PKOTECTIVB MIMICRY. 215 



common Whites, Clouded Yellows, Brimstone, etc)., is totally 

 different from tlie Heliconidee, not only in colour but in the 

 shape of the wings ; the Wood White, however {Leucophasia 

 sinapis), presents a certain likeness to the Heliconidce in the 

 shape of the wings, and also in the comparative feebleness of 

 its flight — a circumstance which will perhaps cause its extinc- 

 tion in the future. The feeble flight is perhaps a necessary 

 concomitant of the shape of the wings ; it is not at all needful 

 to believe — the suggestion has been made — -that the genus 

 Leptalis imitates the Heliconida3 in its wretched powers of 

 flight as well as in the form of the wings and their markings ; 

 it is much more likely the insect is a poor flyer on account of 

 the shape of its wings, and that this very circumstance rendered 

 the mimicry necessary as an alternative to extinction. But, 

 even starting with a form like the Wood White, there is a 

 very considerable distance to be traversed before the Leptalis 

 becomes at all like an Ithomia. 



This difficulty has been met by Fritz Miiller * ; a com- 

 parative study of a number of different species of Leptalis led 

 him to believe that the banded forms are the most archaic ; 

 comparatively little modification, therefore, was needed to pro- 

 duce the detailed resemblance between the several species of 

 Leptalis and the protected Mechanitis and AcrcBa. 



If this is generally the case, it is far easier to believe in the 



efficiency of natural selection in bringing about mimicry ; but 



it raises the further question — How are we to account for the 



initial likeness ? The answer to this question may bring us 



back to the neighbourhood of the theory of "like conditions " 



— to the view expounded by Murray, and the earlier opinion of 



Mr. AVallace. 



* Jeiiaischs Ziitsclirift, 1870. This important paper is noticed in the 

 American Naturalist for 1876, p. 535. 



