northern, and when the group extends into the new world as well, 

 the conclusion that its original home was in the far north, and that 

 it has spread downwards is irresistible. Such a group is that under 

 present consideration, and the Coliads would furnish another similar 

 instance. Of course, the formation of the contments of both 

 hemispheres has undergone great changes since the dispersal of 

 these groups began, but these changes are not so great as to 

 discount the extreme probability that the migration took place from 

 east to west in the case of genera predominant in America, and 

 from west to east in those predominant in Asia and Europe. The 

 genus Melit((a comes under the latter head, and it is fairly certain 

 that we must look for its ancestral home in the far north of Asia. 

 The entire absence of butterflies from Iceland, and even the lack of 

 any indigenous butterfly in the Shetlands, certainly both point to 

 the Siberia-Alaska route as the only one available for the stream of 

 migration in whichever direction, so far as the two hemispheres are 

 concerned ; but except when impeded by some impassable barrier, a 

 species, or some members of a group of species, would probably 

 spread in every available direction, as was evidently the case with 

 the genus now under consideration. The forms which inhabit the 

 mountains would be the first to spread along the mountain-chains, 

 reaching greater and greater elevations as they progressed south- 

 wards, and sending down offshoots to the lower slopes and gradually 

 to the plains below. Many of these modified forms might reason- 

 ably be expected to confine themselves to small, or comparatively 

 small districts, and might even be found at great distances apart 

 when similarity of climate, food- plants, and environment generally, 

 had tended to guide evolution on similar lines ; others, again, which 

 had originally been suitably placed and had developed into a strong 

 and aggressive stock, might again spread widely, even though they 

 were not constitutionally fitted for life in an extremely cold climate, 

 since it would not be necessary for them, being developed further 

 south, to go again into high northern latitudes or to cross the higher 

 passes of the mountains. We shall expect then in these cases that 

 the more ancestral forms will be found high on the mountain 

 ranges in widely separated localities, that many forms of later 

 development will be restricted in their localities, especially if those 

 localities are among mountain masses, that comparatively recent 

 forms will in some instances be wide- spread, but that their distribu- 

 tion will be continuous, whilst the most modern of all (probably the 

 most southern) will again be very restricted in their habitats. 



Now, does the distribution of the species of Melitaa tally in any 

 degree with these a priori expectations ? I think it will be found to 

 do so to a remarkable extent. Three species are common to the 

 high mountains of Europe and Asia, vierope, asteria and varia, three 

 of the most ancestral forms of the genus ; four only of the lowland 

 or sub-alpine species are to be found from the east to the west of 

 the old world, athalia, dictynna, j^ha-be &nd didyuia, all of which may 



