“ PALAZEONTOLOGICAL SERIES.” 79 
The Crustacea, being mostly aquatic, and in virtue of their hard. 
shells, are fossilised in great numbers. 
The Arachnida and the Insects, owing to their air-breathing habit,. 
are chiefly represented by chance individuals that have been drowned, 
or enclosed within tree-stumps and amber. 
The Molluscs and Brachiopods are perhaps better preserved tham 
any other animals, since nearly all of them are possessed of a shell 
specially suitable for preservation. 
Among the Vertebrates some of the lowest are without scales, teeth, 
or bony skeleton ; such forms have therefore left almost no traces. 
Fishes, which are usually furnished with a firm outer covering, or 
with a bony internal skeleton, or with both, are well represented. 
The primitive Amphibians were furnished with an exoskeleton of 
bony plates, and are fairly numerous as fossils. ‘The bones and teeth 
of the others have been fossilised, though more rarely. Of some the 
only record is their footprints. 
The traces of Reptilia depend upon the habits of the various orders,. 
those living in water being oftenest preserved, but the strange flying 
Reptiles have also left many skeletons behind them. 
Of the Birds, the wingless ones are best represented, and then those- 
that lived near seas, estuaries, or lakes. 
The history of Mammals is very imperfect, for most of them were 
terrestrial. But the discoveries of Marsh, Cope, and others show how 
much may be found by careful search. The aquatic Mammals are- 
fairly well preserved. 
“Paleontological series.”—In spite of the imperfection 
of the “geological record,” in spite of the conditions un- 
favourable to the preservation of many kinds of animals, it 
is sometimes possible to trace a whole series of extinct forms. 
through progressive changes. Thus a series of fossilised 
fresh-water snails (Planorbis) has been worked out; the 
extremes are very different, but the intermediate forms link 
them indissolubly by a marvellously gradual series of transi- 
_tions. The same fact is well illustrated by another series of 
fresh-water snails (Paludina, Fig. 34), and not less strikingly 
among those extinct Cuttle-fishes which are known as. 
Ammonites, and have perfectly preserved shells. Similarly, 
though less perfectly, the modern crocodiles are linked by 
many intermediate forms to their extinct ancestors, for it is- 
impossible not to call them by that name. In short, as 
knowledge increases, the evidence from Paleontology 
becomes more and more complete. 
In a general way it is true that the simpler animals pre- 
cede the more complex in history as they do in structural 
rank, but the fact that all the great Invertebrate groups are 
