rom the structure of the
country, that these animal remains were deposited in the beds in which
they occur at a time when the lake extended over the region in which
they are found. This involves the conclusion that they lived and died
before the falls had cut their way back through the gorge of Niagara;
and, indeed, it has been determined that, when these animals lived,
the falls of Niagara must have been at least six miles further down the
river than they are at present. Many computations have been made of
the rate at which the falls are thus cutting their way back. Those
computations have varied greatly, but I believe I am speaking within
the bounds of prudence, if I assume that the falls of Niagara have
not retreated at a greater pace than about a foot a year. Six miles,
speaking roughly, are 30,000 feet; 30,000 feet, at a foot a year, gives
30,000 years; and thus we are fairly justified in concluding that no
less a period than this has passed since the shell-fish, whose remains
are left in the beds to which I have referred, were living creatures.

But there is still stronger evidence of the long duration of certain
types. I have already stated that, as we work our way through the great
series of the Tertiary formations, we find many species of animals
identical with those which live at the present day, diminishing in
numbers, it is true, but still existing, in a certain proportion, in the
oldest of the Tertiary rocks. Furthermore, when we examine the rocks
of the Cretaceous epoch, we find the remains of some animals which the
closest scrutiny cannot show to be, in any important respect, different
from those which live at the present time. That is the case with one of
the cretaceous lamp-shells (_Terebratula_), which has continued to exist
unchanged, or with insignificant variations, down to the present
day. Such is the case with the _Globigerinae,_ the skeletons of which,
aggregated together, form a large proportion of our English chalk. Those
_Globigerinae_ can be traced down to the _Globigerinae_ which live
at the surface of the present great oceans, and the remains of which,
falling to the bottom of the sea, give rise to a chalky mud. Hence
it must be admitted that certain existing species of animals show no
distinct sign of modification, or transformation, in the course of a
lapse of time as great as that which carries us back to the Cretaceous
period; and which, whatever its absolute measure, is certainly vastly