ZOOGENESIS 



ing forms in adjacent areas over a broad territory, or 

 in a very narrow belt, or they may be wholly distinct 

 though very similar. This last is especially the case 

 if they occur on islands. Thus in one of the common 

 Aristolochia (or pipe-vine) swallowtails a single sub- 

 species is found from the south Atlantic states through 

 Central and South America as far as Buenos Aires, 

 while on each of the West Indian islands there is a 

 well marked and distinctive form. Very many sub- 

 species have been described, especially in birds, mam- 

 mals, butterflies and mollusks. 



Geographical variation usually affects both sexes 

 equally, the sexes varying together. Thus the com- 

 mon northern butterfly known as the white admiral 

 in southern New England and New York in both sexes 

 passes over into a southern form which is much larger 

 and in which the conspicuous white band is wholly 

 lacking, while the hind wings are slightly angulated. 



But sometimes one sex only is affected, or one sex 

 is affected to a much greater extent than is the other. 

 In the northern United States and Canada the common 

 yellow swallowtail is rather small, and both sexes 

 are almost alike in color. From New York south- 

 ward the insect is much larger. The males remain 

 almost the same in color, but the females become 

 black, or if remaining yellow they acquire a large 

 amount of blue on the hind wings. In the extreme 

 south they are always black, and in some regions 

 there are more or less well marked intermediate forms 

 between the black and yellow females. 



In several Indo-Malayan swallowtails the males are 



^37] 



