1302 THE BUTTERFLIES OF NEW ENGLAND. 



Dimorphism. This InitterHy presents one of the most interesting exam- 

 ples of dimorphism known, since it is limited not only sexually but geo- 

 graphicallj' ; it is found only where the insect develops more than one 

 brood, at first, whei-e there are but two broods annually, in a very feeble 

 way, but subsequently, and apparently only when a third brood becomes 

 common, to such an extent that nearly or quite all the females share it and 

 the species becomes completely antigenic. Now, though it appears to 

 require the occurrence of a third or at least a second brood to develop this 

 antigenic quality (which consists in almost complete melanochroism), the 

 feature is in no way confined to these later generations but occurs, accord- 

 ing to Edwards, to an equal extent in the spring brood from wintering 

 chrysalids. From its geographical limitation to regions where the species 

 is more than single brooded, we should naturally presume that this varia- 

 tion first arose in a summer brood. That it should have extended so as to 

 include the spring brood to an equal extent, as appears to be the case, and 

 yet never to include the male nor to have spread to monogoneutic regions 

 is certainly surprising and demands study. It does not appear to me that 

 there is the slightest ground for Weismann's presumption that this is a case 

 of sexual selection, and Edwards seems to doubt it, since he says that "the 

 yellow females taken by me at Coalburgh have as surely been fertilized as 

 the blacks and have as readily laid eggs ; and on the w'mg the males may 

 be seen coquetting with the yellow as freely as with the blacks." Why is 

 it not rather a case similar to others of protective resemblance or of mimi- 

 cry, which is not infrequently confined to the female as to the most needy? 

 Surely the black female must be a less conspicuous object than its gaily- 

 banded sister to an insectivorous enemy ; and its restriction to the south is 

 in keeping with the greater abundance of examjjles of protective coloring 

 in the south (where insectivorous creatures more abound) than in the 

 noi'th. 



The exact distribution and comparative abundance of this black female 



has been partially told above, but the following extract from Edwards's 



Butterflies of North America, vol. ii., will still further elucidate the 



matter : — 



In Turnus the males are always yellow, and to the north of a certain latitude, about 

 41° 30' on the Hudson river, and 42° 30' in Wisconsin, all the females are yellow. 

 Below these lines, as one goes southward, the black females appear, at flrst but rarely, 

 then increase gradually in proportion to the yellow, until an equilibrium is somewhere 

 reached, apparently between 39° and 38°. But I cannot learn that, after that, the black 

 everywhere continue to increase at the expense of the yellow, though they seem to do 

 80 in certain districts or large sections of country. In this part of West Virginia, lat. 

 38°, I have often taken yellow females in the garden and field, and while they seem to 

 be never so common as the black, yet they cannot in most seasons be called at all 

 uncommon. But I am certain that in some years, or rather in particular broods of 

 some years, the black form does greatly outnumber the other. This was so in mid- 

 summer of 1876. For some cause the species was exceedingly scarce in the spring of 

 that year, quite the reverse of what usuaUy happens. During the month of July, how- 



