16 STKATIGEAPHICAL GEOLOGY 



species and another, and would connect the generic forms of the 

 present with those which have long been extinct. Unfortunately, 

 however, though many such links have been found, and though 

 more come to light every year, so that the genealogy of many 

 genera can be satisfactorily made out, yet there are reasons why 

 we can never hope to find all the links in the chain. 



Imperfection of the Geological Record. That the 

 truth of Darwin's theory of the origin of species cannot at once be 

 proved, and the complete genealogy of modern genera be constructed 

 from the records of the past, is due to the imperfection of that very 

 geological record which would otherwise contain the obvious proof 

 or refutation of the hypothesis. This imperfection results from 

 two causes, (1) the conditions which limit the entombment and 

 preservation of organic remains, (2) the frequent erosion and 

 destruction of the deposits in which they have been buried. 



1. Conditions of Entombment. If we consider the manner in 

 which deposits are now taking place, we shall perceive that the 

 preservation of many kinds of organic remains in these deposits is 

 likely to be an exceptional and accidental occurrence. Deposition 

 of sediment is not an uninterrupted process likely to ensure the 

 conservation of specimens of the greater number of plants and 

 animals living at any one time, but is, on the contrary, so 

 discontinuous that only a small proportion of any fauna or flora 

 would be entombed in such a way as to be permanently preserved, 

 either in terrestrial or marine deposits. 



On land, generations of animals may die and moulder away, 

 shells, bones, and all, without any remains being transported to 

 places where deposits are accumulating. Mr. F. C. Selous remarks 

 that in South Africa " no organic matter lasts very long when 

 exposed to the weather, and even the skull and leg bones of an 

 elephant would, I think, crumble to dust and absolutely disappear 

 in less than fifteen years from the date of the animal's death." l 

 In some countries where the climate is very dry they may last 

 longer, but still ultimately decay and crumble into dust. 



Of the larger land animals a few are occasionally buried in bogs 

 or drowned in lakes and rivers, and still fewer are carried down 

 by rivers to the sea. The animals which live in fresh water and 

 the leaves of plants stand a better chance of preservation in those 

 parts of the country where lacustrine and fluviatile deposits are 

 being formed ; but these purely freshwater beds occupy but small 

 areas, and are particularly liable to subsequent destruction, so that 

 as a rule our knowledge of the terrestrial and fluviatile fauna of 

 any period is derived from the specimens which were carried down 

 1 Sport and Travel, by F. C. Selons, 1900, p. 156. 



