220 Annals Entomological Society of America [Vol. II, 



Danaidas. Furthermore, such changes as have taken place in 

 the older invader, D. plexippns, during its residence in the New 

 World, are also retained in those colonies which, during the half 

 century, have been re-establishing themselves in the Old World. 

 These facts support Darwin's conclusion that the physico- 

 chemical influences of soil, climate, etc., are of comparatively 

 slight importance, a conclusion which made him feel "inclined 

 to swear at the North Pole, and * * * to speak disrespectfulh' of 

 the Equator. "-1 



The mimics on the other hand are derived from characteristic 

 and ancient inhabitants of the northern land-belt. If, as the fol- 

 lowers of the theory of External Causes (see p. 205) maintain, 

 species are the expression of the physical and chemical forces of 

 the environment, then the Danaidas express the Old World 

 tropics and the species of Limenitis the northern land-belt. We 

 might expect on this theory that the Danaidas, when they in- 

 vaded the northern zone, might come to resemble the Limenitis; 

 but the transformation that has actually occurred is entirely 

 inconsistent with any such hypothesis. Although the Danaidas 

 have undergone no important change in the new environment, 

 their presence has entirely transformed and brought into a close 

 superficial resemblance to themselves the descendants of a mem- 

 ber of an ancient group. Such a fact is inconsistent with any 

 interpretation as yet offered except that which refers the change 

 to the accumulation by selection of variations which promote a 

 likeness to the Danaidas. 



The facts also bear upon the two theories of Mimicry associated.. 

 with the names of H. W. Bates and Fritz Miiller. According to 

 Bates' theory. Mimicry is a special form of protective or cryptic 

 resemblance. In the ordinary examples of this principle, species 

 are aided in the struggle by concealment, by a likeness to some 

 objects of no interest to their enemies (such as bark, earth, etc.) ; 

 in these special examples (called mimetic) species are aided by 

 resembling some object which is unpleasant or even dangerous 

 to their foes. Fritz Mliller's theory of Mimicry includes the 

 cases in which the mimics, as well as their models, are specially 

 defended, although generally to an unequal degree. The" 

 resemblance is due to the advantages of a common advertise- 



2' In a letter to Sir Charles L3'ell, Oct. 11, 1859. — Life and Letters, ii. 212. 



-- It is probable that relative abundance may determine the relationship of 

 model and mimic in ca.ses where there is no reason for suspecting any difference in 

 the degree of unpalatability. 



