42 BIOLOGICAL SOCIETY OP WASHINGTON. 



As a rule, too, he finds that the questions which arise in connec- 

 tion with the probable manner in which the various groups of ani- 

 mals have originated and become distributed over the earth are too 

 complex to allow of their reduction to even approximately simple 

 propositions. Still, the accumulated results of the various and ex- 

 tensive investigations which naturalists have made of late years 

 have placed the general subject of the evolution of organic forms 

 in such a condition as a working hypothesis, that someof its various 

 divisions may be treated with considerable detail, even with refer- 

 ence to extinct faunas. Furthermore, in a great number of cases, 

 the facts which have been observed are of such a character as to 

 warrant opinions of so important a nature that they may be legiti- 

 mately used as a basis for philosophical discussions. It is mainly 

 upon such facts and opinions as these that the following remarks 

 are based ; and while the conclusions and opinions which are here 

 expressed are believed to be fully warranted by known facts, it is 

 only too evident that much, which it is very desirable to know in 

 this connection, still remains beyond our reach. 



That I may more clearly present my subject, I submit the follow- 

 ing statement of certain views which I hold in relation to it; and, 

 for the purpose of greater conciseness of statement in the remarks 

 which are to follow, I shall refer to those views somewhat as if 

 they were supported by established and acknowledged facts. 



The general subject of the probable origin of the different forms 

 of animal life, and their perpetuation through geological time being 

 so broad, I must confine myself to such small portions of it as my 

 studies have led me to understand as having an important bearing 

 upon the geological history of the North American continent. I 

 must, furthermore, confine myself to such portions of the subject 

 as relate to terrestrial life only, leaving largely out of consideration 

 the su])ject of marine life. 



Since the points I wish to present are somewhat disconnected by 

 our present imperfect knowledge, I must trust to making their rele- 

 vancy more apparent when I come to make the proposed application 



