354 AFFINITIES CONNECTING [CHAP. XIV 



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 variety, 

 from its resemblance to a flourishing and little-persecuted kind, 

 has a better chance of escaping destruction 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 illustration of natural selection. 



Messrs. Wallace and Trimen have likewise described several 

 equally striking cases of imitation in the Lepidoptera of the Malay 

 Archipelago and Africa, and with some other insects. Mr. Wal- 

 lace has also detected one such case with birds, but we have none 

 with the larger quadrupeds. The much greater frequency of 

 imitation with insects than with other animals, is probably the 

 consequence of their small size ; insects cannot defend themselves, 

 excepting indeed the kinds furnished with a sting, and I have 

 never heard of an instance of such kinds mocking other insects, 

 though they are mocked; insects cannot easily escape by flight 

 from the larger animals which prey on them ; therefore, speaking 

 metaphorically, they are reduced, like most weak creatures, to 

 trickery and dissimulation. 



It should be observed that the process of imitation probably 

 never commenced between forms widely dissimilar in colour. But 

 starting with species already somewhat like each other, the closest 

 resemblance, if beneficial, could readily be gained by the above 

 means ; and if the imitated form was subsequently and gradually 

 modified through any agency, the imitating form would be led 

 along the same track, and thus be altered to almost any extent, 

 so that it might ultimately assume an appearance or colouring 

 wholly unlike that of the other members of the family to which 

 it belonged. There is, however, some difficulty on this head, for 

 it is necessary to suppose in some cases that ancient members 

 belonging to several distinct groups, before they had diverged to 

 their present extent, accidentally resembled a member of another 

 and protected group in a sufficient degree to afford some slight 

 protection ; this having given the basis for the subsequent acquisi- 

 tion of the most perfect resemblance. 



On ilie Nature of ilie Affinities connecting Organic Beings.-^- 

 As the modified descendants of dominant species, belonging to 

 the larger genera, tend to inherit the advantages which made the 

 groups to which they belong large and their parents dominant, 

 they are almost sure to spread widely, and to seize on more and 

 more places in the economy of nature. The larger and more 

 dominant groups within each class thus tend to go on increasing 



