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 Conchifera, and the Brachiopods), the 
Echinodermata, 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 Red Sand- 
stone, up to the Chalk, reptiles (chiefly Sawia) 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 +. 
* The late Prof. Jameson remarks that Werner, his master, already made 
the observation that “different formations can be discrimimated by the 
petrifactions they conta, 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 followmg 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 géognostique sur le 
Gisement des Roches, Paris et Strasbourg, 1826, 8vo, p. 37), the first point 
—that different 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 caractéristiques 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. 
+ 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 Développement des Etres organisés a la 
surface du Globe terrestre dans les differents 4ges de la Nature’ (Neu- 
chatel, 1841, 8vo). In this work we have the periods, (1) of Fishes, (2) of 
Reptiles, and (3) of Birds. 
