84 Mr, Huxley on the Progressive [April 20, 



raneous beds. Lingulce^ again, are very aberrant BracMopoda, 

 in nowise comparable to the embryonic forms of any mollusk ; 

 Phyllopods are the highest Entomostraca ; and the Hymenocaris 

 vermicauda discovered by Mr. Salter in the Lingula beds, is closely 

 allied to Nebalia, the highest Phyllopod and that which approaches 

 most nearly to the Podopthalmia. And just as Hymenocaris stands 

 between the other Entomostraca and the Podopthalmia, so the 

 Trilobita stand between the Eidjmostraca and the Edriopthalmia. 

 Nor can anything be less founded than the comparison of the Trilo- 

 bita with embryonic forms of Crustacea ; the early development of 

 the ventral surface and its appendages being characteristic of the 

 latter ; while it is precisely these parts which have not yet been 

 discovered in the Trilobita, the dorsal surface, last formed in order 

 of development, being extremely well developed. 



The hivertebrata of the earliest period, then, afford no ground 

 for the Progressionist doctrine. Do the Vertebrata ? 



These are cartilaginous fish. Now Mr. Huxley pointed out that 

 it is admitted on all sides that the brain, organs of sense, and re- 

 productive apparatus, are much more highly developed in these 

 fishes than any others ; and he quoted the authority of Prof. Owen,* 

 to the effect that no great weight^is to be placed upon the cartilagi- 

 nous nature of the skeleton as an embryonic character. There 

 remained, therefore, only the heterocercality of the tail, upon which 

 so much stress has been laid by Prof. Agassiz. The argument 

 made use of by this philosopher may be thus shortly stated : — 

 Homocercal fishes have in their embryonic state heterocercal tails ; 

 therefore, heterocercality is, so far, a mark of an embryonic state as 

 compared with homocercality ; and the earlier, heterocercal fish are 

 embryonic as compared with the later, homocercal. 



The whole of this argum.ent was based upon M. Yogt's examina- 

 tion of the development of the Coregonus, one of the Salmonidce ; 

 the tail of Coregonus being found to pass through a so-called hetero- 

 cercal state in its passage to its perfect form.f For the argument 

 to have any validity, however, two conditions are necessary. 

 1. That the tails of the Salmonidce should be homocercal, in the 

 same sense as those of other homocercal fish. 2. That they should 

 be really heterocercal, and not homocercal, in their earliest con- 

 dition. On examination, however, it turns out that neither of these 

 conditions holds good. In the first place, the tails of the Salmonidce, 

 and very probably of all the Physostomi are not homocercal at all, 

 but to all intents and purposes intensely heterocercal : the chorda 

 dorsalis in the Salmon, for instance, stretching far into the upper 

 lobe of the tail. The wide difference of this structure from true 

 homocercality is at once obvious, if the tails of the Salmonidce be 



♦ Lectures on the Comparative Anatomy of the Vertebrata, pp. 146-7. 



t Von BUr had already pointed out this circumstance in Cyprinus, and the 

 rqhition of the foetal tail to the permanent condition in cartilaginous fishes. — See 

 his ** Entwickelungsgeschichte der Fische," p. 36. 



