OF THE AMAZON VALLEY. 531 



inquired into*. I found no clear instance on tlie banks of the Amazons of two forms 

 of 3£. Polymnia having this relationship ; but, to judge from the collections received, it 

 exists between M. Lysimnia and 31. Nesoea, in the middle parts of the Brazilian coast- 

 country, at Bahia. 



Local var. Meclianitis JEJgaensis (PL LVI. fig. 7 a). 



Expanse, <S 2" 2"''-2" 7'", ? 2" 4"'-3". d . Eore vsdng similar in shape to that of M. 

 Folymnia : above, dark orange-tawny or reddish brown ; the basal part of the costa and 

 a broad stripe along the hind margin, extending three-fourths the length of the wing, and 

 touchiag the edge, black ; there are, besides, six black spots, namely, a large quadrate one 

 about the middle of the cell, a faintly marked one near the base, two (sometimes united) 

 over the end of the cell, and two between the 1st and 2nd median branches. Across the 

 wing, a little beyond the middle, is a very irregular bright -yellow belt, which begins on 

 the costa and terminates about the middle of the outer margin : this band generally 

 includes a quadrate spot lying within the end of the cell ; its outer margin is deeply 

 bisinuated, and broadly edged with black : in the middle of the brownish apical part of 

 the wing, near the costa, is a large ill-defined yellowish spot. Beneath, the same, except 

 that there is on the outer margin, near the apex, a series of five whitish spots, in some 

 examples reduced in size and numlier, in others altogether wanting. 



Sind wing short and quadrangular, the angularity most pronounced in small examples : 

 above, same colour as the fore wing; there is a broad, black, zigzag stripe along the disk, 

 behind the cell, and generally a series of marginal lunules of the same colour. The 

 marginal lunules are wanting in some examples ; in others they are large, and blended 

 partly with the black discal stripe. Beneath, the same, the margin spotless ; there is a 

 yellow spot at the base of the costa. 



? . Similar to the cJ in colours : the hind wing is not angular, but elliptical in shape, 

 and has generally a trace of the marginal white spots on the under surface. Individuals 

 occur much darker in colour, with the black marks spreading partly over the surface. 



Extremely abundant all the year round at Ega, in thinned parts of the forest ; but found 

 in no other part of the Amazon region. It occurred in company with the typical M. 

 Polymnia and numerous individuals exhibiting all the gradations between the two 

 extreme forms. I add descriptions of some of these intermediate varieties. 



* I have given a case iu the first paper published of this series, " On the Insect Fauna of the Amazon Valley," in 

 ' Transactions of the Entomological Society of London,' n. s. vol. v. p. 354, PajtUio Fertumnus and P. Hierocles. It 

 seems to me that, as only those local varieties become species which maintain themselves distinct vphen brought by 

 redistribution in contact with their sister-forms, natural selection comes here into play. It is an advantage to a 

 form to have a sphere of life different from its allies : when two sister forms keep themselves distinct in a locality, it 

 is a sign they have acquired sufficient difference to fill two separate spheres ; if they paired together, they would soon 

 become one again. Nature may be said to place a premium on diversity ; for she thus destrovs the incompletely formed 

 race, and preserves the completely formed one. 



The case of Mechanitis Polymnia diiFers from that of Leptalis Theonoe, in exhibiting the production, generally, of 

 only one local form in a district, instead of many. As far as my observations go, this seems to have been the most 

 frequent course in nature. More than one new race would with difficultv be formed in a limited area, where the 

 individuals live in close neighbourhood, except in such cases as our Leptalis, where rigid destruction of intermediate 

 forms is going on, thus restricting the choice of mates to the surviving forms, or iu such genera as Ithomia, where 

 there is no doubt the insects carefully select their exact counterparts in pairing. 



