His original way of dealing with subjects is well illustrated in his studies of fossil bivalve mollusca. It had been customary to describe the external markings of the shell and when possible the muscular impressions within. Agassiz soon realized the importance of studying the interior contour of the shell, and forthwith proceeded, by means of casts, to bring to light the relations of these fossils with their living representatives. His maxim was to have abundant material—thousands of specimens, if necessary—for any proper research. In studying glaciers he literally rode on the back of one for weeks at a time. He furthermore urged his students to read all that had been written on a subject before publishing.
Agassiz not only defined many new species of animals in various classes, but he was continually dwelling on the affinities and homologies among the various groups; more particularly their classification and their geographical and stratigraphical distribution. His studies in embryology and his familiarity with the work of Von Baer led him to recognize the general truth that the young of higher animals in their respective groups resembled the mature forms of animals lower down in the scale. From these studies he soon grasped the greater conception that this principle was carried out in time as well, and that fossil animals in the early horizons of geological history resembled the embryonic or early condition of higher animals now living and hence the idea of comprehensive or prophetic types. This same broad grasp of fundamental principles was remarkably illustrated in his studies of glacial phenomena in the Alps. One of his biographers says, "With his power of quick perception, his unmatched memory, his perspicacity, and acuteness, his way of classifying, judging and marshaling facts, Agassiz promptly learned the whole mass of irresistible arguments collected patiently during seven years by Charpentier and Venetz, and with his insatiable appetite and that faculty of assimilation which he possessed in such a wonderful degree he digested the whole doctrine of the glaciers in a few weeks," and added a great many new and important facts.
From his study of the glaciers of the Alps he soon announced his belief that the whole northern hemisphere had at one time been covered by an ice sheet. The various records of this vast sea of ice which had been interpreted by the most eminent geologists as the result of diluvial action and flowing mud he rightly attributed to the action of ice. In the face of the most strenuous and even bitter opposition he triumphantly established the former existence of the Great Ice Age. Subsequent studies, while modifying the limitation of the great ice sheet, have only strengthened the views of Agassiz.
Agrassiz's absorbing interest in the structural relations of animals led him to define with greater accuracy the limitation of various groups.
