The next spring, the Lawrence Scientific School was organized at Cambridge, in connection with Harvard University, and Agassiz was offered the chair of Natural History (zoölogy and geology), with a salary of fifteen hundred dollars. The school owed its existence to Abbott Lawrence, formerly our minister to England. Agassiz accepted the position, and opened his first course in April, 1848. Here he found congenial friends, Longfellow, Lowell, Prescott, Motley, Gray, Holmes, and others. M. Christinot, who had so generously helped to send him to Paris years before, came to the Cambridge home and was put in charge of it. "If your old friend," he said, "can live with his son Louis, it will be the height of his happiness." The small plot of ground about the house became a zoölogical garden, with its tank for turtles and an alligator, its cage for eagles, a tame bear, and a family of opossums. Agassiz had already begun his Museum of Comparative Zoölogy, on the banks of the Charles River, in an old shanty. The outlook was hopeful; but he was sad at heart, for Cecile, his wife, had died since he came to America, and his children seemed too young to bring into a home where there was no mother. In the summer of 1848, Agassiz organized an expedition of students and naturalists for the examination of the eastern and northern shores of Lake Superior. At Niagara, he saw for the first time a living garpike, the only representative among modern fishes of the fossil type of Lepidosteus. He made a careful study of the fauna and geology of the lake, and the results were published in a book. Charles Darwin wrote, "I have seldom been more deeply gratified than by receiving your most kind present of 'Lake Superior.' ... I had heard of it, and had much wished to read it, but I confess it was the very great honor of having in my possession a work with your autograph as a presentation copy that has given me such lively and sincere pleasure." Agassiz had published another book in America, in 1848, "Principles of Zoölogy," which had a large sale, and was much used in schools. In 1849, his only son, fifteen years old, came to live with his father. The following year, 1850, Agassiz married Elizabeth Cabot Gary, of Boston, a cultivated and lovely woman. His daughters, much younger than their brother, arrived from Europe the same year. M. Christinot, though urged to remain, now preferred to find another home, settled in New Orleans as pastor, and later died in Switzerland.