METHODS — GEOLOGICAL 5 



tailed Deer and the Prong-horned Antelope are of Old World 

 origin, but their ancestors came in at a far earlier period and 

 the modern species are greatly changed from the ancestral 

 migrants. The Armadillo of Texas and the Canada Porcupine 

 are almost the only survivors, north of Mexico, of the great 

 migration of South American mammals which once invaded 

 the northern continent. On the other hand, the raccoons 

 and several families of rodents are instances of indigenous types 

 which may be traced through a long American ancestry. 



Fully to comprehend the march of mammalian development, 

 it thus becomes necessary to reconstruct, at least in outline, 

 the geography of the successive epochs through which the 

 developmental changes have taken place, the connections and 

 separations of land-masses, the rise of mountain ranges, river 

 and lake systems and the like. Equally significant factors in 

 the problem are climatic changes, which have had a profound 

 and far-reaching effect upon the evolution and geographical 

 spread of animals and plants, and the changes in the vegetable 

 world must not be ignored, for, directly or indirectly, animals 

 are dependent upon plants. To one who has paid no atten- 

 tion to questions of this kind, it might well seem an utterly hope- 

 less task to reconstruct the long vanished past, and he would 

 naturally conclude that, at best, only fanciful speculations, 

 with no foundation of real knowledge, could be within our 

 reach. Happily, such is by no means the case. Geology offers 

 the means of a successful attack upon these problems and, 

 although very much remains to be done, much has already 

 been accomplished in elucidating the history, especially in its 

 later periods, with which the story of the mammals is more 

 particularly concerned. 



It is manifestly impossible to present here a treatise upon the 

 science of Geology, even in outline sketch. Considerations of 

 space are sufficient to forbid any such attempt. Certain things 

 must be taken for granted, the evidence for which may be 

 found in any modern text-book of Geology. For example, 



