s--Present Condition of the Earth one of Exceptional Stability as
Regards Climate--Date of last Glacial Epoch and its Bearing on the
Measurement of Geological Time--Concluding Remarks.

The subjects discussed in the last three chapters introduce us to a
difficulty which has hitherto been considered a very formidable one--that
the maximum age of the habitable earth, as deduced from physical
considerations, does not afford sufficient time either for the geological
or the organic changes of which we have evidence. Geologists continually
dwell on the slowness of the processes of upheaval and subsidence, of
denudation of the earth's surface, and of the formation of new strata;
while on the theory of development, as expounded by Mr. Darwin, the
variation and modification of organic forms is also a very slow process,
and has usually been considered to require an {100} even longer series of
ages than might satisfy the requirements of physical geology alone.

As an indication of the periods usually contemplated by geologists, we may
refer to Sir Charles Lyell's calculation in the tenth edition of his
_Principles of Geology_ (omitted in later editions), by which he arrived at
240 millions of years as having probably elapsed since the Cambrian
period--a very moderate estimate in the opinion of most geologists. This
calculation was founded on the rate of modification of the species of
mollusca; but much more recently Professor Haughton has arrived at nearly
similar figures from a consideration of the rate of formation of rocks and
their known maximum thickness, whence he deduces a maximum of 200 millions
of years for the whole duration of geological time, as indicated by the
series of stratified formations.[30] But in the opinion of all our first
naturalists and geologists, the period occupied in the formation of the
known stratified rocks only represents a portion, and perhaps a small
portion, of geological time. In the sixth edition of the _Origin of
Species_ (p. 286), Mr. Darwin says: "Consequently, if the theory be true,
it is indisputable that before the lowest Cambrian stratum was deposited
long periods elapsed, as long as, or probably far longer than, the whole
interval from the Cambrian age to the present day; and that during these
vast periods the world swarmed with living creatures." Professor Huxley, in
his anniversary address to the Geological Society in 1870, adduced a number
of special cases showing that, on th