96 THE CASE OF PAPILIO POLYTES [ch. 



total is now reduced to one-quarter. After 18 more 

 generations the proportions 4:4:1 are reached, the 

 recessives being only one-ninth of the total; after 40 

 further generations of the process they become reduced 

 to one-fortieth. In other words a selective advantage 

 of 10% operating against the recessives will reduce 

 their numbers in 70 generations from nearly one-half 

 of the population to less than one-fortieth. 



With a less stringent selective rate the number of 

 generations elapsing before this result is brought about 

 will be larger. If, for example, the selective rate is 

 diminished from 10 % to 1 % the number of genera- 

 tions necessary for bringing about the same change is 

 nearly 700 instead of 70 — roughly ten times as great. 

 Even so, and one can hardly speak of a 1 % selective 

 rate as a stringent one, it is remarkable in how brief 

 a space of time a form which is discriminated against, 

 even lightly, is bound to disappear. Evolution, in so 

 far as it consists of the supplanting of one form by 

 another, may be a very much more rapid process than 

 has hitherto been suspected, for natural selection, if 

 appreciable, must be held to operate with extra- 

 ordinary swiftness where it is given established varia- 

 tions with which to work. 



We may now consider the bearing of these theo- 

 retical deductions upon the case of Papilio polytes in 

 Ceylon. Here is a case of a population Hving and 

 breeding together under the same conditions, a popu- 

 lation in which there are three classes depending upon 

 the presence or absence of two factors, X and Y, 



