NATURAL SELECTION 185 



(2) It is implicit in Darwin's presentation of the theory 

 that single variants will be ' swamped ' by intercrossing, and 

 that the swamping of new variants is only avoided if they 

 happen to be serviceable and if there are enough of them to 

 reach maturity and breed together. Though even on the 

 particulate theory of inheritance a character depending on 

 several genes would undoubtedly run the risk of being 

 ' swamped ' by intercrossing, much of the risk envisaged by 

 Darwin is seen, in the light of more exact knowledge, to be 

 non-existent. There is, however, at the present time an 

 increasing emphasis laid on the effects of wholesale elimination, 

 and in particular on the slight chance that a single mutant 

 will have of surviving unless it has some selective advantage. 

 A tendency has thus arisen to stress the importance of selection 

 in serving to multiply or ' spread ' variants, as opposed to its 

 value as a means of preventing the ' swamping ' process. 

 This valuation of selection has gained ground correlatively 

 with the estimation of mutation-rates based on those of Droso- 

 phila. Whether this estimation has any general application is 

 discussed on p. 220, but in all probability the revised valuation 

 of the selective process is a just one and failure to recognise 

 its cogency vitiates such criticism of Natural Selection as that 

 of Hogben (1931, p. 180), who, in contrasting the Darwinian 

 conception of selection with that of the modern experi- 

 mentalist, suggests that a given mutant may spread and attain 

 a representation in a population, without discussing how it 

 survives the incidence of the normal death-rate. 



In addition to the important developments just mentioned, 

 a number of inquiries all relevant to the theory have been 

 developed since Darwin's time, the results of which have 

 enlarged the field of inquiry. It is needless to mention them 

 in detail, but it will be apparent that the advances in the 

 experimental study of heredity, in animal ecology and in the 

 intensive study of variation in natural populations — to mention 

 the more outstanding developments — have profoundly altered 

 our views on the efficacy of selection. It is perhaps per- 

 tinent to add that study of the living organism as a whole, 

 its development, reactions and organisation, has also modified 

 our estimate of selection as an important agency in evolution. 



It would take us very far from our course of inquiry to 

 describe the changes in the attitude of students of biology and 



