26 PROBLEMS OF GENETICS 



meet the fact that the central silvery mark undergoes endless 

 variation. "Truly no two are alike," says Mr. Tutt, "and to 

 look down a long series of inter rogationis is something like looking 

 at a series of Chinese characters." In contrast to this we have 

 the fact that in Plusia gamma the very similar silvery mark is 

 by no means variable. 



I have taken this series of cases from the Noctuid moths, 

 but it would be as easy to illustrate the same proposition from 

 the Geometridae or the Micro-Lepidoptera. 15 I have a long 

 series of Peronea cristana,ior example, which was given tome by 

 Mr. W.H. B. Fletcher, of Bognor. All were beaten out of the same 

 hedge, and their polymorphism is such that no one unaccustomed 

 to such examples could suppose that they belonged to a single 

 species. Another common form, P. schalleriana, which lives in 

 similar circumstances, exhibits comparatively slight variability. 



It should be expressly noted that the variation of which I am 

 speaking is a genuine polymorphism. Several of the species 

 enumerated exhibit also geographical variation, possessing defi- 

 nite and often strikingly distinct races peculiar to certain 

 localities; but apart from the existence of such local differen- 

 tiation, stands out the fact upon which I would lay stress, that 

 some species are excessively variable while others are by com- 

 parison constant, in circumstances that we may fairly regard 

 as comparable. 



This fact is difficult to reconcile with the conventional view 

 that specific type is directly determined by Natural Selection 



16 The statements made above are for the most part taken from Barrett, C. G. f 

 Lepidoptera of the British Islands, and from Tutt, J. W., The British Noctuae and 

 their Varieties. The reader who is unfamilar with the amazing polymorphism 

 exhibited by some of these moths should if possible take an opportunity of looking 

 over a long series in a collection, or, if that be impossible, refer to the admirable 

 coloured plates published by Barrett. It may not be superfluous to observe that 

 plenty of similar examples are known in other countries. For instance Plotheia 

 frontalis, a Noctuid which often abounds in Ceylon, shows an equally bewildering 

 wealth of forms. If a dozen specimens of such a species were to be brought home 

 from some little known country, each individual would almost certainly be described 

 as the type of a distinct species. (See the coloured plate published by Sir G. Hamp- 

 son, Cat. Brit. Mus., Heterocera, Vol. IX.) 



