METHODS — GEOLOGICAL 21 



crepancies between them are concerning former land-connec- 

 tions and extensions. 



The only kind of indirect evidence bearing upon ancient 

 land-connections, now broken by the sea, that need be con- 

 sidered here is that derived from the study of animals and plants, 

 both recent and fossil. All-important in this connection is 

 the principle that the same or closely similar species do not 

 arise independently in areas between which there is no con- 

 nection. It is not impossible that such an independent origin 

 of organisms which the naturalist would class as belonging to 

 the same species may have occasionally taken place, but, if so, 

 it must be the rare exception to the normal process. This 

 principle leads necessarily to the conclusion that the more 

 recently and broadly two land-areas, now separated by the 

 sea, have been connected, the more nearly alike will be their 

 animals and plants. Such islands as Great Britain, Sumatra 

 and Java must have been connected with the adjacent mainland 

 within a geologically recent period, while the extreme zoological 

 peculiarity of Australia can be explained only on the assumption 

 that its present isolation is of very long standing. The princi- 

 ple applies to the case of fossils as well as to that of modern 

 animals, and has already been made use of, in a preceding 

 section, in dealing with the ancient land-connections of North 

 America. It was there shown that the connection of this 

 continent with the Old World and the interruptions of that 

 connection are reflected and recorded in the greater or less 

 degree of likeness in the fossil mammals at any particular epoch. 

 Conversely, the very radical differences between the fossil 

 mammals of the two Americas imply a long-continued separa- 

 tion of those two continents, and their junction in the latter 

 half of the Tertiary period is proved by the appearance of 

 southern groups of mammals in the northern continent, and of 

 northern groups in South America. 



Inasmuch as the connection between North and South 

 America still persists, the geology of the Isthmus of Panama 



