THE ORIGIN OF INSECTS 87 



of external characters necessarily implies their direct use- 

 fulness to the species concerned. External characters, 

 useless in themselves, may be mysteriously bound up with 

 important chemical or physiological conditions. For ex- 

 ample, Darwin states that male white cats with blue eyes 

 are deaf; and Sir Kay Lankester remarks that " if deafness 

 were ever an advantage (a difficult thing to imagine), you 

 would get a species of cat with white hair and blue eyes, 

 and be led to distinguish the species by those characters, 

 not by the real cause of survival — namely deafness." 

 Thus, obvious but useless characters may be established by 

 natural selection because of their correlation with others 

 that are useful but obscure, and there can be little doubt 

 that the peculiarities of many species can only be satis- 

 factorily explained in this way. 



We have still to see how varieties, established through 

 the agency of natural selection, may be consolidated into 

 species. In order to make this clear, I cannot do better 

 than repeat an instance cited by Professor Carpenter, 

 especially as it concerns the well-known brown argus 

 butterfly {Polyommaius astrarche). This insect frequents 

 sunny hillsides in the south of England in June and again 

 in August. The upper surface of the wings is dark sepia- 

 brown in colour, while there is a black spot in the centre 

 of each fore-wing. Further, within the margin of each 

 fore-wing there is a row of orange spots. On the under- 

 side, the ground colour of the wings is light greyish fawn, 

 while around the margins, inside a series of white lunules 

 and black specks, there are rows of orange spots sur- 

 rounded by white rings — " eye-spots " as they are called. 



As we travel northward we find that the brown argus 

 butterfly is much rarer in the English midlands than in 

 the south ; but in many places in the northern counties 

 it again becomes common. We find, however, that most 



