IMPERFECTION OF PAL^EONTOLOGICAL RECORD. Gl 



remains are chiefly, but not exclusively, to be found in 

 fluviatile deposits. - 



As regards Birds, their powers of flight, as pointed out by 

 Sir Charles Lyell, would save them from many destructive 

 agencies, and the lightness of their bones would favour the 

 long floating of the body in water, and thus increase the 

 chances of its being devoured by predaceous animals. In 

 accordance with these considerations the most abundant re- 

 mains of Birds are referable to large wingless forms, to which 

 the power of saving themselves from their enemies by flight 

 was denied, whilst most of their bones were filled with 

 marrow instead of air. Next in abundance after these come 

 the remains of birds which frequent the sea-shore, lakes, ) 

 estuaries, or rivers, or which delight in marshy situations. 



Lastly, as regards Mammals, the record is far from being 

 a full one, and from obvious causes. The great majority of 

 Mammals live on land, and therefore are not likely to be 

 buried in aqueous, and especially in marine, accumulations. 

 That this cause is the chief one which has operated against 

 the frequent preservation of Mammalian remains is shown by 

 the fact that when we exhume an old land-surface, the re- 

 mains of Mammals may be found in tolerable plenty. The 

 strictly aquatic Mammals such as Whales, Dolphins, and 

 the like are, of course, much more likely ibo have been pre- 

 served as fossils than the strictly terrestrial forms ; but their 

 want of integumentary hard structures places them at a 

 disadvantage in this respect as compared with fishes. In a 

 general way, we may conclude that the preservation of the 

 terrestrial Mammals as fossils is due to the comparatively 

 rare occurrence of a stray individual being killed whilst 

 swimming a river or some other piece of water, or being 

 mired in a bog, or to the bones of one that had died on land 

 being washed into some stream, and thence into a lake or 

 into the sea, by floods ; but there are other cases for which a 

 different explanation must be sought. The most abundant 

 remains of Mammals have been found in deposits which have 

 been laid down in lakes. 



II. UNREPRESENTED TIME. In the second place, we have 

 seen that the geological record is very imperfect, and this of 



