PART IV. 



FOSSILIZED ORGANIC BODIES. 



In sedimentary accumulations now forming at river-mouths and 

 in lakes and seas, the shells of mollusca, bones of vertebrates, and 

 hard parts generally of other animals, are being constantly enclosed 

 together with many sea-weeds, and the leaves and stems of various 

 land plants. In like manner, the remains of many of the plants 

 and animals which lived upon the earth or in its waters in former 

 ages, became entombed in the sedimentary rock-matters then being 

 deposited ; and they have thus been fossilized and preserved, although 

 very commonly in the form of casts or impressions only. Many 

 stratified rocks are thus found to contain fossilized organic bodies, 

 either entire or in a fragmentary condition. In these fossil bodies, 

 therefore, we see representatives of some, at least, of the life-forms 

 of former geological periods; but we must guard against the sup- 

 position that more than the merest vestiges of the floras and faunas 

 of the Past are thus brought before us. Thousands of living forms, 

 it is clear from their conditions of life and general surroundings, 

 have little chance of being preserved in a fossilized state : and 

 equally, in former periods, must the great majority of the types, 

 then living, have passed away without leaving behind them any 

 record of their existence. Imperfect, however, as must necessarily 

 be our knowledge of the life-forms of past ages, the vestiges thus 

 preserved to us reveal many points of great physical and biological 

 interest. Their study enables us to distinguish the formations of 

 one geological age or period from those of other periods each 

 holding its own proper and separate forms. These fossil forms, 

 moreover, fill up many breaks or gaps in the existing life-series, 

 shewing connections between living forms apparently remote in 

 structural affinities, and indicating a gradual passage from earlier 

 and comparatively generalized types, into higher and more specialized 

 forms of existence. Their study reveals also in many instances the 

 great changes in the distribution of land and sea that have taken 

 place both in early and in comparatively recent times. 

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