( Originally Published 1917 )
Migration seems to be a general instinct in the animal world, developed when a species becomes enormously abundant. At such times this instinct apparently over-comes all others and the creatures move on regardless of obstacles and conditions that may mean certain death to the vast majority. Such migrations among mammals have often been recorded, one of the most notable examples being that of the little lemmings which migrate at periodical intervals in a way which has often been described. Among the insects such migrations have been frequently noticed, and the phenomenon has apparently been observed oftener among the butterflies than in any other group. Entomological literature during the last hundred years contains a great many records of enormous flights of butterflies over long distances, extending even from Africa into Europe or from one part of America to another far remote. As such migration is likely to happen whenever a species becomes extremely abundant it probably is Nature's way of providing for an extended food supply for the succeeding generations. That it results in the death of the great majority of the migrants is doubtless true, but it must lead to vast experiments in extending the geographic area inhabited by these species. Numerous examples of such migrating swarms will be found in the pages of this little book.
The migrations thus considered are only exceptional occurrences. There is, however, a regularly recurring annual migration on the part of some butterflies which is also a phenomenon of extraordinary interest. The most notable example is that of the Monarch which apparently follows the birds southward every autumn and comes northward again in spring. There is much evidence to indicate that in some slight degree other butterflies have a similar habit, although the present observations are inadequate to determine to what extent this habit has be-come fixed in most of these species.