The Principles of Palaeontology. 185 



But naturalists are less ambitious, and often evidence even 

 wanting in precision is suiRcient for nearly exact conclusions. 

 They would be extremely fortunate to ascertain the terms of 

 transition which must have existed between the grand divisions, 

 and which would permit the establishment, otherwise than by 

 theoretic inductions,, of the origin of classes and orders. But 

 it is precisely these interesting forms which are of tenest want- 

 ing. This is the case with the Birds, Chelonians, Lamellibranchs, 

 Cephalopods, Brachiopods, Trilobites, Corals, Sponges, not to 

 mention those whose ancient types are known with some detail. 

 The method of geologic continuity is, in such instances, 

 altogether defective. 



Order of appearance of new forms. — The difficulty is some- 

 times even still greater, and the chronologic order of appearance 

 is in complete discordance with what we might be permitted to 

 expect from the method of Comparative Anatomy applied to 

 Palaeontology. If the ontogenic evolution is unknown, and if 

 we can not bring in evidence any fact of regression, we are 

 obliged to reserve our conclusion for the epoch when new evi- 

 dence shall permit us to elucidate the question ; until then we 

 should formulate hypotheses with great reserve. Thus the suc- 

 cession of forms among the Brachiopods, the appearance of the 

 orders of Mammals, etc., raise difficulties of this kind. 



In a general way, the result of recent discoveries has been 

 to cause us to refer to epochs more and more remote the pre- 

 sumable date of appearance of the various groups. To confine 

 ourselves to facts most recently brought to light, we will cite 

 the discovery, by Brady, of Nummulites in the Carboniferous ; 

 that of Ammonites in the Per mo-carboniferous system (Waagen, 

 Gemellaro) ; that of Sponges, belonging to the types of Hexac- 

 tinellids and of Tetractinellids in the Cambrian (Hinde) ; of the 

 Arachnids and Insects in the Silurian ; of the Myriapods in the 

 Carboniferous (Scudder, flagen, Ch. Brongniart, Fayol) ; Fishes 

 have lately been found in the Middle Silurian (Kohon). 



It can be seen how dangerous it is to attach an exclusive im- 

 portance to the order of appearance; every theory which in any 

 way rests on these data, when the question concerns the pri- 

 mordial forms of large groups, incurs the risk of being ere long 

 contradicted by some unexpected discovery. 

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