vm] THE CASE OF PAPILIO POLYTES 103 



the conditions appear to be more favourable to exuber- 

 ance of variation and where generations succeed one 

 another in more rapid succession. At present, however, 

 we are without data. A form reported by an old col- 

 lector as common is now rare ; a variety once regarded 

 as a great prize is now easily to be found. Such to-day 

 is the sort of information available. For the solution 

 of our problem it is, of course, useless. The develop- 

 ment of Mendelian studies has given us a method, 

 rough perhaps but the best yet found, of testing for 

 the presence, and of measuring the intensity, of natural 

 selection. Much could be learned if some common 

 form were chosen for investigation in which, as in 

 P. polytes, there are both mimetic and non-mimetic 

 forms. Large numbers should be caught at stated 

 intervals, large enough to give trustworthy data as to 

 the proportions of the different forms, mimetic or non- 

 mimetic, that occurred in the population. Such a 

 census of a polymorphic species, if done thoroughly, 

 and done over a series of years at regular intervals, 

 might be expected to give us the necessary data for 

 deciding whether the relative proportion of the different 

 forms was changing — whether there were definite 

 grounds for supposing natural selection to be at work, 

 and if so what was the rate at which it brought the 

 change about. 



