344 ^^^^ Theory of Natural Selection 



the appearance of another species. Darwin expHcitly re- 

 pudiated the suggestion that there was any inward aspiration 

 to the favorable mask. He supposed only that, in so far as 

 individuals of one species do resemble another species already 

 favored, they would stand a better chance to survive and 

 leave progeny. With succeeding generations this increased 

 resemblance to the protected species would accumulate until 

 the existing conditions of mimicry had evolved. 



As a matter of fact, it must be conceded that there are 

 striking resemblances between butterflies of one family and 

 butterflies of another family, and that even some flies re- 

 semble bees and wasps which belong to a totally different 

 order. The argument, however, that these resemblances 

 were developed in the course of natural selection takes al- 

 together too much for granted. It assumes, first of all, that 

 the model species is of itself protected by a disagreeable 

 taste, or otherwise, so that it is relatively free from molesta- 

 tion by the natural enemies of the group. This protection, 

 to be sure, is not automatic. It is necessary for the natural 

 enemies in a given region (for example, birds or frogs) to 

 discover each for himself that insects of a given appearance 

 are to be avoided. We can watch young chicks pecking at 

 " worms " and bugs, and learning in a few days to avoid 

 certain creepy things and to feed exclusively upon others. 

 Each generation of a species protected by disagreeable taste, 

 for example, must sacrifice a considerable number in order 

 to educate the enemy to take warning from the distinctive 

 appearance. To be of protective value to the mimic, 

 under such circumstances, it would be necessary that the 

 model be much more numerous in a given area. Otherwise 

 the mimic species would have to sacrifice a disproportionate 

 number of individuals toward the education of the common 

 enemy and would in fact neutralize excessively the education 

 carried on by the distasteful model. 



The theory assumes, in the second place, that the pro- 

 tective value of the resemblance would be acquired through 

 natural selection in an area in which the model had already 



