504 PEOF. E. B. POULTON : NATURAL SELECTIOIS" 



and tells me that it supports a theory of mimicry as due to 

 internal developmental causes, compelling different species to pass 

 through similar phases. The hypothesis that " laws of growth" 

 may cause these resemblances also falls into this category. The 

 suggestion that such laws may account for certain phenomena 

 which are usually explained by the theories of natural and sexual 

 selection was made by Prof. D'Arcy Thompson at the Oxford 

 meeting of the British Association in 1894. This may be called 

 the Theory of Internal Causes. 



(3) The operation of Sexual Selection, cautiously suggested as 

 a possibility by Mr. Blandford, who did not, on this occasion, 

 develop the details of the manner in which the principle may be 

 supposed to act. Fritz Miiller had suggested this idea in a 

 letter to Darwin, who wrote not unfavourably of it to Prof. 

 Meldola, on Jan. 23rd, 1872. " You will also see in this letter 

 a strange speculation, which I should not dare to publish, 

 about the appreciation of certain colours being developed in 

 those species which frequently behold other forms similarly 

 ornamented. I do not feel at all sure that this view is as in- 

 credible as it may at first appear. Similar ideas have passed 

 through my mind when considering the dull colours of all the 

 organisms which inhabit dull-coloured regions, such as Patagonia 

 and the Galapagos Islands " (' Charles Darwin and the Theory of 

 Natural Selection,' Poulton (London, 1896), p. 202.) 



In the present paper the attempt will be made to show that 

 many of the known facts of mimetic resemblance do not admit 

 of interpretation by any of the three theories mentioned above, 

 while they do receive a ready explanation on the supposition 

 that the resemblances are useful and have been produced by 

 natural selection. Certain new observations upon the details of 

 the resemblances in a large group of insects, undertaken specially 

 to test these rival theories, will be found to point strongly in the 

 same direction (see Section 12). 



In order to render the argument as complete as possible, 

 various considerations which have been urged before will be 

 included as well as those which now appear for the first time. 



The objection may be raised that such detailed treatment is 

 unnecessary, and that biologists generally agree in attributing 

 these resemblances to natural selection. To this contention the 

 discussion at the Entomological Society is sufficient answer. 



