466 



THE GENERAL PRINCIPLES OF ZOOLOGY 



example is the butterfly Methona psidii, which is imitated by the 

 butterfly Leptalis orise{¥'\g. 425 ). Some moths closely resemble 

 wasps, the wings being perfectly transparent. Among verte- 

 brates some perfectly harmless snakes mimic poisonous species, 

 and in the East Indies harmless and helpless birds are known 

 which resemble certain powerful species. It is an essential con- 

 dition of all mimicry 

 that the two species 

 which resemble 

 each other should 

 have the same habi- 

 tat in order that the 

 resemblance maybe 

 protective. 



It will be readily 

 seen what an im- 

 portant role such 

 protective resem- 

 blances must play 

 in the struggle for 

 existence, and how 

 natural selection 

 may well account for 

 their permanence. 

 There is one other 

 aspect of Darwin's 

 theorv which we 

 must mention. It 

 may happen that 

 certain structures 

 which have long 

 been carefullv pre- 

 served by natural 

 selection are no 

 longer necessary 

 maintenance of the species; natural enemies 

 may have disappeared, food may have become much more 

 abundant. The result would be that many more individuals 

 would reach maturity than before; there would be free crossing 



FlG, 424. Mygnimia aviculus, upper figure, a wasp imitated 

 by a beetle, Coloborhombm fasciatipennis,ihe lower figure; 

 both insects are black with large white spots (indicated by 

 dotted lines) near the tips of the wings. (After Wallace.) 



for the vigorous 



