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266 These he is always ready to assist and encourage to the extent of his ability. As will be seen from the foregoing sketch, he is an indefatigable worker, and, to the brief notices of his articles given above, space will only allow a few other references to the discoveries he has made and the theories he has advanced in the various lines of zoölogical and geological research.

When Agassiz came to this country, he brought with him not only an interest in zoölogical subjects, but, as well, that enthusiasm which made his name famous in connection with the study of glaciers. He pointed out the existence of local glaciers in the White Mountains, but Dr. Packard traced out further than ever before the extent of this local system, following these rivers of ice from Mount Washington and the adjacent peaks down the valleys of western Maine. This work on glaciers was still further elaborated in his large memoir on Labrador, mentioned above, and led to other speculations of a zoölogical rather than of a purely geological character.

These were that the existing insect fauna of at least the Northeastern United States had its origin from a circumpolar Tertiary fauna. The facts for this conclusion were in part the following: Oswald Heer and Dr. Asa Gray had conclusively shown that the plants of the same region had thus originated, the Tertiary rocks of Greenland containing many genera which are characteristic of the American flora of to-day. Now, as is well known, there is the most intimate connection between the distribution of many insects and the plants on which they feed, and the habitats of many insects can only be accounted for upon some such supposition. For these in detail the student should seek Dr. Packard's "Monograph of the Geometrid Moths," but we can mention one instance. Certain butterflies and moths are known to-day only from the colder regions. They are found in Labrador and farther north, while in the United States they only occur in the widely separated mountain-regions of New Hampshire and Colorado. These, it is assumed, must have lived near the edge of the great continental ice-sheet of glacial times, and must have occurred in all the intervening extent of country. As the ice retreated and the territory became warmer, the plants on which the larvæ fed could only find conditions favorable to their existence on these high mountain-regions or the isothermal but lower lands of Labrador. This view of the origin of the fauna of the United States has since been adopted by many writers, and receives its most complete exposition in Dr. A. R. Wallace's "Geographical Distribution of Animals," but without credit to Dr. Packard, who advanced it several years before.

In morphological work, the studies on the development of the sting of the bee must be mentioned. Dr. Packard pointed out