220 The Succession and Development of Animal Organization, 



without giving a right to deduce from that negative evidence 

 any conclusion as to the absence of those animals. 



From the foregoing remarks it follows that our knowledge of 

 the former species of organic beings is imperfect, and that it will 

 ever be so, even when it is enlarged and newly remodelled by 

 the most splendid future discoveries. General comparisons must 

 thus be restrained to some classes and groups. Such are, in the 

 animal kingdom, the reptiles, fishes, and the mollusks (chiefly 

 the Cephalopods, the Conehifera, and the Brachiopods), the 

 Echinodcrmata, and the Corals. A comparison in such a limited 

 direction will certainly give some interesting results. A funda- 

 mental point for these investigations has already been gained in 

 the conclusion, deduced from a great number of facts, that the 

 different formations are characterized by their respective fossils*, 

 which, indeed, is but another formula for the statement that the 

 various species have a distinct term of duration, and that their 

 existence ended sooner or later. It will also be seen that the 

 oldest strata contain remains chiefly of non-vertebrate animals, 

 that only in later strata a greater number of Vertebrata appear, 

 and that in the strata which embrace the Lower New lied Sand- 

 stone, up to the Chalk, rej)tilcs (chiefly Sauria) are predominant. 

 It is first in Tertiary strata that the remains of Mammalia be- 

 come numerous, of which class, as we have already said, remains 

 are indeed not entirely absent in older strata, but are in that 

 case in a subordinate proportion to the remains of reptiles -f. 



* The late Prof. Jameson remarks that Werner, his master, already made 

 the observation that "different formations can be discriminated by the 

 petrifactions they contain, that petrifactions appear first in transition rocks, 

 that these are but few in number and of animals of the zoophytic or testa- 

 ceous classes. In the older floetz rocks they are of more perfect species, 

 as of fish or amphibious animals ; and in the newest floetz and alluvial 

 rocks, of birds and quadrupeds, or animals of the most perfect kind." See 

 his notes following his translation of the ' Discours ' of Cuvier, ' Essay on 

 the Theory of the Earth,' 3rd ed. Edinb. 1817, pp. 232, 233. But already, 

 long before Werner, as is stated by Humboldt (Essai geognostique sur le 

 Gisement des Roches, Paris et Strasbourg, 1826, 8vo, p. 37), the first point 

 — that diiFerent formations can be distinguished by their fossils — was ac- 

 knowledged by Lister in reference to fossil shells. It is this peculiarity 

 which gave occasion to the so-named Coquilles caracteristiques of French 

 authors, or Zeitmuscheln, as they are named by the German geologists, 

 which were duly appreciated by the great Leopold von Buch in several of 

 his latest papers. 



t These general remarks on the succession of animal life at the surface 

 of our globe were proposed, in 1841, by the eminent paleontologist, 

 L. Agassiz, in his address at the inauguration of the University of Neuf- 

 chatel, ' De la Succession et du Developpement des litres organises a la 

 surface du Globe terrestre dans les differents ages de la Nature ' (Neu- 

 chatel, 1841, Svo). In this work we have the periods, (1) of Fishes, (2) of 

 Reptiles, and (3) of Birds. 



