(4) That the Imitators Differ From the Bulk of Their Allies
What importance we attach to this condition must depend upon our interpretation of the word “allies”—whether, for example, we use it for a small group of closely connected species, for a genus, for a group of genera, or in an even wider sense. Perhaps an example will serve to make the difficulty more clear. As already noticed, the S. American genus Dismorphia belongs to the family of Pieridae or “whites.” Also certain species of Dismorphia bear a close resemblance to certain species of Ithomiines, a noteworthy example being D. praxinoe and Mechanitis saturata (Pl. X, figs. 3 and 7), in which the pattern, colour, and shape of the two species are all far removed from what is usually understood by a “white.” It must not be forgotten, however, that these matters are generally discussed by European naturalists who have grown up in a region where the majority of the “whites” are more or less white. For this reason the statement that D. praxinoe differs from the bulk of its allies is likely to meet with general acceptance, especially as some of the species of the genus itself (e.g., D. cretacea, Pl. X, fig. 1) are regular whites in appearance. But when we come to look at the genus Dismorphia as a whole the matter assumes another complexion. Seitz29 recognises 75 species of which about a dozen are predominantly white. The rest present a wonderful diversity of colour and pattern. Black predominates on the fore wings, and the insect is frequently marked with gay patches of yellow, bright brown, scarlet, or blue. Forms which from their colour are clearly not mimics present nevertheless the general pattern and shape of other forms which bear a strong resemblance to some Ithomiine. Sometimes a change of colour in certain patches from blue or yellow to bright brown would make all the difference between a non-imitative and an imitative species. Moreover, the non-imitative forms frequently have the peculiar narrow wing, so unusual in a Pierine, which enhances the resemblance of the mimicking species to the Ithomiine model, and which to some extent occurs even in D. cretacea. Clearly we are not justified in saying that D. praxinoe differs from the bulk of its allies, for inside the genus there are many non-imitative species which differ from it in some particulars and are alike it in others. There is a distinct family resemblance among the bulk of the Dismorphias, including practically all the mimetic forms, and on the whole the resemblances between the imitative and the non-imitative forms are as noteworthy as the differences. Though not exhibited in so striking a fashion, the same is to a large extent true of a large proportion of the cases of mimicry. It is on the whole unusual to find cases where a single species departs widely from the pattern scheme of the other members of the genus and at the same time resembles an unrelated species. Two of the best instances are perhaps those of Limenitis archippus (p. 49) and of the Pierid Pareronia (p. 23). Of the total number of mimicry instances a high proportion is supplied by relatively few groups. In each region several main series of models and mimics run as it were parallel to one another. In Asia, for example, we have the Papilio-Danaine series where the colour-patterns of a series of Danaines, all nearly related, are closely paralleled by those of a section of the genus Papilio, and by those of the Satyrid genus Elymnias. In Africa there is a similar Papilio-Danaine series though of less extent. Africa has a group of models not found in Asia, and the Papilio-Danaine series is as it were curtailed by the Papilio-Planema series with which to some extent runs parallel the genus Pseudacraea. These phenomena of parallel series have been mentioned here as shewing that mimicry tends to run in certain groups and that in many cases at any rate little meaning can be attached to the statement that the imitators differ from the bulk of their allies.
The fifth of Wallace’s conditions is clear and needs no discussion.
It is evident that at any rate a large proportion of the instances of close resemblance do not fulfil all of the conditions laid down by Wallace. Nevertheless we should expect them to do so if the resemblance has been brought about by the cumulative effect of natural selection on small favourable variations. Clearly there is a prima facie case for doubting whether we must of necessity ascribe all resemblance of the kind to natural selection, and in the next few chapters we shall discuss it in more detail from several points of view.