
Boox V.] IMPERFECTION OF GEOLOGICAL RECORD. 619 
less limited also in past time. Similarity or identity of fossils among 
_ formations geographically far apart, instead of proving contem- 
poraneity, may be compatible with great discrepancies in the 
relative epochs of deposit. For on any theory of the origin of 
species, the spread of a species, still more of any group of species, to 
a vast distance from the original centre of dispersion, must in most 
cases have been inconceivably slow. It doubtless occupied so pro- 
longed a time as to allow of almost indefinite changes in physical 
geogtaphy. A species may have disappeared from its primeval 
birthplace while it continued to flourish in one or more directions in 
its outward circle of advance. The date of the first appearance and 
final extinction of that species would thus differ widely according to 
the locality at which we might examine its remains. 
The grand march of life, in its progress from lower to higher 
forms, has unquestionably been broadly alike in all quarters of the 
globe. But nothing seems more certain than that its rate of ad- 
vance has not everywhere been the same. It has moved unequally 
over the same region. A certain stage of progress may have been 
reached in one quarter of the globe thousands of years before it was 
reached in another; though the same general succession of organic 
types might be found in each region. At the present day, for 
example, the higher fauna of Australia is more nearly akin to that 
which flourished in Hurope far back in Mesozoic time than to the 
living fauna of any other region of the globe. There seems also to 
be now sufficient evidence to warrant the assertion that the progress 
of terrestrial vegetation has at some geological periods and in some 
Tegions, been in advance of that of the marine fauna (see p. 626). 
In fine, in every country where the fossiliferous geological 
formations are well displayed and have been properly examined, the 
same general order of organic succession can be made out among 
them. Their relative age within a limited geographical area can 
be demonstrated by the law of superposition. When, however, 
the formations of distant countries are compared, all that we can 
safely affirm regarding them is that those containing the same or a 
representative assemblage of organic remains belong to the same 
epoch in the history of biological progress in each area. They are 
homotaxial; but we cannot assert that they are contemporaneous 
unless we are prepared to include within that term a vague period 
of many thousands of years. 
3. Imperfection of the Geological Record—sSince 
the fact was insisted upon by Darwin, geologists have more fully 
recognized that the history of life has been very imperfectly pre- 
served in the stratified parts of the earth’s crust. Apart from 
the fact that, even under the most favourable conditions, only 
a small proportion of the total flora and fauna of any period 
could be preserved in the fossil state, enormous gaps occur where 
from non-deposit of strata no record has been preserved at all. tis 
as if whole chapters and books were missing from an historical work. 
