;02 DEVELOPMENT OF ANIMAL LIFE IN TIME 



The Progression theory, however, after all, resolves itself very 

 nearly into a question of the structure of fish-tails. If, in fact, we 

 enumerate the oldest known undoubted animal remains, we find them 

 to be Graptolites, LingulcE, Phyllopoda, Trilobites, and Cartilaginous 

 fishes. 



The Graptolites, whether we regard them as Hydrozoa, Anthozoa, 

 or Polyzoa (and the recent discoveries of Mr. Logan would strongly 

 favour the opinion that they belong to the last division), are certainly 

 in no respect embryonic forms. Nor have any traces of Spongiadce 

 or Foraininifera (creatures unquestionably far below them in organiza- 

 tion), been yet found in the same or contemporaneous beds. Lingulcs, 

 again, are very aberrant Brachiopoda, in nowise comparable to the 

 embryonic forms of any mollusk ; Phyllopods are the highest Ento- 

 iiiostraca ; and the Hyiiienocaris veruiicauda discovered by Mr. Salter 

 in the Lingula beds, is closely allied to Nebalia, the highest Phyllopod, 

 and that which approaches most nearly to the Podopthaliiiia. And 

 just as Hyinenocaris stands between the other Entoviostraca and the 

 Podopthaliiiia, so the Trilobita stand between the Entoinostraca and the 

 Edriopthalviia. Nor can anything be less founded than the com- 

 parison of the Trilobita with embryonic forms of Crustacea ; the 

 earl}^ 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 Invertebrata 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 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. 



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



