all geological history showing
continuity of life, must be limited within some such period of past time as
one hundred million years. The first question which suggests itself,
supposing Sir W. Thomson's views to be correct, is, Is this period anything
like enough for the evolution of all organic forms by "Natural Selection"?
The second is, Is this period anything like enough for the deposition of
the strata which must have been deposited if all organic forms have been
evolved by _minute_ steps, according to the Darwinian theory?

In the first place, as to Sir William Thomson's views, the Author of this
book cannot presume to advance any opinion; but the fact that they have not
been refuted, pleads strongly in their favour when we consider how {137}
much they tell against the theory of Mr. Darwin. The last-named author only
remarks that "many of the elements in the calculation are more or less
doubtful,"[134] and Professor Huxley[135] does not attempt to _refute_ Sir
W. Thomson's arguments, but only to show cause for suspense of judgment,
inasmuch as the facts _may be_ capable of other explanations.

Mr. Wallace, on the other hand,[136] seems more disposed to accept them,
and, after considering Sir William's objections and those of Mr. Croll,
puts the probable date of the beginning of the Cambrian deposits[137] at
only twenty-four million years ago. On the other hand, he seems to consider
that specific change has been more rapid than generally supposed, and
exceptionally stable during the last score or so of thousand years.

Now, first, with regard to the time required for the evolution of all
organic forms by merely accidental, minute, and fortuitous variations, the
useful ones of which have been preserved:

Mr. Murphy[138] is distinctly of opinion that there has not been time
enough. He says, "I am inclined to think that geological time is too short
for the evolution of the higher forms of life out of the lower by that
accumulation of imperceptibly slow variations, to which alone Darwin
ascribes the whole process."

"Darwin justly mentions the greyhound as being equal to any natural species
in the perfect co-ordination of its parts, 'all adapted for extreme
fleetness and for running down weak prey.'" "Yet it is an artificial
species (and not _physiologically_ a species _at all_), formed by
long-continued selection under domestication; and there is no reason to
suppose that any of the variations which have been se