ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 519 
to know that a vast period (even in the geological sense) of time 
and physical changes of almost unprecedented extent separate 
the two. 
But if it be a demonstrable fact that strata containing more than 
60 or 70 per cent. of species of Mollusca in common, and compara- 
tively close together, may yet be separated by an amount of geolo- 
gical time sufficient to allow of some of the greatest physical changes 
the world has seen, what becomes of that sort of contemporaneity the 
sole evidence of which is a similarity of facies, or the identity of half 
a dozen species, or of a good many genera? 
And yet there is no better evidence for the contemporaneity as- 
sumed by all who adopt the hypotheses of universal faunze and flore, 
of a universally uniform climate, and of a sensible cooling of the 
globe during geological time 
There seems, then, no escape from the admission that neither 
physical geology nor paleontology possesses any method by which the 
absolute synchronism of two strata can be demonstrated. All that 
geology can prove is local order of succession. It is mathematically 
certain that, in any given vertical linear section of an undisturbed 
series of sedimentary deposits, the bed which lies Jowest is the oldest. 
In any other vertical linear section of the same series, of course, 
corresponding beds will occur in a similar order ; but, however great 
may be the probability, no man can say with absolute certainty 
that the beds in:the two sections were synchronously deposited. 
For areas of moderate extent, it is doubtless true that no practical 
evil is likely to result from assuming the corresponding beds to be 
synchronous or strictly contemporaneous ; and there are multitudes 
of accessory circumstances which may fully justify the assumption 
of such synchrony. But the moment the geologist has to deal with 
large areas or with completely separated deposits, then the mischief 
of confounding that “homotaxis” or “similarity of arrangement,” 
which caz be demonstrated, with “synchrony” or “identity of 
date,” for which there is not a shadow of proof, under the one com- 
mon term of “contemporaneity” becomes incalculable, and proves 
the constant source of gratuitous speculations. 
For anything that geology or paleontology are able to show to 
the contrary, a Devonian fauna and flora in the British Islands may 
have been contemporaneous with Silurian life in North America, and 
with a Carboniferous fauna and flora in Africa. Geographical pro- 
vinces and zones may have been as distinctly marked in the Paleozoic 
epoch as at present, and those seemingly sudden appearances of new 
