222 THE VARIATION OF ANIMALS IN NATURE 



strong theoretical case against the survival of non-advantageous 

 gene-mutations. But at the same time, by stressing the rarity 

 of mutation of any sort, Fisher introduces a serious doubt as 

 to the fate of mutations, even if Natural Selection is operative. 

 For if gene-mutations are infrequent and often injurious, as 

 Wright (193 1, p. 143) points out, what are the chances that 

 a viable and useful mutation of this order of rarity will always 

 occur in those individuals which are allowed to survive by a 

 death-rate which is probably always at least 50 per cent, 

 random in its incidence ? 



It is most unfortunate that all our exact knowledge of the 

 rate, nature and hereditary behaviour of gene-mutations is 

 founded on studies in which the mutations are mainly disad- 

 vantageous and even lethal (eye- and wing-mutations of 

 Drosophila, eye-mutations of Gammarus). Exactly how many 

 of the mutations in Drosophila are of this nature it is not easy 

 to say. We have taken the list of 389 mutations given by 

 Morgan, Bridges and Sturtevant (1925, p. 218 and foil.) and 

 analysed them as far as possible, with the following result : 



389 



These figures are only approximate, as it is not possible 

 to be certain which should be regarded as defective ; also we 

 are uncertain whether the reduction of pigment in the eyes 

 (e.g. ' pink ') is to be treated as defects : we have accord- 

 ingly grouped them in a separate category. In ' Uncertain 

 or normal ' are included a fairly large number of types (e.g. 

 ' ebony 3,' ' dusky ') which are plainly normal from the point 

 of view of their viability. Speaking generally, it may be said 

 that nearly 60 per cent, of the mutants are certainly defective, 

 and a certain small percentage is normal. Sexton, Clark and 

 Spooner (1930, p. 189) say of the Gammarus mutants that they 

 ' would have but little chance, in normal conditions of nature, 

 of survival through the early critical period. Each new 



