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��THE SCIENTIFIC MONTHLY
��all these now familiar phenomena as observed in other insects — ^aroused the conviction that my correspondent must have been stationed upon some such crowded avenue of travel as has alreadv been described in other parts of the country. Only a cursory glance at the relief map suf- fices to show the geographical features responsible for these recurring spectacles. For the Ozark Mountains, extending from the northeast to the southwest, lie at right angles to, and form a barrier across, the great sulphur's southeastward migration. So, held back by the hills, they are forced to turn aside and seek an egress to the warmer zone beyond these obstacles. But not until the gateway of the Arkansas Valley opens are the flocking multitudes able to move in the desired direction, and to stream past Mt. Nebo, which stands on the valley border, only to scatter again, no doubt, and move in more open order after passing the contracted flight-way of the hills.
���Fig. 5. Early Spbixq Appearances op *' Monarchs " in the North. Their distribution shows that spring routes toward the North are probably some'wliat similar to the flight-lines of autumn. It is significant that, in Europe, both hutterffff and dragonfly records of Spring are the most profuse : here, the autumn flights are the greatest. (Map from the V. S. Geological Survey.)
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