118 NATURE SKETCHES IN TEMPERATE AMERICA 



generally from very diverse groups of insects, according to the 

 general resemblance which existed before the commencement 

 of the process of adaptation, and to the variations made possible 

 by the physical nature of the species concerned." 



Bates was the first to suggest that when two species exhibited 

 similar patterns and one was distasteful to birds, the other 

 had acquired a protective resemblance to it through natural 

 selection. Darwin, as mentioned in his "Life and Letters," 

 had praised Bates' paper with enthusiasm. Fritz Miiller, 

 in 1879, and later, Lloyd Morgan, attempted to show that 

 birds have no intuitive knowledge of what forms of insects 

 should be avoided. Young birds are said to destroy many 

 distasteful forms before learning to leave them unmolested. 

 It is also believed that when birds learn that individuals of a 

 species, which may serve as prey, are disagreeable, other species 

 which closely resemble the protected species profit by the 

 mimicry, because each of the two species will need to contribute 

 only a portion of the sacrificed ones instead of the whole. It is 

 presumed that the greater the number of forms that mimic a 

 favored species, the smaller would be the destruction of indi- 

 viduals, and conversely, the greater are the chances for existence 

 for all. In this way associations of species called "Mtillerian 

 groups," resembling one another, yet being of diverse genera, 

 may coexist and enjoy common immunity. 



These suggestions of Bates and Miiller were enlarged by 

 Wallace, Trinien, Mcldola, and Poulton, and as a result, much 

 light has been thrown on the problem of the complicated rela- 

 tions of many forms. These views, like all theories when first 

 propounded, have been called in question by a number of 

 writers, principally because of lack of direct evidence that birds 

 actually eat butterflies. Weismann, Judd, and others have 

 made limited investigations of this character, and very recently 

 Marshall ^ has shown that many birds eat butterflies. On the 

 other hand, we find that Gadow, of England, has propounded 

 a chemical theory of animal coloration to explain mimicry, 

 maintaining that the pigments are physiological products of 

 the organism, liable to chemical transformation with corre- 

 sponding changes in color. 



* Trans. Entomological Society of London, 1908, p. 329. 



