THE CAUSE OF MIMETIC RESEMBLANCE. 583 



of accidental resemblance due to internal causes, as has been 

 suggested by Beddard ('Animal Coloration,' London 1892, p. 252); 

 but such resemblances will ditter from those under discussion iu 

 this among other things — that they will not be characteristically 

 local. The theory of internal causes oilers us a valid interpretation 

 of such cases, which are, as a rule, readily distinguished from those 

 which are here considered. 



There is another aspect of locality which only receives an 

 explanation on the theory of natural selection. Why should 

 examples of mimicry and common warning colours be so much 

 more abundant and perfect in one country than another ? The 

 physico-chemical influences, the effects of luxuriant vegetation, 

 as Wallace has pointed out, are very similar in tropical 

 8. America, Malaya, and W. Africa, and yet the first-named 

 country is pre-eminent iu affording examples of the resemblances 

 under consideration. This is not only true of likenesses within 

 the Order of Lepidoptera, it appears to be equally true within the 

 Coleoptera ; it is true of the resemblances of moths to wasps. 

 It is even more marked in Miillerian resemblance between pro- 

 tected forms than in the examples of Batesian mimicry. If the 

 direct action of forces connected with locality cannot explain the 

 immense predominance of tropical South America in this respect, 

 we are driven to enquire whether insect-life is especially luxuriant 

 and remarkable in this part of the world, and whether it is not 

 probable that the struggle for existence is especially keen. 

 There is no doubt about the answer to the former question ; 

 the variety, peculiarity, and abundance of insect life is far greater 

 than iu any other pai't of the world, and it is a fair inference 

 that the conditions are in an equally marked degree favourable 

 for rapid and complete modifications under the operation of 

 natural selection. 



We have not as yet sufficient evidence that mimetic groups 

 and groups with a common warning coloration appear at the 

 same time of the year. Such evidence as we have points in this 

 direction. The rarity of the mimetic species is usually stated to 

 be due to their being lost in the swarms of the abundant model. 

 There are a large number of cases in which the forms have been 

 caught together by a collector who has passed a limited time in 

 a given locality. 



We are collecting at Oxford as many examples as possible of 

 such species, captured upon the same day iu the same place. 



