78 BIOLOGICAL LECTURES. 



Still another department of investigation may be carried on 

 with the aid of very imperfect material, namely, the correlation 

 of geological horizons in different continents. This is a neces- 

 sary preliminary to the study of the problems of distribution, 

 in order to determine the place of origin of the group in ques- 

 tion. In the present state of knowledge this correlation is 

 difficult, if not impossible, in the case of continents which are 

 and long have been completely separated from each other, and 

 therefore have no common elements in their faunas. It is this 

 fact which makes the correlation of the South American Ter- 

 tiaries with those of North America and Europe so puzzling. 

 But when the continents have been repeatedly, or for long 

 periods, connected by land bridges, as is true of the land-masses 

 of the Northern Hemisphere, the problem may be attacked 

 with every prospect of success, and many North American 

 formations seem to have their exact equivalents in Europe. To 

 make out these equivalences, it is only necessary that the 

 fossils shall be determinable, generically and specifically. 



You have listened with exemplary patience to a dry and dull 

 exposition of methods, but if the listening has convinced you 

 that the methods of modern palaeontological investigation are 

 truly scientific and trustworthy, and that its results are entitled 

 to a respectful hearing on the part of morphologists, I shall not 

 feel that the dullness and dryness stand in need of any apology. 



PRINCETON UNIVERSITY. 



