to limitations of the Milllerian Hypothesis of Mimicry. 131 



etc, while Bates' factor is in force at all times. But if 

 we further consider the life of an individual bird it is 

 apparent that its experimental period is only of short 

 duration, while for the rest of its life its selective action 

 will be simply in a Batesian sense ; its increasing experience 

 giving it added efficacy in this direction. As against this 

 view Professor Poulton has urged (Proc. Ent. Soc. 1903, 

 p. ix) that Milllerian selection is probably more keen than 

 it appears ; that we must not estimate it by a comparison 

 of the relative numbers of mature and immature enemies 

 at any given time, but that we must remember that there 

 is a considerable elimination of the young of these enemies, 

 so that a large percentage never reaches maturity, and it 

 is apparently assumed that these individuals only operate 

 as Milllerian factors. 



Taking the case of birds, which are, almost certainly, the 

 principal agents in the production of mimicry among 

 butterflies, let us examine this matter of the destruction 

 of the young. We have no exact knowledge on the sub- 

 ject at all, but it is reasonable to suppose that the most 

 critical and dangerous period of a young bird's life is 

 during the time when it is a helpless nestling and also 

 for a few days after it has left the nest, while it is still 

 unable to fly properly, to recognise its enemies or to 

 provide itself with food. It is probable, therefore, that 

 by far the greatest destruction of life among young birds 

 will take place between the time when they leave the egg 

 and the time when they are able to fly properly and forage 

 for themselves. But throughout this period young birds 

 do not operate as Mlillerian factors ; such selective influence 

 as they may have upon the insects in their neighbourhood 

 will be exercised entirely through the medium of their 

 parents, and as these latter will be mature and, pre- 

 sumably, experienced birds, their effect will be solely in 

 a Batesian direction. This aspect of the question there- 

 fore does not, as it seems to me, add weight to the 

 Mlillerian argument, but rather the reverse. 



Tenacity of life in unpalatable species. 



Many observers have called attention to the fact that 

 various butterflies exhibit a very remarkable tenacity of 

 life, accompanied by a toughness of the integuments. In 

 all these cases the insects belonsf to oenera in which the 



