Boox V.| PALAONTOLOGY AND EVOLUTION. 623 
Until this is done detailed paleontological classification may prove 
to be worthless. | 
From what has been above advanced it must be evident that, even 
if the several groups in a formation or system of rocks in any district 
or country have been found susceptible of minute subdivision by 
means of their characteristic fossils, and if, after the lapse of many 
years, no discovery has occurred to alter the established order of 
succession of these fossils, nevertheless the subdivisions may only 
hold good for the region in which they have been made. They 
must not be assumed to be strictly applicable everywhere. Advancing 
into another district or country where the petrographical characters 
of the same formation or system indicate that the original conditions 
of deposit must have been very different, we ought tv be prepared to 
find a greater or less departure from the first observed, or what we 
unconsciously and not unnaturally come to look upon as the normal 
order of organic succession. There can be no doubt that the 
appearance of new organic forms in any locality has been in large 
measure connected with such physical changes as are indicated by 
diversities of sedimentary materials and arrangement. ‘The Upper 
Silurian formations, for example, as studied by Murchison in Shrop- 
shire and the adjacent counties, present a clear sequence of strata 
well defined by characteristic fossils. But within a distance of sixty 
miles it becomes impossible to establish these subdivisions by fossil 
evidence. If we examine corresponding strata in Scotland, we find 
that they contain some fossils which never rise above the Lower 
Silurian formations in Wales and the west of England. Again, in 
Bohemia and in Russia we meet with still greater departures from 
the order of appearance in the original Silurian area, some of the 
most characteristic Upper Silurian organisms being there found far 
down beneath strata replete with records of Lower Silurian life. 
Nevertheless the general succession of life from Lower to Upper 
Silurian types remains distinctly traceable. Such facts warn us 
against the danger of being led astray by an artificial precision 
of paleontological detail. ven where the palxontological sequence 
is best established, it rests probably in most cases not merely upon 
the actual chronological succession of organic forms, but also, 
far more than is usually imagined, upon original accidental 
differences of local physical conditions. As these conditions have 
constantly varied from region to region, it must comparatively seldom 
happen that the same minute paleontological subdivisions, so 
important and instructive in themselves, can be identified and 
paralleled, except over comparatively limited geographical areas. 
The remarkable “zones” of the Lias have been recognized over 
central and western Europe, but cease to be traceable as we recede 
from their original geographical province. - 
v. Bearing of paleontological data upon Evolution.—Since 
the researches of William Smith at the end of last century it has 
