lOO PAST HISTORY OF ANIMALS 



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 than 

 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. 



" Palaeontological series." — In spite of the imperfec- 

 tion of the " geological record," in spite of the conditions 

 unfavourable 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 (Planorbts) has been worked 

 out ; the extremes are very different, but the intermediate 

 forms link them indissolubly by a marvellously gradual 

 series of transitions. The same fact is well illustrated by 

 another series of fresh- water snails {Paludina, Fig. 44), 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 Palaeontology 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 

 represented in the oldest distinctly stratified and fossili- 

 ferous rocks — the Cambrian system — shows that this corre- 



