32 INTRODUCTION. 



aquatic Mammals such as Whales, Dolphins, and the like 

 are, of course, much more likely to have been preserved 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 Mam- 

 mals 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 

 by floods ; but there are other cases for which a different ex- 

 planation must be sought. 



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

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

 necessity causes vast gaps in our palasontological knowledge. 

 In this connection we may briefly consider the evidence which 

 we possess as to the immensity of the "unrepresented time" 

 between some of the great formations, and no better example 

 can be chosen than that of the Cretaceous and Eocene Rocks. 

 In considering such a case, the evidence may be divided into 

 two heads, the one palaeontological, the other purely physical, 

 and each may be looked at separately. 



The Chalk, as is well known, constitutes in Britain the 

 highest member of the Cretaceous formation, and is the highest 

 deposit there known as appertaining to the great Secondary or 

 Mesozoic series. It is directly overlaid in various places by 

 strata of Eocene age, which form the base of the great Tertiary 

 or Kainozoic series of rocks. The question, then, before us is 

 this, What evidence have we as to the lapse of time repre- 

 sented merely by the dividing-line between the highest beds of 

 the Chalk and the lowest beds of the Eocene ? 



Taking the palsontological evidence first, it is found that 

 out of five hundred species of fossils known in the Upper Cre- 

 taceous beds, only one Brachiopod and a few Foraminifera 

 have hitherto been detected in the immediately overlying 

 Eocene beds. These latter, on the contrary, are replete with 

 organic remains wholly distinct from those of the Cretaceous 

 beds. It may be said, therefore, that the very extensive as- 

 semblage of animals which lived in the later Cretaceous seas 

 of Britain had entirely passed away and become a thing of the 

 past, before a single grain of the Eocene Rocks had been de- 

 posited. Now, it is of course open to us to believe that the 

 animals of the Chalk sea were suddenly extinguished by some 

 natural agencies unknown to us, and that the animals of the 

 Eocene sea had been in as sudden and as obscure a manner 



