lo 
Dritte allgemeine Sitzung. 
The theory of External Causes is probably more commonly re- 
ceived as an explanation than any of the others except Natural 
Selection itself. 
IT now propose to bring before you several illustrations, setting 
forth the main aspects of mimetic resemblance, and to inquire how 
far each of them is consistent with these four theories. All the 
coloured illustrations were made by Mr. Sanger-Shepherd who 
took photographs of the actual specimens in the Hope Department 
of the Oxford University Museum and prepared from the negatives 
the three-colour films which are superposed in the slides. Hence the 
exact patterns of the insects are faithfully represented by physico- 
chemical means. 
The too-exclusive study of mimicry in Lepidoptera alone is 
probably responsible for a common belief in the theory of External 
Causes. Thus when we contemplate a group of many species of Heli- 
conine, Ithomiine, Danaine, Erycinid, and Pierine butterflies from 
British Guiana and Surinam, and find that all tend to develope dark 
hind wings, it is plausible to suggest as an interpretation that we 
are witnessing the common effect of local influences. But this 
and every other explanation, except natural selection, leave as a 
mere coincidence the fact that the first-named three groups con- 
tribute the vast majority of the species, and undoubtedly provide 
the models for the others. Under Natural Selection the inter- 
pretration is easy; the groups in question are specially defended 
by unpalatable qualities and it is to their advantage to warn their 
enemies by a common advertisement. The Erycinid and Pierine 
species may also be unpalatable and fall into the same Miillerian 
(Synaposematic)!) combination, or they may be edible and gain 
advantage by living on the reputation of the three nauseous 
Nymphalid sub-families (Batesian mimicry or Pseudaposematic 
resemblances). Taking all available facts into consideration 
the former is the more probable view. Not in Guiana alone, 
but whereever we may travel in tropical America groups of 
species of these three Nymphalid sub-families tend to resemble 
each other and to act as models for butterflies of other families 
and sub-families. In Venezuela, for example, they are amber- 
coloured black-barred insects as in Guiana but without the ten- 
dency towards preponderant black in the hind-wings; in South 
Eastern Brazil they all possess an especially light stripe, frequently 
bright’ yellow, along the hind- wings, and a light spot, frequently 
white, at the apex of the fore-wings; at Ega, on the upper Ama- 
zon, they all gain a rich chestnut brown ground colour; still further 
west, the brown ground colour is much less dark than at Ega, 
and of a very characteristic shade. Why should these three sub- 
families be so conspicuously subject to the common influence of 
1) Proceedings Entom. Soc, London, 1897, p. X XIX. 
