DARWIN AND BERGSON ON EVOLUTION 



brought out in the Proceedings of the Entomological Society of 

 London (1879, p. xx). The suddenness of the imaginative leap 

 which produced the new hypothesis is made clear in the opening 

 paragraph of the earlier paper, where Fritz Miiller speaks of a 

 problem over which one puzzles and puzzles, while the solution is 

 all the time lying close at hand and obvious. 



The Miillerian hypothesis is briefly as follows. Insect-eating 

 animals do not come into the world with an instinctive knowledge 

 of specially protected insects, and it is only by experiment that they 

 gain a knowledge of the patterns which advertise unpalatability or 

 danger. Experimental tasting means injury or death to insects, 

 and anything which facilitates the education of enemies reduces the 

 destruction of insect-life. Now the resemblance between specially 

 protected forms does tend to lessen the amount of experimental 

 tasting. If A and B possess the same pattern, an enemy, having 

 tasted A, may not require to taste B at all, or may approach it 

 more cautiously and do little injury. Another enemy will taste 

 B first and then the dangers of experiment will be reduced for A. 



Such, in few words, is the Miillerian hypothesis, and on the present 

 occasion, I can say no more about it except to point out that much 

 support is afforded by recent investigations showing that extra- 

 ordinary intricacy is a common feature of mimetic combinations. 

 The conclusions, originally stated by Fritz Miiller, and in later years 

 arrived at independently by F. A. Dixey, that mimetic resemblance 

 may often be attained by reciprocal approach, is a matter of contro- 

 versy, but there can be no dispute over the statement that mimetic 

 species are often themselves the Models for mimicry. 



(3) The third place is given to the hypothesis by which Bates 

 sought to explain the resemblance between specially protected species 

 as the result of climate or of food, the effect at any rate of one or more 

 of the physico-chemical influences at work in the locality. This 

 explanation may be called the hypotheses of External Forces. I 

 formerly spoke of it as the hypothesis of " External Causes," but 

 the Batesian and Miillerian hypotheses also depend upon external 

 causes. The word " Force," on the other hand, points more directly 

 to physico-chemical influences than to the selective elimination 

 brought about by enemies. 



A. R. Wallace at first, but not later, followed Bates in both 



56 



