Paleontological Speculations. — Gratacap. 99 
been in progress, but has not taken place uniformly over the 
whole earth, but with local modifications depending upon local 
environment." White (Correlation Papers; Cretaceous) re- 
iterates this in stating that "general biological evolution, while 
it has been progressive, has not progressed at a uniform rate 
throughout geological time, and in all parts of the world." 
A less conservative and, it would seem, more reasonable 
reliance upon fossil indications is expressed by Walcott, Will- 
iams, Dall and Harris. Wolcott says : "The method of cor- 
relation by the comparison of fossils, or, as it has been called, 
'matching,' is the one that affords the best results. It includes 
the comparison of species, genera, families, and the general 
facies of a fauna. It is the basis of paleontologic correlation 
and geologic classification of the sedimentary rocks, with the 
exception of the stratigraphic and lithologic correlation of lo- 
cal formations ;" "all paleontologic reasoning is based upon 
known data. By the discovery of a new grouping of fossils, 
or a different range of known species, the identification of 
horizons may be materially modified." Dall and Harris write : 
"Paleontology holds the key to the problems of local and com- 
parative stratigraphy," admitting also that there should be no 
neglect of "broad and general stratigraphic changes." Will- 
iams writes : "The law of paleontologic succession did not be- 
come a factor of correlation till the idea of the evolution of 
species furnished a rational basis of confidence in the natural- 
ness of the observed order of sequence of forms. The idea 
of evolution suggests the true biologic system of correlation, 
in which the data of the classification are fossils, and the dis- 
tinctions made are into periods in the history of organisms, 
the strata taking their relative position in the series according 
to the period in this history which their contained fossil re- 
mains may indicate." 
The validity of fossils as determinants of geological posi- 
tion cannot be questioned, nor is it rational in our existing 
knowledge to deny the correlation strictly of horizons having 
the same or very similar fossils. Any possible chronological 
disparity is disqualified as an objection inasmuch as such dis- 
parity can be neglected, a few thousand years even, in a mat- 
ter of geological contemporaneity being unimportant. 
Whether such an apparently easy rule of judgment as the 
