Mr. Huxley on the Progressive Development of Animal Life. 71 



The Invertebrata of the earliest period, then, aiford 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 repro- 

 ductive 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 cartilaginous 

 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, hetero- 

 cercality 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 argument was based upon M. Vogt'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 SalmonidcB 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 condition. 

 On examination, however, it turns out that neither of these con- 

 ditions hold good. In the first place, the tails of the Salmonidcey 

 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 SalmonidcB be 

 compared with those of Scomber scombrusy Gadus ceglefinuSi &c. 

 In the latter, the tail is truly homocercal, the rays of the caudal fin 

 being arranged symmetrically above and below the axis of the spinal 

 column. 



All M. Vogt's evidence, therefore, goes to show merely that a 

 heterocercal fish is heterocercal at a given period of embryonic life ; 

 and in no way affects the truly homocercal fishes. 



In the second place, it appears to have been forgotten that, as 

 M. Vogt's own excellent observations abundantly demonstrate, this 

 heterocercal state of the tail is a comparatively late one in Coregonus^ 

 and that, at first, the tail is perfectly symmetrical, i. e, homocercal. 



In fact, all the evidence on fish development which we possess, is 

 to the effect that Homocercahty is the younger, Heterocercahty the 

 more advanced condition : a result which is diametrically opposed 

 to that which has so long passed current, but which is in perfect 

 accordance with the ordinary laws of development ; the asymmetri- 



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



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

 the relation of the fcetal tail to the permanent condition in cartilaginous 

 fishes. See his " Entvvickelungsgeschichte der Fische/* p. d^. 



