NATURAL SELECTION 219 



evolution rather than the origin of the peculiarities of a few 

 species. 



We believe that the study of the Drosophila mutations has 

 led to a wrong conception of adaptation, which reacts in turn on 

 the present form of the Natural Selection theory. The Fisher- 

 Haldane modification of the Natural Selection theory requires 

 that animals should be extraordinarily closely adapted to their 

 environment. Direct evidence of this is hard to obtain. Much 

 use has been made of the well-known fact that most of the 

 mutations in Drosophila are less viable than the wild type. From 

 this it is argued that even the relatively slight changes involved 

 in most of these mutations are more than the delicate adjust- 

 ments of the animal can tolerate. Thus it is assumed that the 

 material with which Natural Selection works consists of much 

 smaller mutations, not large enough to upset the general 

 adaptation of the animal, but still big enough to affect the 

 chance of survival of the mutants. Small beneficial mutants of 

 this type have not (or scarcely ever) been observed, but Fisher 

 (1930, p. 19) says : ' In addition to the defective mutations, 

 which by their conspicuousness attract attention, we may 

 reasonably suppose that other less obvious mutations are 

 occurring which, at least in certain surroundings or in certain 

 genetic combinations, might prove to be beneficial.' 



It seems to us a somewhat questionable procedure to 

 postulate the occurrence of beneficial mutations when in fact 

 we are so much more familiar with harmful ones. But the 

 argument appears to be open to a much more serious criticism. 

 Both the wild type and the mutants of Drosophila are kept in 

 exceedingly artificial conditions. The greater viability of the 

 wild type in these conditions provides no evidence as to close- 

 ness of its adaptation to natural conditions — in fact the insect 

 can evidently survive in a wide range. All we can safely say 

 is that the internal adjustments of the mutants are in some 

 way less perfect, and we may deduce, only, that the internal 

 adaptations of Drosophila are very complex and delicate (which 

 we might have suspected previously), not that Drosophila is 

 highly adapted to its external environment. We do not, of 

 course, maintain that animals are never selected for life in a 

 particular environment, but we think that in many cases it is 

 more important for an animal to be able to survive in all or 

 many environments. To accomplish this, an evolution of 



