The Zoologist— February, 1867. 573 



6 Because the theory assumed that the Beliconiidas existed before the attempt at 



mimicry commenced on the part of the Pierid* ; whereas Mr. Bates' statement 



would lead to the inference that the Heliconiidas were so unstable a group that 



the manufacture of species is still going on among them. 



7. Because, according to the doctrine of chances, it was in the highest degree 



improbable that a casual variation of any given species of Pierids should by 



constant modification, assisted by hereditary descent, gradually assume the form, 



colour and markings of another species, especially of so remarkable a type as the 



Heliconiids. But for an entire group to be simultaneously engaged in such a 



process, each species tending towards distinct and equally peculiar species, would 



by a logician be pronounced impossible. The admission that the God of Nature 



created these species in their present mimetic condition for some w.se butb.dden 



purpose disposed of all difficulty. 



Mr Alfred B. Wallace followed, with an exposition of the theory of mimicry or 

 adaptive resemblances as explaining anomalies of sexual variation. He began by 

 pointing out what was meant by mimicry ; when moths or beetles so closely resembled 

 the bark of the trees they were accustomed to rest on that it was difficult to distinguish 

 them, or when the curious Phasmid* were ^distinguishable from the sticks or leaves 

 among which they lived, no one doubted that the resemblance was serviceable to the 

 creature,-it was a protective adaptation. So with the moths of the genus Trochil.um 

 which resembled stinging Hymenoptera, but were themselves helpless sluggish 

 creatures, the protection gained was no less clear; and this was termed mimicry 

 because one insect was, as it were, dressed to imitate another. Mr. Bates first showed 

 how extensively this prevailed in nature, especially among the Lepidoptera, aud 

 argued that if the imitated forms had any special immunity from attack, the species 

 of other groups which resembled them would to some extent be free from attack also 

 and would thus gaiu an advantage in the struggle for existence. He then shewed 

 that the forms imitated always belonged to dominant groups, or those excessively 

 abundant in species and individuals, and therefore presumptively free from the attacks 

 of those insect-enemies that kept down the numbers and threatened the extinction of 

 other species; and that in the case of the Danaids and Heliconiidte (the groups most 

 frequently imitated all over the world), the protection was probably the powerful odour 

 they emitted. The theory of natural selection, or the preservation of useful variations, 

 was shown to be fully capable of explaining these facts, and it bore the test of a true 

 theory by also explaining other anomalies as they arose. A species of D.adema was 

 then 'exhibited, in which the female was glossed with blue, while the male was dull 

 brown, thus reversing the usual sexual characters of the genus; and it was observed 

 that the male in insects was usually more active, the female more sluggish ; the male 

 ,ailv coloured, the female dull ; and these fads were connected by the consideration 

 that .he female, having to cam a heavy load of ova, and to deposit them in places 

 favourable for their development, required protection for a much longer period than 

 the male, whose duty of fecundation was very speedily performed. Thus dull colours 

 were useful to female insects, since it rendered them less conspicuous. It followed 

 that any other kind of protection would he also more necessary for the female than for 

 the male, and, to show that this really was so, a male specimen of the well-known 

 leaf-insect (P/ujllium, sp.) was exhibited, having none of that wonderful protective 



