30 THE VARIATION OF ANIMALS IN NATURE 



found that the amount of non-disjunction of the X-chromo- 

 some in Drosophila was also materially increased. Many papers 

 have been published on this subject during the last few years, 

 but these seem to be the essential facts. 



Huxley (1926) and Haldane (in Robson, 1928) at one 

 time suggested that naturally occurring radiations might 

 cause the apparently spontaneous mutations. But Muller and 

 Mott Smith (1930) have shown that this is highly improbable. 



C. Effect of Radium. — Hanson and Heys (1928) obtained lethal 

 mutations in Drosophila by exposing the males to the whole 

 radiation of radium or to the y-rays only. Similar results have 

 been obtained in plants. On the whole it appears much more 

 difficult to obtain positive results with radium than with X-rays. 



D. Experiments with Salts of Lead and Manganese. — Harrison 

 and Garrett (1926) and Harrison (1928a) claimed to have 

 produced melanic mutations in certain Lepidoptera by 

 feeding the larvae on food-plants which had absorbed these 

 metallic salts. Plunkett (1927) criticised the 1926 results 

 chiefly on the score of the low number of individuals 

 involved in the experiments. Recently, Hughes (1932) and 

 Thomsen and Lemche (1933) have repeated the experiments 

 on a very large scale without producing any melanics. It 

 appears probable either that melanic mutations occurred as a 

 very rare coincidence in the stock that Harrison was using or, 

 as suggested by Haldane (in Hughes, I.e.), that the original 

 parent was heterozygous and the recessive melanic factor is 

 linked with a lethal. (Cf also Harrison, Proc. Roy. Soc, 

 London, 117 B, 1935.) 



We see therefore that in a few cases the mutation-rate has 

 been directly affected by external agencies. It must not be 

 forgotten, however, that some of the agencies used (e.g. X-rays) 

 are not likely to be influential in nature. In the same way we 

 should disregard the experimental induction of hereditary 

 defect by such toxic agencies as alcohol (Stockard and Papa- 

 nicolaou, 1916) and lead acetate (Cole and Bachuber, 1914), 

 which really amount to a direct poisoning of the reproductive 

 organs. 



5. The Inheritance of Induced Modifications 



(a) General Considerations. — This subject has been dis- 

 cussed almost ad nauseam and there are numerous critical 



