CERTAIN PHASES IN THE GEOLOGICAL HISTORY OF 

 THE NORTH AMERICAN CONTINENT, BIOLOGI- 

 CALLY CONSIDERED.* 



By Charles A; White. 



It is quke certain that there has never been a time in the history 

 of mankind when the thoughts of men were so eagerly turned to 

 biological subjects as they are to-day ; nor has there ever before 

 been a time when an intelligent knowledge of them was so broadly 

 diffused among cultivated persons. An earnest desire is everywhere 

 manifested by such persons to obtain substantial knowledge con- 

 cerning the animal and vegetable life of the earth, and of the broad 

 significance of that life, which is revealed by a comparative study 

 of its myriad forms. The investigator immediately finds that this 

 subject, although it is so comprehensive and so complex, is only a 

 fragment of a great history of life, which extends back through 

 unnumbered ages. He finds himself at once confronted by ques- 

 tions concerning successive multitudes of foniier denizens of the 

 earth, the physical conditions which prevailed when they existed, 

 the probable lines of descent by which they came into being, and 

 by which their successors have come down to the present time ; and 

 the manner in which those lines have probably originated and been 

 preserved from destruction through successive geological periods. 



This prevalent spirit of inquiry among men has been the cause 

 of a vast amount of patient and exhaustive research, and it has 

 also resulted in a large accumulation of knowledge. But it cannot 

 be denied that every investigating naturalist, although he may fully 

 accept the doctrine of evolution, finds the subject of the origin 

 and derivation of the various groups of animals that now inhabit 

 the earth, and those which have inhabited it during past geological 

 time, to be beset with many difficulties and uncertainties. 



* Presidential Address delivered at the Fourth Anniversary Meeting of tlie 

 Society, January 25, 1884, in the Lecture Room of the U. S. National Museum. 



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