ii 4 MODERN IDEAS OF EVOLUTION 



different states of our planet ? Let us suppose a state 

 of things in which far more forms were being destroyed 

 than were reproduced, another in which introduction 

 of species was more rapid than extinction. In the 

 latter case we may suppose an exuberance of new 

 species to have been produced. In the former there 

 would be a great clearance of these, and perhaps only 

 a few types left to begin new series. Do we now 

 live in one of the periods of diminution or of increase? 

 Perhaps in the former, since there seems to have been, 

 in the case of the mammalia of the Post-Pliocene, an 

 enormous amount of extinction of the grandest forms 

 of life, apparently without their replacement by new 

 forms. If so, how far can we judge from our own 

 time of those which preceded it ? They may have 

 been far more fertile in new forms, or perhaps farther 

 in excess in the work of extinction. The question is 

 further complicated with that which asks if these 

 differences arise from merely physical agencies acting 

 on organic beings from without, or if there is in the 

 organic world itself some grand law of cycles inde 

 pendent of external influences. The answers to such 

 questions are being slowly and laboriously worked 

 out by geologists and naturalists, and all the more 

 slowly that so many inevitable errors occur as to the 

 specific or varietal value of fossils and the relative 

 importance of geological facts, while the great gaps 

 in the monumental history are only little by little 

 being filled up. 



