306 THE POPULAR SCIENCE MONTHLY 



the adult, replaced by almost smooth ridges. Now, Semon in his 

 beautiful studies on the development of Neoceratodus* has shown that 

 the teeth of this fish at one stage in ontogeny, are represented by rows 

 of denticles even more discrete than the denticles in the Devonic Dip- 

 terus; then the denticles gradually merge at their bases, the separate 

 cusps, however, still showing — a stage comparable with the Carbonifer- 

 ous Ctenodus; then they merge still more and assume the ridge-like 

 form seen in the adult Neoceratodus. 



Another example : In many sharks the alimentary canal is 

 longer in the embryo than in the adult, the anal opening being situ- 

 ated near the posterior end of the trunk. From such cases one is in- 

 clined to believe that in the ancestral sharks this must have been the 

 condition in the adult form; that is to say, the anal opening probably 

 was near the posterior termination of the trunk. We may therefore 

 ask: are there any early fossil sharks which show such a condition? 

 Recently Professor Dean has described 5 a remarkable specimen of 

 Cladoselache from the Upper Devonic of Ohio which seems to indicate 

 such a condition. In this specimen remnants of both kidneys are pre- 

 served. They extend in the posterior half of the fish and by their direc- 

 tion indicate that they were drawn together, toward their external 

 opening, not far from the posterior termination of the trunk. This 

 shows that the anal opening in this ancestral shark was very much as in 

 the early shark embryo to-day. 



In conclusion perhaps I may venture to make one other point 

 in regard to this question. A vast amount of skepticism concern- 

 ing the doctrine of recapitulation is to be found in the literature 

 of to-day ; and if we study the reasons for this skepticism we find 

 that it is in some measure justified. It is clearly established that among 

 vertebrates as well as among invertebrates there are many examples of 

 structures appearing during embryonic growth which are identical 

 with structures found in the adult of some remote ancestor. But 

 when we reflect on the amount of adaptation which any embiwo 

 has undergone in its long evolutional history; when we remember 

 how palingenetic characters are on every hand overlaid by ceno- 

 genetic ones ; who will say that recapitulation is a principle of gen- 

 eral application, or that it is safe to draw conclusions from all em- 

 bryos concerning their long extinct ancestors? Who will believe that 

 a bony fish which runs through its embryonic development in a few 

 days repeats its ancestral history, when we see at every stage of its 

 ontogeny how it has been adaptively modified for this and for that 

 special need? Only when series of related forms have certain onto- 



4 " Die Zahnentwickelung des Ceratodus forsteri," " Zool. Forsch. in Austral. 

 u. Malay. Archipel.," 1899, pp. 115-135, pis. xviii-xx. 



5 Mem. Amer. Mus. Nat. Hist., Vol. IX.. p. 232. 



