FOSSIL PLANTS AS AN AID TO GEOLOGY. 369 



to its growth, by a natural barrier such as a lofty, unbroken 

 mountain chain or a broad expanse of water, the chances are 

 against the species finding its way quickly to the remote areas. 

 As an example of this may be cited the flora of the Hawaiian 

 Islands. This flora, exclusive of the species introduced since 

 the discovery of the Islands by Cooke in 1779, embraces 860 

 species of phanerogams and vascular cryptogams. Of this num- 

 ber no less than 653, or 75.93 per cent, are endemic or peculiar 

 to the Islands. On account of the vast expanse of the Pacific 

 by which the Hawaiian Islands are separated from the nearest 

 land, the flora has been unable to extend its distribution. 



It is but reasonable to suppose that similar conditions existed 

 in past geologic ages, but by the obliteration of barriers, such as 

 the shallowing of the water or emergence of direct land connec- 

 tion, the plants may have been enabled to invade new territory, 

 and thus extend from area to area or from continent to conti- 

 nent. If now an examination is made of the remains of vegeta- 

 tion in two or several widely separated areas, the succession will 

 be found to have been the same, but they may not have been 

 strictly contemporaneous. 



What now is the deduction to be made since the formulation 

 of this principle regarding the value of paleontologic evidence? 

 Does it immediately follow that all correlations based upon sim- 

 ilarity of fossil remains fall to the ground ? By no means. It 

 has simply introduced an additional element of caution into the 

 problem of correlation between widely separated areas. And 

 even here it has been, and must continue to be, of the greatest 

 importance, for, as Professor Ward has well said,^ "What we 

 possess is the general fact that a similar flora once existed in 

 two parts of the world very widely separated, and until some 

 other facts are discovered which complicate and vitiate such a 

 conclusion, it is both safe and useful for the geologist to regard 

 the two deposits as belonging to the same geologic age. There 

 are certain limitations within which this must be true, and when 

 these limitations are recognized the paleontologist m.ay as safely 

 ■ ' loc. cit. p. 47. 



