52 BIOLOGICAL SOCIETY OF WASHINGTON. . 



As to the manner in which the great vertebrate and articulate 

 land faunas which now exist, and which have existed in former 

 geological periods, have been respectively developed from lower forms, 

 I have, at present, no suggestions to offer; but it is, nevertheless, 

 assumed that they have been thus developed. It may be remarked, 

 however, concerning their origin and perpetuation, that while cer 

 tain minor faunas of terrestrial animals may have originated and been 

 perpetuated upon comparatively small areas, it seems certain that 

 the development and perpetuation of the great reptilian faunas, 

 which existed during Mesozoic time, and also that of the wonder 

 ful mammalian faunas, whose remains are found in Tertiary strata, 

 required large and congenial continental areas. It also seems nec 

 essary to infer that those continental areas, although they, from 

 time to time, suffered material losses, and received considerable 

 accessions of land, have been of true continental dimensions from 

 early geological time. 



The general subject of the geological history of continents is so 

 complex and far reaching, and our present knowledge of that sub 

 ject so fragmentary, that I shall not now attempt even an outline 

 of such a history for North America ; but I shall confine myself to 

 what I conceive to be some of the more salient points of that 

 history from a biological standpoint only. 



If the geological history of animal life is incomplete and frag 

 mentary, a like history of continental areas, especially as regards 

 their location and outlines at different periods, is much more so. 

 In the former case, we are guided in our investigations and conclu 

 sions by a known zoological system, which is based upon the abun 

 dant and diversified existing life of the earth. In the latter case, 

 we are apparently without any available systematic guide ; and the 

 various phases of geological history of continents seem to have 

 been the result of fortuitous movements of the earth's crust, in 

 connection with sedimentation and sub-gerial and aqueous erosion. 

 Not that those movements and processes have not been governed 

 by physical laws, but the events, so far as we have yet learned in- 



