Page:Popular Science Monthly Volume 27.djvu/394

378 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 Mississippi. The "cotton-worm moth," which, in its caterpillar state, inflicts great damage at times upon the plantations, is a case in point. Individual specimens or flocks of other moths, such as the "great eyespot" (Erebus odora), the "blue and green hawk" (Argeus labruscæ), 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, perhaps, 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 vegetables 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 Alleghanies 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 valleys 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