The Challenger left Sheerness December 17, 1872, and crossed the Atlantic four times, making a course of nearly twenty thousand miles during 1873; in 1874 she went southward from the Cape of Good Hope, spent nearly a month among the southern ice, dipping into the Antarctic Circle as far as she could with safety, then traversed the seas of Australia and New Zealand, made observations in the Malay Archipelago, and reached Hong-Kong in November, after making a course of more than seventeen thousand miles; in 1875 she traversed the Pacific, making a course of about twenty thousand miles, and in the early part of 1876 she crossed the Atlantic for the fifth time, to fill up blanks in her former observations, finally reaching England in May.
During this course of 68,890 miles, 362 stations were established, and observations and collections made at them. The magnitude of the collections is illustrated in a statement made by Professor Alexander Agassiz, that, "if a single individual, having the knowledge of eighteen or twenty of the specialists into whose hands they were to be placed, were to work them up, he would most certainly require from seventy to seventy-five years of hard work to bring out the results which the careful study of the different departments ought to yield." They were assigned to various gentlemen recognized as authorities in different departments for description group by group.
The most prominent and remarkable result of the voyage was the final establishment of the fact that the distribution of living beings has no depth-limit, but that animals of all the marine invertebrate classes, and probably others also, exist over the whole of the flora of the ocean. But, although life is thus universally extended, probably the number of species, as of individuals, diminishes after a certain depth is reached.
Professor Thomson had been led by his researches in the Lightning to the belief that the chief formation now going on in the bed of the Atlantic was a chalk, "the chalk of the Cretaceous period." This belief grew more firm with continued investigations, but was modified after the Challenger Expedition, when the species deposited were found to be in very few instances identical with those of the chalk, or even with those of the modern tertiaries. "But," he added, in his address on the subject before the British Association, in 1876, "although the species, as we usually regard species, are not identical, the general character of the assemblage of animals is much more nearly allied to the cretaceous than to any recent fauna."
Professor Thomson had been elected in 1870, previous to the dispatch of the Challenger Expedition, Professor of Natural History in the University of Edinburgh, succeeding Dr. Allman. He was only relieved from duties during the expedition, and held the professorship until October, 1881, when he resigned it upon a retiring allowance granted him by the Senatus. Immediately on Professor Thomson's