In the spring of 1830, Agassiz received the degree of Doctor of Medicine, being in his twenty-third year, and at the end of the same year left Munich for Switzerland, where he remained for a year working on the fossil fishes and fresh-water fishes, and practicing medicine as often as opportunity offered. But he was restless for the larger life to be found in the scientific circles of a great city, and in the autumn of 1831 started for Paris, though not without a certain dread of the future, as his financial prospects were anything but cheering.

But his scientific life in his new home was so inspiring that it repaid all the loss he suffered in personal deprivations. Humboldt and Cuvier received him with the greatest kindness, and the museum at Paris offered inexhaustible resources in the prosecution of his work on the fossil fishes. In this work Agassiz's aim was to determine to what geological period the different specimens belonged, and to trace the connection between the fishes of the past ages and those of the present time.

This was not an easy task, as in many cases it was impossible to obtain enough of the skeleton to distinguish the specimen without great difficulty. But Agassiz was undeterred by this circumstance. A tooth, a scale, or a spine served him as a guide into this wide field of research, and from these trifles his patient energy would reconstruct the entire skeleton, and bring back to life again, as it were, the dead animal which the long centuries had carve din stone. The importance of this work, which would service the twofold purpose of explaining the development of the different classes of fishes, and the succession of the layers of rock, as told by their presence or absence, was well understood by Agassiz, and it was a matter of great seriousness to him that his limited means should stand in the way of carrying on his studies to the best advantage.

During the year he spent in Paris he received a generous loan from Humboldt, whose high appreciation of Agassiz's talent never diminished, but Agassiz felt more and more the impossibility of depending upon chance for a livelihood, and in 1832 accepted a professorship at Neuchatel. His work on fossil fishes occupied him ten years, during which time he visited England, Germany, and France, for the of studying the fossils in the various museums.

The publication of the first volume in 1833 at once placed Agassiz among the greatest living naturalists, and was received with the most distinguished favor by all the scientific societies of Europe.

In this work Agassiz made the very important discovery that the natural succession of the different classes of fishes, as regarded their development, also corresponded with the succession of the geological epochs, as marked out by the recent studies in geology.

While this work was in progress Agassiz also made some very interesting studies on the nature of the action of glaciers. Up to this time the theory about those great fields of moving ice had been based upon the convulsionist theories of the older geologists, and the presence of vast ice fields, and great boulders, in places where there seemed no apparent reason for their existence, was explained by supposing that nature worked by fits and starts, and that there could be no other way of accounting for her actions.

But from the year 1836 to 1846 Agassiz visited all the glaciers of Europe, and studied them with the greatest care. In these excursions he was accompanied by other men of science, who gave him help in special ways, and he was thus able to make the most thorough study of glacial action. One member made a microscopic study of the red snow, and the animal life it contained, another studied the flowers, another the temperature of the interior of the glaciers, and another the deposits or débris left by this slow movement.

The geologists of all countries had long been puzzled over the presence of boulders, fossils, and the quantity of loose unstratified material called drift, which were scattered over various places, where their appearance did not correspond with the geological formation of the rocks, and Agassiz's bold theory of glacial action, which explained these phenomena on simple and reasonable grounds, was received with unmistakable satisfaction and admiration.

Agassiz laid aside the theory of sudden of nature, and claimed that the glacial phenomena could be explained upon principles more in harmony with the ordinary workings of nature; and his ten years' study of glaciers only confirmed a conclusion to which he had been led early in his investigations.

According to this theory, the whole of the northern continent was once covered with ice which extended from the North Pole to the boundaries of Central Europe and Asia. Before this ice period the whole of that region had been covered with a luxuriant vegetation and was inhabited by the great animals which are now found only in the torrid zone. Elephants, hippopotami, and enormous flesh-eating animals wandered through the vast forests, and the rivers which flowed into the Arctic Ocean were the haunts of fishes and waterfowl, now only to be found in the streams of the tropics.

This condition of things existed for long ages, during which the earth was covered with verdure from the equator to the uttermost north, and the teeming life of the tropics extended to the polar regions. Then, by degrees, the whole aspect of nature changed, and from some unknown source, cold succeeded heat, and death came to take the place of life. Lakes, seas, and rivers were frozen, and the myriad living creatures they contained were changed to inanimate forms; over the vast plains stretched a great mantle of ice, which touched the flowers, shrubs, and trees, as if by magic, and turned them to stone, while the huge beasts, wandering through the forests, or basking in the sunlight of the northern shores, were overtaken by the same dreadful fate, which spread a shroud over the living face of nature and turned a scene of beauty to ruin and desolation.

Ages after this catastrophe, the sun's beams melted the ice and snow, which slowly began their retreat toward the north, and the ice-fields and glaciers of Central Europe along remain to remind the student of nature that the story they tell was a living reality, and not the fanciful imagining of the poet or romancer.

The acceptance of this theory accounted for the presence in the Siberian rivers of those of gigantic animals whose counterparts are now to be found only in the tropics, and explained the appearance of fossils, boulders, and other deposits in places where their presence had been hitherto unexplainable. And although it was elaborated during the years when Agassiz was busy upon zoölogical studies of the gravest importance, it lacked nothing of that conciseness, vigor, and attention to detail which distinguished all the work of this master, who considered every part of creation of equal interest and found zoölogy and geology alike but the means of reading more clearly the great design of the universe.

In 1846 Agassiz came to the United States on a visit, having for its object the pursuit of his scientific studies. At this time, when his face was world-wide, his theories were nowhere received with greater enthusiasm than in America, and it was a matter for no surprise that two years after his landing in the New World he was offered the chair of Natural History in Harvard University.

From that time his scientific work was confined to the continent and islands of America, and his many journeys, having for their object the study of zoölogy and geology, were all made in the interests of science in connection with those studies in the New World.