378 THE POPULAR SCIEXCE MONTHLY. 



If we take a map giving a bird's-eye view of the continent, with 

 the elevations marked, we can understand the problem better. Ranges 

 of mountains obstruct, valleys and river-channels assist, the dispersion 

 of moths. They travel on the wings of the wind, and an important 

 factor is the prevailing seasonal direction of the air-currents. There 

 is in North America a summer migration of many species from the 

 South to the North, so that, toward the autumn, several tropical kinds 

 have crept up along the coast, or inland, up the valley of the Missis- 

 sippi. The " cotton-worm moth," which, in its caterpillar state, in- 

 flicts great damage at times upon the plantations, is a case in point. 

 Individual specimens or flocks of other moths, such as the " great eye- 

 spot " {Erebus odora), the " blue and green hawk " (Argeus labritscce), 

 visit us yearly, coming up from the West Indies. They die out in the 

 winter here, and leave no progeny behind them to continue the species 

 in our high latitudes. 



Rivers assist in the dispersion of insects, and, in a less degree, per- 

 haps, the particular insects we are here discussing. Nevertheless, upon 

 leaves and sticks the eggs of moths are floated on the current, while 

 the commerce of the water-routes brings the cocoons with the vege- 

 tables and fruits which it carries from place to place in boats and ships. 



A bird's-eye view of our continent shows us the elevations of the 

 Rocky Mountains and parallel spurs in the West, and the Allegha- 

 nies in the East. Mountain-ranges stand in the way of the spreading 

 of moths, which perish in the cold atmosphere and the storms which 

 gather about the rocky summits. Our fauna? can be understood by 

 studying the formation of the land in this way. Over the vast plain 

 east of Colorado the same kinds of moths generally prevail. The val- 

 leys in the West, on the other hand, contain a majority of peculiar 

 species or kinds, often more local than in the East. In New York we 

 are cut off, again, by the Alleghanies from many species which are 

 plentiful in Ohio and Indiana. Our tropical wanderers come to us up 

 and along the coast. I have met, sailing on the Gulf Stream, flights of 

 moths, mostly of a few kinds, which fell on the rigging and sides of 

 the vessel in great numbers. In the autumn, on Staten Island, I have 

 captured specimens whose true home was Cuba and Jamaica. Although 

 smaller fauna?, or limits of particular species, are traced by naturalists, 

 our mountain-ranges are the best general guide as to the changes in 

 the sorts of moths which we may expect. From Ohio to Louisiana we 

 meet much the same kind of moths, with a difference in the rarity of 

 certain species, and in the presence of others dependent on particular 

 kinds of plants. But, when we get into the valleys of the Rocky 

 Mountains, we shall have taken leave of the most of our dusty-winged 

 Eastern friends. Some kinds take the voyage with us completely 

 across the continent, but these are comparatively few in number, and 

 are sometimes almost cosmopolitan. 



So true is it that one branch of a subject leads to quite different 



