1869.] DAWSON — IDEAS OF DERIVATION. 127 



alternations of long periods of physical repose and slow subsidence 

 in which our more important fossiliferous formations have been 

 deposited, with others of physical disturbance and elevation, with 

 extinction of species. Dana has well shown how completely this 

 view is established by the series of Geological formations as seen 

 on the broad area of the American continent. Now the question 

 arises, how would the law of derivation operate in these two 

 diiFerent 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 for 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 iu excess in the work of 

 extinction. The question is further complicated with that which 

 asks if these differences arise from merely physical agencit'S 

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

 world itself some grand law of cyles independent 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 tilled up. 



Nothing can more forcibly illustrate the amount of work 

 remaining to be done toward the settlement of these questions 

 than a glance at the elaborate and most valuable " Thesaurus 

 Silur-cus" of Dr. Bigsby, recording, as it does, nearly 9,000 

 species of animals already found in the Silurian rocks. The 

 rapid increase in the number of known species shows that we 

 know as yet but a fraction of this ancient fauna, while the facts 

 relating to introduction, extinction, geographical distribution 

 and distribution in time, show that we are still a very long way 



