Mimicry and Natural Selection. 
By E. B. Poulton (Oxford). 
I feel itto be a great honour and pleasure to be called on to 
deliver the address on behalf of the English-speaking nations at the 
fifth International »Zoological Congress at Berlin. At the same 
time Iam sensible of the great difficulty of the task, the attempt 
to say anything adequate on so wide a subject in the narrow 
compass of five and forty minutes. 
In attempting to arrive at a decision upon the origin and 
cause of mimetic resemblance we have no direct evidence to 
assist us. We are driven to base our opinion upon the same 
ground as that upon which the belief in the theory of gravi- 
tation is founded. This theory finds acceptance, not because 
of direct evidence in its favour, but because the facts of the cos- 
mos, so far as we know them, are consistent with the theory 
and none of them inconsistent with it. 
It is necessary therefore first to give a brief account of 
the theories which have been advanced to account for the ori- 
gin of Mimicry, secondly to enquire how far each one of them 
is consistent with the main facts of Mimicry. 1) The theory 
of Natural Selection as an explanation, assumes that these 
resemblances have been produced because they are and have been 
useful in the struggle for existence. There has been according 
to this interpretation a greater average survival in successive 
generations of the forms in which these useful likenesses were 
more strongly developed as compared with those in which they 
were less strongly developed, and thus in process of time a very 
high degree of resemblance has been attained. 2) The theory 
of External Causes assumes that mimicry has been produced 
by the direct action upon the organism of some one or more of 
the various influences which exist in the locality, such as food, 
moisture. dryness, heat, coldice:--3). Phe theory of In- 
ternal Causes assumes that mimetic resemblances are due to 
the independent arrival of different species at the same evo- 
lutionary stage, as regards the characters in which such resem- 
blances are manifest. 4) The theory of Sexual Selection has 
occasionally been invoked to account for mimicry, the assumption 
being that the selection of mates has been influenced by the 
colours and patterns of other species living in the same country. 
The last mentioned theory is believed to account for mimicry 
by comparatively few naturalists, although it was deemed to be 
worthy of consideration by Charles Darwin and Fritz Miller’). 
1) See a letter from Charles Darwin describing Fritz Miiller’s thoughts 
on this subject. ,,Charles Darwin and the Theory of Natural Selection.‘‘ Poulton, 
London 1896, p. 202, 
