516 ANNIVERSARY ADDRESS TO THE GEOLOGICAL SOCIETY 
or false ; but to direct your attention to a much simpler though very 
essential preliminary question—What is their logical basis? what 
are the fundamental assumptions upon which they all logically de- 
pend? and what is the evidence on which those fundamental propo- 
sitions demand our assent ? 
These assumptions are two: the first, that the commencement of 
the geological record is coeval with the commencement of life on the 
globe ; the second, that geological contemporaneity is the same thing 
as chronological synchrony. Without the first of these assumptions 
there would of course be no ground for any statement respecting the 
commencement of life; without the second, all the other statements. 
cited, every one of which implies a knowledge of the state of dif- 
ferent parts of the earth at one and the same time, will be no less 
devoid of demonstration. 
The first assumption obviously rests entirely on negative evidence. 
This is, of course, the only evidence that ever can be available to: 
prove the commencement of any series of phenomena; but, at the 
same time, it must be recollected that the value of negative evidence 
depends entirely on the amount of positive corroboration it re- 
ceives. If A B wishes to prove an a/zbz, it is of no use for him to 
get a thousand witnesses simply to swear that they did not see him 
in such and such a place, unless the witnesses are prepared to 
prove that they must have seen him had he been there. But the 
evidence that animal life commenced with the Lingula-flags, e.g., 
would seem to be exactly of this unsatisfactory uncorroborated sort. 
The Cambrian witnesses simply swear they “haven’t seen anybody 
their way ;” upon which the counsel for the other side immediately 
puts in ten or twelve thousand feet of Devonian sandstones to make 
oath they never saw a fish or a mollusk, though all the world knows. 
there were plenty in their time. 
But then it is urged that, though the Devonian rocks in one part 
of the world exhibit no fossils, in another they do, while the lower 
Cambrian rocks nowhere exhibit fossils, and hence no living being 
could have existed in their epoch. 
To this there are two replies: the first, that the observational 
basis of the assertion that the lowest rocks are nowhere fossiliferous 
is an amazingly small one seeing how very small an area, in com- 
parison to that of the whole world, has yet been fully searched: the 
second, that the argument is good for nothing unless the unfossili- 
ferous rocks in question were not only contemporaneous in the geo- 
logical sense, but syzchronous in the chronological sense. To use 
the a/éz illustration again. If a man wishes to prove he was. 
