EVIDENCES FROM PALAEONTOLOGY 137 



mollusks and crustaceans are represented in almost all rock deposits 

 which contain any kind of fossil remains." — Jordan and Kellogg.^ 



The study of geology teaches us that the earth's outer zones have 

 undergone within the period of vertebrate history numerous profound 

 changes which in general we may term climatic changes. There have 

 been periods of continental subsidence, accompanied by ocean-floor 

 elevations, during which great continental plains have been covered 

 with comparatively shallow seas. The marine faunas of the seas have 

 migrated into these shallows and representatives of them have been 

 buried in sediment. When the reverse change has occurred and the 

 continental plain has been again elevated, the sedimentation of the 

 shallow-sea period forms a great rocky stratum laden with marine 

 fossils. Between periods of subsidence millions of years elapsed, and 

 therefore a break in the continuity of the entombed fossils is to be 

 expected. Discontinuity between the fossil faunas in adjacent strata 

 is the invariable rule. Were it not for this periodicity of subsidence 

 and elevation there would be no boundaries between consecutive 

 geologic strata. 



In addition to the methods of fossilization mentioned, a few others 

 deserve notice. Many animals of the arid plains have been fossilized 

 by becoming imbedded in dust or sand drifts which have piled up 

 against rocky outcrops or have filled in dried-up arroyos. Some very 

 valuable fossils have been recovered from asphaltic deposits as the 

 result of animals falling into liquid or semiliquid lakes or pools of 

 asphalt. 



Not only are external organs preserved with precision, but even 

 delicate internal structures, such as the brains or the viscera of verte- 

 brates, have been found in such a perfectly natural shape that the 

 comparative anatomy could be worked out with confidence. 



On the whole, then, we must conclude that the earlier pessimism 

 regarding the inadequacy and insufficiency of fossil data is giving way 

 before a steadily increasing optimism, due to the very rapid advance 

 in technique and the surprisingly abundant discoveries of the modern 

 palaeontologist. The more enthusiastic of the new scliool of fossil- 

 hunters do not despair of ultimately bringing to light all of the really 

 essential links in the chain of evidence necessary to place the evolution 

 theory beyond the reach of controversy. 



'From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copy- 

 right 1907). Used by special permission of the publishers, D. Appleton & Company. 



