which err on the side of being too slow
rather than too fast. If, then, the _maximum_ thickness of the _known_
sedimentary rocks is taken to represent the _average_ thickness of _all_
the sedimentary rocks, and we also know the _amount_ of sediment carried to
the sea or lakes, and the _area_ over which that sediment is spread, we
have a means of calculating the _time_ required for the building up of all
the sedimentary rocks of the geological system. I have here inquired how
far the above suppositions are correct, or on which side they probably err;
and the conclusion arrived at is, that the time required is very much less
than has hitherto been supposed.

Another estimate is afforded by the date of the last glacial epoch if
coincident with the last period of high excentricity, while the Alpine
glaciation of the Miocene period is assumed to have been caused by the next
earlier phase of very high excentricity. Taking these as data, the {539}
proportionate change of the species of mollusca affords a means of arriving
at the whole lapse of time represented by the fossiliferous rocks; and
these two estimates agree in the _order_ of their magnitudes.

It is then argued that the changes of climate every 10,500 years during the
numerous periods of high excentricity have acted as a motive power in
hastening on both geological and biological change. By raising and lowering
the snow-line in all mountain ranges it has caused increased denudation;
while the same changes have caused much migration and disturbance in the
organic world, and have thus tended to the more rapid modification of
species. The present epoch being a period of very low excentricity, the
earth is in a phase of _exceptional stability_ both physical and organic;
and it is from this period of exceptional stability that our notions of the
very slow rate of change have been derived.

The conclusion is, on the whole, that the periods allowed by physicists are
not only far in excess of such as are required for geological and organic
change, but that they allow ample margin for a lapse of time anterior to
the deposit of the earliest fossiliferous rocks several times longer than
the time which has elapsed since their deposit to the present day.

Having thus laid the foundation for a scientific interpretation of the
phenomena of distribution, we proceed to the Second Part of our work--the
discussion of a series of typical Insular Faunas and Floras with a view to
explai