvae 

 in PI. 17 of the book Evolution in the Genus Leptinotarsa for there the larva of 

 undecimlineata is shown as white in the second stage, but yellowish in the third 

 stage; perhaps there is an error in printing. 



