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FOSSIL PLANTS AS AN AID TO GEOLOGY.
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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 number 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 connection, the plants may have been enabled to invade new territory, and thus extend from area to area or from continent to continent. If now an examination is made of the remains of vegetation 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 similarity 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,[1] "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 may as safely

  1. ↑ loc. cit. p. 47.
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