as 
618 - PALAONTOLOGICAL GEOLOGY. — [Boo Y. 
cannot be asserted of any strata merely on the ground of similarity 
or identity in fossils. 
But the phrase “geologically contemporaneous ” is too vague to 
have any chronological value except in a relative sense. To speak 
of two formations as “contemporaneous” which may have been 
separated by thousands of years seems rather a misuse of language, 
though the phraseology has now gained such a footing in geological 
literature as probably to be inexpugnable. If we turn again for 
suggestions to the existing distribution of life on the earth (though 
it is probable that formerly, and particularly among the earlier geo- 
logical periods, there was considerably greater uniformity in zoological 
distribution than there is now) we learn that similarity or identity 
of species and genera holds good on the whole only for limited 
areas, and consequently, if applied to wide geographical regions, 
ought to be an argument for diversity rather than for similarity of 
age. If we suppose the British seas to be raised into dry land, so 
that the organic relics preserved in their sands and silts could be 
exhumed and examined, a general or common facies or type would be 
found, though some species would be more abundant in or entirely 
confined to the north, while others would show a greater development 
in the opposite quarter. Still there would be such a similarity 
throughout the whole that no naturalist would hesitate to regard 
the organisms as those of one biological province, and belonging to 
the same great geological period. The region is so small, and its 
conditions of life so uniform and uninterrupted, that no marked 
distinction can be drawn between the forms of life in its different 
parts. 
Widening the area of observation, we perceive that as we recede 
from any given point on the earth’s surface the existing forms of life 
eradually change. Vegetation alters its aspect from climate to 
climate, and with it come corresponding transformations in the 
characters of insects, birds, and wild animals. A lake bottom would 
preserve one suite of organisms in England, but a very different 
group at the foot of the Himalaya Mountains, yet the deposits at 
the two places might be absolutely coeval, even as to months and 
days. If, therefore, in the geological past there has been, as there is 
now, a grading of plants and animals in great biological provinces, 
marked off by differences of contour, climate, and geological history, 
we must conclude that, while strict contemporaneity cannot be pre- 
dicated of deposits containing the same organic remains, it may 
actually be true of deposits in which they are quite distinct.’ 
If, then, at the present time, community of organic forms, except 
in the case of a few almost world-wide species, obtains only in re- 
stricted districts, regions, or provinces, it may have been more or 
1 The present geographical distribution of plants and animals has a profound 
geological interest, but cannot be properly discussed in this volume. The student will 
find it luminously treated in Darwin's “ Origin of Species,” chapters xii. and xiii, ; Lyell’s 
“Principles of Geology,” chapters xxxviii—xli.; and in Wallace’s “Geographical Distri- 
bution of Animals,” 2 yols, 1876, and his ‘‘ Island Life,’ 1880, 
o . ae 
