THE LAWS OF EVOLUTION EMPHASIZED. 367 



are obliged to assume a degree and amount of change in them 

 of which the facts of geology give no evidence. 



If the conditions which have changed with the geological 

 ages have been the organisms themselves, and they have con- 

 stituted the environment, then it becomes necessary to ex- 

 plain the more powerful contestants before their selecting 

 agency can result in the survival of fitter races. 



But leaving aside for the present the philosophical argu- 

 ment, the burden of these pages is to show what is in fact 

 the testimony on these questions furnished by the organic 

 history as found in the best-preserved parts of the record. 



As previously explained, the records which are made at 

 the place and time of the formation of the rocks are those 

 which must on that account be the most perfect we can con- 

 sult. The rocks bearing fossils are not wholly, but are in the 

 large majority of cases, of marine origin. This determined 

 the selection of the evidence from among marine animals. 

 The animals of which the best records could be preserved in 

 the rocks are those secreting hard parts — shells, or corals, or 

 similar parts ; hence the examples have been taken chiefly 

 from the corals, the Mollusca, and Brachiopods. 



The Geological Evidence does not Emphasize the Importance of 

 Natural Selection as a Factor of Evolution. — What has already 

 been said is sufficient to show that the emphasis of the 

 testimony brought forward differs from the emphasis drawn 

 by the embryologist, or by the student of living organisms, as 

 to the relative prominence of the several factors in the evolu- 

 tional history of organisms. 



That which has seemed most conspicuous to the latter 

 class of observers has been the intimate relationship existing 

 between morphological difTerence and environmental condi- 

 tions; paleontological facts point to the greater importance 

 of the continuous and progressive process of differentiation 

 and specialization of structure and function with the passage 

 of geological time. 



The law of natural selection, suggested to explain the evo- 

 lution from the first point of view, calls for an extremely slow 

 rate of modification, but uniform and continuous. The facts 

 of the history itself point to the reality of rapid strides at 



