pods (1858-81): of this period is to be mentioned his beautiful monograph on the pantopods. In 1875 appeared his paper "On the Origin of the Vertebrata and on the Principle of the Change of Function"; it was a small brochure, but it touched with a master's hand some of the great problems of its day. This paper Brooks declared should be read by every one of his pupils before he would give him his doctorate—this in spite of Brooks's lack of sympathy with the tenets which Dohrn had laid down. It was this paper which paved his way for researches which were to extend over a third of a century. And one may follow the development of the subject—and of Dohrn himself—in a series of twenty-five voluminous memoirs.
It was the "momentous problem of the beginnings of the back-boned animals" which Dohrn sought to solve. And upon this matter his writings are encyclopedic. Vertebrates were to him descendants of chætopod worms; their typical organs did not arise de novo but as transformations of organs having a different function; animals of simple structures were not as often primitive as Haeckel, for example, would teach, but were frequently degenerate; Amphioxus and Ascidians were not in the line of descent of the higher vertebrates, nor were lampreys—the latter were rather degenerate bony-fishes. In his early papers his readers are carried along on the crest of a splendid wave of enthusiasm. "The problem is solved, . . . all but solved, . . . soon to be solved." But this stage of conviction comes to pass darkly into the background, new difficulties keep appearing, and in later papers one hardly realizes the genetic bearing of the data which Dohrn is bringing together. The method of the paper of 1875, which was essentially deductive, gradually gave place to an elaborate inductive method; separate organs of the vertebrata are studied in turn, especially in the head region, and the difficulties are traced gradually into finer and finer ramifications, until in the end he deals with what appears to be purely the puzzle of nerve histogenesis. For Dohrn would not admit that the head problem in the vertebrate was not to be solved with the present materials or by the aid of the embryo-
