OF THE AMAZON VALLEY. 531 



inquired into*. I fmmd no clear iustanei' on the banks of tlie Amazons of two forms 

 of J/. Poh/iiijua haviny tliis relationship; but, tojudgv from the c(jlleetions received, it 

 exists between M. Li/simnla and JI. Xcauut, in the middle parts of the Brazilian coast- 

 country, at Bahia. 

 Local var. Mcchuiiittn Egoensis (PI. LVI. llg-. la). 



Expanse, d 2" 2"'-'2" '"', ? 2" l"'-;3". <?. Tore wint;- similar in shape to thai oi' J/. 

 Pol>/»i)/i<! : afjorr, dark oranu'e-tawny or reddish brown ; the bas;d i)art of tiu- eosia and 

 a l)road stripe along the hind margin, extending three-l'ourths the length of the win--, and 

 touching the edge, black ; there are, besides, six black spots, namely, a large (piadrate one 

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

 over the end of the cell, aiul two between tlu> 1st and 2nd )nediau l)ranehes. Across the 

 wing, a little lieyond the middle, is a very irr(>gular l)right-yellow bi-lt, whieli begins on 

 the costa and terminates about the middle of the outer mar-in : this band -encn-ally 

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

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

 the wing, near the costa, is a larg(> ill-detined yellowish spot. BciirKlh, the same, excejjt 

 that there is on the outer margin, near the apex, a series of live whitish s])o1s, in somr 

 examples rcnluced in siz(> aiul number, in others altogether wanting. 



ITiiul ichi(jv\wvi and (piadrangular, tiie angularity nu)st pronounced in small examples ; 

 ahorc, same colour as the fore wing; there is a broad, black, zigzag stripe alon^• the disk, 

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

 marginal lunules ar(> wanting in some examples ; in others they are large, ami blended 

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

 yellow spot at the base of the costa. 



9. Similar to the 6 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 })artly over the surface. 



Extremely abundant all the yeai- round at Ega, in thinned parts of the ioresl ; init lound 

 in no other part of the Amazon region. It occurred in company with the ty{)ical M. 

 Pohjiiuno aiul numerous individuals exhibiting all the gradations between the two 

 extreme forms. 1 add descriptions of some of these internKHliate varieties. 



* I liave given a case in the first |)ai)er ])uljlislie(l of tliis series, " On the Insert Kanna iil' tlie .Vnia/.on Vallev," in 

 ' Transactions of tlie Entomological Society of London,' n. s. vol. v. |.. ,;.V1, PiiihUo I'l-rtumniix and P. Ilu-ror/cx. It 

 seems to me that, as only those local varieties become sjiecies which nniintain themselves distinct when hronirlit li\ 

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

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

 is a sign they have accpiired siifHcient difference to fill two separate sjdiercs ; if they paired together, thiy wuulil s("'ii 

 become one again. Nature may be said to place a |)reminin on diversity ; lor she thus destroys the incompletely finnuil 

 race, and preserves the completely formed one. 



The case oi Mechanitis Pohjmina differs from that i>l' Lepfalix Theonoe, in exhibiting the prodnclion, gencially, ol 

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

 frequent conrsc in nature. More than one new race would with dithcnlly be formed in a limited area, where (he 

 individuals live in close neighbourhood, except in such cases as our Leptulis, where rigid destruction of inlcrmcihiUe 

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

 there is no doubt the insects carefully select their exact counter]iarts in pairing. 



