70 PAST HISTOR Y OF ANIMALS. 



served 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. 



The armoured and bony Fishes have left many repre- 

 sentatives. 



The Amphibia have bones and teeth, and those living in 

 fresh water have left footprints as traces. 



The traces of Reptilia depend upon the habits of the 

 various orders, those living in water being often preserved, 

 while the monstrous flying Reptiles have left many skeletons 

 behind them. 



Of the Birds, the wingless ones are best represented, and 

 then those that live near seas, estuaries, or lakes. 



The history of Mammals is very imperfect, for most of 

 them were terrestrial. But the discoveries of Marsh and 

 others show how much may be found by careful search. 

 The aquatic Mammals are fairly well preserved. 



" Palceontological Series." — In spite of the imperfection of 

 the " geological record," in spite of the conditions unfavour- 

 able 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 trans- 

 itions. The same fact is well illustrated by another series of 

 fresh-water snails ( Paludina), and not less strikingly among 

 those extinct Cuttlefishes which are known as Ammonites, 

 and have perfectly preserved shells. Similarly, though less 

 perfectly, the modern crocodiles are linked by many inter- 

 mediate forms to their extinct ancestors, for it is impossible to 

 avoid calling them by that name, and the modern horse to 

 its entirely different progenitors. For a time it was thought 

 that paleontology yielded somewhat negative evidence in 

 regard to progressive evolution ; now, with fuller knowledge 

 of the facts, evolutionists would be almost content to base 

 the doctrine of descent on the palaeontological evidence alone. 



In a general way, it is true that the simpler animals 

 precede the more complex in history as they do in structural 



