736 EVENING DISCOURSES. 
MONDAY, AUGUST 5. 
Recent Developments in the Theory of Mimicry. 
By ¥. A. Drxty, WA., M.D. 
The remarkable resemblances that exist between certain insects belonging to 
widely different orders, as, for instance, the likeness borne by some of the ‘ clearwing 
moths’ to wasps and hornets, have long been known to naturalists. They were 
interpreted by the older observers as cases of ‘repetition’ and ‘analogy’ in Nature. 
Kirby and Spence were the first to attempt a rational explanation. These authors 
got so far as to suggest that one species might gain an advantage by resembling 
another ; but the first really scientific account of the matter was given by Bates, 
who pointed out that certain kinds of butterflies in South America escaped attacks 
from birds by mimicking the appearance of other conspicuous species which were 
immune from persecution on account of the possession of distasteful qualities, 
This resemblance to a distasteful model he considered had been gained by a gradual 
selection of varieties tending in the appropriate direction. 
Bates’s theory of mimicry, which was at once accepted by Darwin and met with 
general approval, marked an important step in advance, It left, however, unex- 
plained the fact that these resemblances occurred, not only between distasteful 
models and their presumably edible mimics, but also between the distasteful 
models themselves. ‘To account for this he could only suggest that there must be 
something in the local or geographical conditions which had a direct effect upon 
forms inhabiting the same region, causing them, even if widely separated in 
affinity, to assume a common aspect. 
But the existence of large groups of insects with various affinities and a common 
facies was felt as a stumbling- block in the way of the theory of mimicry until in 
1879 Fritz Miller found the key to unlock the difficulty. He showed that if (as 
experiments, chiefly by Lloyd Morgan, have subsequently proved to be the case) 
birds had no instinctive knowledge of what forms would be suitable for food and 
what should be avoided, so that each bird had to gain its knowledge by experience, 
a certain number of the distasteful forms would have to be sacrificed by each 
generation of birds until these enemies had learned to leave such iorms alone. In 
other words, each distasteful form would have to pay a tax for itsimmunity. ‘Now 
if two distasteful species resembled each other so closely that birds or other 
enemies did not distinguish between them, the disagreeable experience gained by 
tasting an individual of one species would be applied to the benefit of the other, 
and so each of the two species would only need to contribute a portion of the tax, 
instead of each paying the whole. And what is true of a combination of two 
species would be equally true of a larger assemblage: the greater number of 
forms that could be got to share the tax, the better for all. Hence the forma- 
tion of these large Miillerian groups, or, as they might be called, ‘ inedible associa- 
tions,’ giving room, no doubt, for a certain amount of Batesian mimicry side by 
side with them or within their own ranks. It is obvious that the resemblances 
shown between members of these groups, constituted as they are by insects of 
widely separated orders, cannot be explained by affinity; while the fact (amongst 
others) that the resemblances are superficial only, never structural, makes strongly 
against the view which would attribute them to the direct operation of external 
conditions. The Miillerian theory, which is rather a theory of common warning 
marks, or ‘ synaposematism’ (Poulton), than of mimicry proper, may thus be said 
to hold the field as meeting the facts to an extent of which no alternative explana- 
tion has been found capable. Miiller’s suggestion was first brought to the notice 
of British naturalists by Professor Meldola; and in its futher developments at the 
hands of Meldcla himself and of Poulton, it was accepted both by Wallace and by 
Trimen, the two naturalists who had done most by their own observations to con- 
tirm the validity of the original theory of Bates. It is to be observed that both 
theories alike postulate the operation of natural selection, 
