The Annelid theory of Dohrn and Semper was perhaps not so widely accepted as the rival Ascidian theory, but it counted not a few adherents and gave a certain stimulus to comparative morphology. F. M. Balfour, who pointed out about the same time as Semper the analogy between the nephridia of Annelids and the mesonephric tubules of Vertebrates,[405] while not accepting the actual theories of Dohrn and Semper, took up a distinctly favourable attitude to the general idea that Annelids and Vertebrates were descended from a common segmented ancestor. Discussing this question in his classical work on the development of Elasmobranch fishes,[406] Balfour came to the conclusion "that we must look for the ancestors of the Chordata, not in allies of the present Chaetopoda, but in a stock of segmented forms descended from the same unsegmented types as the Chaetopoda, but in which two lateral nerve-cords, like those of Nemertines, coalesced dorsally instead of ventrally to form a median nervous cord. This group of forms, if my suggestion as to their existence is well founded, appears now to have perished."[407] He held that while there was much to be said for the interchange of dorsal and ventral surfaces postulated by Dohrn and Semper, the difficulties involved in the supposition were too great; he preferred, therefore, to assume that the present Vertebrate mouth was primitive, and not a secondary formation. His views as to the phylogeny of the Chordata and the genetic relation of the various classes to one another are exhibited in the following schema,[408] names of hypothetical groups being printed in capitals, names of degenerate groups in italics:--