INTRODUCTION 



21 



reciprocally in both directions and influencing each member of the group.' And again, 

 in a further passage, 'The respective value of the attraction exercised or suffered by any 

 member of a Miillerian group will depend on its numbers, its nauseous qualities, and its 

 notoriety. The stronger any species is in these respects the stronger will be its power of 

 attraction, and the weaker in comparison will be any force tending to draw it in the direction 

 of other members of the group. The actual mimetic path taken by any species will be the 

 resultant of the forces acting upon it. If the form happens to be a dominant one, these 

 external forces will be insignificant in comparison with its own stability ; and it will there- 

 fore resist change to a large extent or perhaps altogether. The most complete intermingling 

 of characters given and taken on both sides may be expected when two species meet on equal 

 terms, neither being strong enough to predominate over the other.' ^ Obviously convergent 

 mimicry, where it occurs, confers a benefit on more individuals than does the mimicry of 

 a distasteful by an innocuous species, for in the latter case only one species is assisted in 

 the struggle for existence. Indeed, if the edible species become numerous they naturally 

 form a positive menace to the protected kind, so that from the nature of the case Batesian 

 mimicry usually exists in cases where the model is abundant relatively to the mimic. That 

 this need not, however, necessarily be so, has been pointed out by Marshall in a letter to 

 Professor Poulton ^ in which he instances the case of the resemblance between Euralia mima ^ 

 and Amauris echeria. The writer describes the latter insect as ' Probably the best protected 

 butterfly here ' (Natal), and continues in the following words, ' Supposing they occur in 

 equal numbers in a given area, and that certain birds, by chancing to catch three or four of 

 the former in succession, were induced to prey upon butterflies with that coloration, then, 

 from a mathematical standpoint, every alternate specimen caught by any bird would be 

 A . echeria. Now I think we are quite safe in assuming that the fact that every other butterfly 

 caught had a nauseating taste and smell would be far and away more likely to create a strong 

 and lasting impression upon a bird's mind than the fact that every second one proved to be 

 edible, and would be quite sufficient to deter the bird from attempting to eat butterflies of 

 that colour. This is from a mathematical standpoint solely, but from what I know of these 

 two forms in life, I believe that, presuming them to occur in equal numbers, a larger pro- 

 portion of echeria would actually be captured, for mima is a much more shy insect, and 

 although it has the same slow sailing flight (when undisturbed) it does not keep on the 

 wing nearly as long as echeria ; moreover, it is much more wary, and always on the alert 

 for danger, going off at a smart pace when frightened, and not returning to the same spot, 

 as echeria frequently does, after being struck at ; altogether it is a much more difficult insect 

 to capture. Indeed I do not see why the mimic should not somewhat surpass the mimicked 

 species in numbers, without upsetting their relations to one another, provided the taste 

 of the latter be sufficiently unpleasant, and particularly if the flavour be of a lasting nature.' 

 This communication from so careful an observer as Mr. G. A. K. Marshall is not only excep- 

 tionally interesting, but shows the immense value of actual observation of the habits and 

 natural surroundings of the insects themselves. Indeed it is not too much to say that 



^ The theory of reciprocal mimicry or Diapose- 

 matism has been criticized at some length by Marshall, 

 for the details of which the reader may be referred to 

 Trans. Ent. Soc, 1908, p. 93 at seq. The criticism 

 affects, not the fact of Miillerian mimicry, but its 

 relative importance, and I therefore feel that, so far 

 as the present work is concerned, the discussion 



may well be left in its present stage pending such 

 additions to our practical knowledge of the subject 

 as may reasonably be expected to accrue from further 

 research. 



2 See Trans. Ent. Soc, 1902, p. 503. 



3 Hypolimnas duhius, f. mima. 



