28 INTRODUCTION. 



aspect of the question, we may advantageously consider some 

 of the main causes of the numerous breaks and gaps in the 

 palaeontological record at some length. 



I. CAUSES OF THE ABSENCE OF CERTAIN ANIMALS IN 

 FOSSILIFEROUS DEPOSITS. In the first place, even if the series 

 of the stratified deposits had been preserved to us in its entirety, 

 and we could point to the sedimentary accumulations belong- 

 ing to every period of the earth's history, there would still be 

 enormous deficiencies in the palaeontological record, owing to 

 the differences in the facility with which different animals may 

 be preserved as fossils. This subject is sufficiently important 

 to render it advisable to consider each of the primary groups 

 of the animal kingdom separately from this point of view : 



a. Protozoa. As regards the sub-kingdom of the Protozoa, 

 the entire classes of the Gregarinidcz and Infusoria* Animal- 

 cules, from their absence of hard parts, must ever remain un- 

 represented in a fossil condition. One or two of the latter, 

 however, possess an integumentary covering capable under 

 favourable circumstances of being preserved in rocks of recent 

 age. The Monera present no structures capable of fossilisa- 

 tion ; and the same may be said of the Amozbea, though one or 

 two of the latter have a carapace which might possibly be pre- 

 served. The remaining Rhizopodous orders viz., the Fora- 

 jninifera, Radiolaria, and Spongidd almost invariably develop 

 hard structures of lime or flint ; and all these orders, therefore, 

 have left abundant traces of their existence in past time. 



b. Coelenterata. Amongst the Ccelenterate animals, the 

 Fresh-water Polypes (Hydra), the Oceanic Hydrozoa, the Jelly- 

 fishes (Medusida), the Sea -blubbers (Litcernarida), the Sea- 

 anemones (Actinidce), and the Ctenophora are destitute of hard 

 parts which could be preserved as fossils. The Sea-blubbers, 

 however, supply us with an instance of how a completely soft- 

 bodied creature may leave traces of its past existence; for there 

 is no doubt that impressions left by the stranded carcasses of 

 these animals have been detected in certain fine-grained rocks 

 (the Lithographic Slate of Solenhofen). On the other hand, 

 the coralligenous Zoophytes or "corals" (comprising the Zoan- 

 t/iaria sderodermata and sclerobasica, and most of the Alcyonaria) 

 possess hard parts capable of preservation, and the same is the 

 case with most of the Hydroid Zoophytes. Accordingly, there 

 are few more abundant fossils than corals ; whilst the large ex- 

 tinct group of the Graptolites is generally placed in the vicinity 

 of the Sea-firs (Sertularians). 



c. Annuloida. In this sub-kingdom the great class of the 

 Echinodermata may be said to be represented more or less 



