CHAPTER VI 

 CRISES IN EVOLUTION 



SUBDIVISION of the foregoing outline of faunal 

 progress into sections, although strictly orthodox 

 and almost inevitable, cannot fail to lay undue 

 emphasis on the apparent discontinuity of evolution. 

 It has been mentioned above that the classification of 

 Stratigraphy was devised, and to some degree elaborated, 

 by workers whose belief in the separate creation of 

 successive faunas was complete. The practical utility 

 of that scheme in these latter days, when the basis 

 for its original adoption sounds almost ludicrous, must 

 awaken questions as to the quality of evolutional pro- 

 gress. It has been argued, in the last chapter of Part I., 

 that the rate of evolution varies in different phases of 

 the phylogeny of a single stock. Is it reasonable to 

 suppose that evolution of the world-fauna is comparably 

 susceptible to acceleration and retardation ? Are there 

 periods during which all but the most sluggish groups 

 are unusually plastic and vigorous in differentiation ? 

 Or is the progress of Evolution uniform ? 



Approximately definite answers to these questions 

 are, as yet, impossible. Until proportionate amounts of 

 material have been collected from every horizon, and 

 from all accessible parts of the world, no such sweeping 

 generalizations could be other than speculative. The 

 purpose of the following sentences (which are primarily 

 addressed to residents in the British Isles) is to 



