THE ORIGIN OF VARIATION 29 



In so far as the vital activities are physico-chemically 

 determined it is impossible to imagine that mutations can be 

 truly spontaneous. Doubtless all that this term has meant in 

 the writings of those who have thought out its implications, is 

 that the agencies responsible for gene-constancy or gene- 

 mutation are so numerous that it is difficult or impossible to 

 speak of any one as the cause. A theoretical discussion has 

 been given by Schmalfuss and Werner (1926) with reference to 

 the hypotheses that the genes are enzymes (Goldschmidt) or 

 autocatalytic substances (Hagedoorn), and the conclusion is 

 favoured by them that mutations are produced by the action 

 of external factors on specific catalysts. 



More recently good experimental evidence has been put 

 forward to show that high temperature, (3-rays (of X-rays) or 

 y-rays (of radium) have a marked effect on the mutation-rate. 

 We shall mention these experiments briefly in the order 

 indicated. 



A. Effect of High Temperature. — Goldschmidt (1929), Jollos 

 (1930) and Rokizky (1930) have shown that the mutation- 

 rate of Drosophila is very much raised when the late larvae 

 are subjected to a temperature so high (35°-37° G.) as to kill 

 most of them. The attempts of other workers (e.g. Ferry and 

 others, 1930 ; cf. also Muller, 1932) have been partially or 

 completely unsuccessful. Apparently the mutations produced 

 are all types that have already been recognised. Jollos obtained 

 evidence that the mutations were largely in one direction and 

 the effect cumulative. This is very suggestive of the actual 

 causation of mutation, but more evidence is required on this 

 point. The results should be compared with the Dauer- 

 modifikation-experiments (p. 35). 



B. Effect of X- Rays. —Muller (1928) showed that the 

 mutation-rate of Drosophila was raised about 150 times by 

 subjection to X-rays. Hanson, Heys and Stanton (1931) 

 have recently shown that the increase in mutation-rate, as 

 measured by the number of sex-linked lethals, is directly 

 proportional to the X-ray dosage. Similar results have been 

 obtained by Little and Bagg (1924) and Dobrovolskaia (1929) 

 with mice. Most of the mutations are not unknown in normal 

 cultures, though some of those in mice are apparently novel. 

 The effect would seem to be one of general disturbance, since 

 Mavor (see Morgan, Bridges and Sturtevant, 1925, p. 116) 



