THE STIMULUS OF INTEREST xlv 



of Oriental butterflies and moths which had been exposed 

 to light for a long period of years. I noticed that the pig- 

 ments of the moths had as a whole faded far more than 

 those of the butterflies. 1 It at once occurred to me that 

 stable pigments are far more necessary for the butterfly 

 exposed to the light of a tropical sun, than for the moth 

 flying in the evening or at night. Hence a much higher 

 level of stability would be selected in the pigments of 

 butterflies than in those of moths. Professor Farmer 

 would of course maintain that this is one of the ' teleo- 

 logical explanations that really explain nothing, but 

 rather bar the way of scientific inquiry ' (see p. 74 n. 2 of 

 the present work). I, on the contrary, believe that it ex- 

 plains a great deal. It explains the reason why pigments 

 with particular qualities were selected, and have now 

 come to be characters of certain species. It does not 

 pretend to explain how it was that pigments with these 

 qualities were there to be selected ; but, so far from bar- 

 ring the way, this suggestion actually points the road to 

 scientific inquiry. As a matter of fact, a further investi- 

 gation into the chemical nature of these pigments, and 

 the steps by which they arise in the individual, is now 

 more probable than it was before the suggestion was made. 

 It is precisely the same with regard to the example 

 brought forward by Professor Farmer in his Address 

 (p. 676) : — ' One of the commonest responses to the sti- 

 mulus of wounding, in the higher plants, is the formation 

 of a layer of cork over the injured and exposed tissue. 

 No one can deny that this is a reaction of great utility, 

 checking as it does the undue evaporation of water, and 

 the entrance of other parasitic organisms. And yet 

 I suppose that no one would go so far as to seriously 



1 Confirmatory evidence was afforded by the condition of the day- 

 flying moths and the shade-loving and crepuscular butterflies. 



