BLUE-GREEN CATERPILLARS 399 



That they are not is urged by Harrison ('20), who ascribes 

 the increasing prevalence of melanism of moths near English 

 industrial centers to ''changed metabolism favoring resistance 

 to, or actually induced by, the use of food contaminated with 

 metallic salts and other compounds." Melanism in moths of 

 the genus Oporabia, however, upon which his paper treating of 

 the possible inheritance of acquired melanism is based, unfortu- 

 nately, is non-mendelian in its inheritance. A persistent blend 

 occurs. In other words, melanism has not affected the chromo- 

 somes, though it apparently has affected the hemolymph. The 

 next step would be to show that the hemolymph may in turn 

 induce mutation in the chromosomes. 



We may well be skeptical of 'proofs' of the inheritance of 

 acquired mutations that are not checked up by control experi- 

 ments running through more than one generation, for the origin 

 of the blue caterpillars shows that to reveal the nature of the 

 genes and bring out latent recessive mutations that might be 

 ascribed to environmental factors, two or more generations of 

 inbreeding may be necessary. 



Suppose, for example, that a culture under the stress of 

 environmental conditions should prove to contain a new 

 recessive mutant and that a control culture of the same 

 stock bred under normal conditions apparently does not. The 

 usual presumption has been that the new variation has been 

 produced by the environmental factors. The presumption, on 

 the contrary, should be that the experimenter is working with 

 heterozygous material, that a pair of heterozygotes have in fact 

 produced his mutants, and that his control culture, being made 

 up of heterozygotes mated, with homozygous dominants and 

 apparently composed exclusively of normal individuals, should 

 be further inbred to show its real genetic constitution. No such 

 carefully controlled experiments giving positive reliable results 

 have yet come to the, writer's attention.* 



* At the time these words were written Dr. M. F. Guyer's and E. A. Smith's 

 very important paper on the inheritance cf an acquired eye defect in rabbits 

 had not been read (vol. 31, no. 2 of this Journal). It should have been in- 

 cluded in this discussion. 



