444 ANALOGICAL RESEMBLANCES. 



"We are next led to inquire wliat reason can be assigned 

 for certain butterflies and moths so often assuming tne 

 dress of another and quite distinct form; why, to the per- 

 plexity of naturalists, has nature condescended to the 

 tricks of the stage? Mr. Bates has, no doubt, hit on the 

 true explanation. The mocked forms, which always 

 abound in numbers, must habitually escape destruction to 

 a large extent, otherwise they could not exist in such 

 swarms; and a large amount of evidence has now been 

 collected, showing that they are distasteful to birds and 

 other insect-devouring animals. The mocking forms, on 

 the other hand, that inhabit the same district, are com- 

 paratively rare, and belong to rare groups; hence, tliey 

 must suffer habitually from some danger, for otherwise, 

 from the number of eggs laid by all butterflies, they would 

 in three or four generations swarm over the whole country. 

 Now if a member of one of these j)ersecuted and rare groups 

 were to assume a dress so like that of a well-protected 

 species that it continually deceived the practiced eyes of an 

 entomologist, it would often deceive predaceous birds and 

 insects, and thus often escape destruction. Mr. Bates 

 may almost be said to have actually witnessed the process 

 by which the mimickers have come so closely to resemble 

 the mimicked; for lie found that some of the forms of 

 Leptalis which mimic so many other butterflies, varied in 

 an extreme degree. In one district several varieties oc- 

 curred, and of these one alone resembled, to a certain 

 extent, the common Ithomia of the same district. In 

 another district there were two or three varieties, one of 

 which was much commoner than the others, and this 

 closely mocked another form of Ithomia. From facts of 

 this nature, Mr. Bates concludes that the Leptalis first 

 varies; and when a variety happens to resemble in some 

 degree any common butterfly inhabiting the same district, 

 this varietv, from its resemblance to a flourishino^ and little 

 persecuted kind, has a better chance of escaping destruc- 

 tion from predaceous birds and insects, and is consequently 

 oftener preserved; " the less perfect degrees of resemblance 

 being generation after generation eliminated, and only the 

 others left to propagate their kind." So that here we have 

 an excellent ilhistration of natural selection. 



Messrs. Wallace and Trimen have likewise described 



