2 INTRODUCTION 



Now, since there are comparatively few places on land 

 where material is being deposited to any great extent, it 

 follows that terrestrial animals will stand but little chance 

 of being preserved ; the greater number after death will 

 remain on the surface and will in a short time be entirely 

 decomposed. A few may become entombed in peat-bogs, 

 in the dust and ashes thrown out by volcanoes, in the sand 

 of sand-dunes, or by a landslip ; some may be sealed up in 

 deposits of carbonate of lime, such as the travertine thrown 

 down by calcareous springs, or the stalagmite formed on 

 the floor of caves ; and lastly, others may be transported 

 by running water and ultimately buried in the bed of a 

 river, of a lake, or of the sea. Such instances, however, 

 are of comparatively rare occurrence. In the case of 

 aquatic animals the conditions for fossilisation are much 

 more favourable, since deposition is more universal in water 

 than on land. Of such aqueous deposits, those formed in 

 the sea will enclose by far the larger number of animals 

 on account of the greater area which these deposits cover. 



The structure and composition of the hard parts vary 

 considerably in different groups of animals and plants ; 

 some are therefore much more readily preserved as fossils 

 than others. Thus in Argon auta the skeleton consists 

 of a thin shell which is easily broken up ; then again in 

 some sponges it is formed of needles of silica, which are 

 held together by the soft parts only and consequently 

 easily become scattered after the death of the animal. But 

 in other cases, as in most of the mollusks and corals, the 

 skeleton is very strong and not easily destroyed, hence 

 these occur abundantly in the fossil form. Perhaps even 

 more important than the structure, is the composition of 

 the hard parts, which, in the case of insects and some 



