840 POPULAR SCIENCE MONTHLY.
been constructed, and was compelled to ask to be relieved of the work of teaching at the end of the year 1892. His successor as director of the institute was appointed at the beginning of the sum- mer semester of 1893, and his own death followed shortly after- ward.
The versatility by which Professor Semper was distinguished, and of which we have spoken at the beginning of this sketch, was rarely favored, in Professor Schuberg's view, by his long sojourn in the tropics; for when one is so situated, as he was then, as to be able to spend seven years and a half in the study of the exceedingly diversified and interesting animal forms of luxuriant tropical nature, without being concerned about outside conditions and without hav- ing any other duties, he enjoys facilities and is assisted to an extent which few zoologists can hope for; and he used these opportunities in a manner which attests his extraordinary energy and his capacity to give equal and impartial attention to every branch of his science. His earlier works, before going to the Philippine Islands, and his first researches there were in comparative morphology and histology. As his investigations continued, they were extended to numerous and diversified animal groups, and gave rise to many important dis- coveries. Of special interest among his publications concerning these researches were his papers on the origin of coral reefs, on the Trochosphcera, and on the alternating generations of stone corals. Of special permanent value likewise are his monographs on holo- thuria and land mollusks. He busied himself, too, with questions of geographical distribution and general biology; and he is credited by Professor Schuberg with having contributed much to the build- ing up of Darwin and Wallace's doctrine of descent, by his efforts to bring some of the questions nearer solution, and by his objective criticisms. Two works are especially mentioned which advanced the discussion of the questions raised by the theory of descent. One of these embodies a series of connected investigations, the results of which go to close a gap which had to be bridged if the theory of de- scent were to be set upon a stable foundation. Professor Semper, almost simultaneously with the Englishman Balfour, had made the important discovery of the presence of structures in sharks, both embryos and full grown, which attested a conformity of the struc- ture of the urogenital system of vertebrates with that of the anne- lids; and he believed that he had at last found the bridge which was to be laid from the vertebrate to the invertebrate type. Thus arose, in the building up of the theory based upon this first observation, a series of researches, in which he tried to demonstrate a conformity in structure of the vertebrates and the articulate worms for other organs than the urogenital apparatus, and materially promoted the