e been periods in
the history of the earth when the changes of organic life occurred in
a far swifter manner than in this last section of the earth's history.
This supposition is inadmissible, for it rests on no kind of proof; it
is, moreover, contraindicated by the evident fact that the advance in
the organic series has been more rapid in recent time than at any
stage of the past. In a word, all the facts with which the geologist
deals are decidedly against the assumption that terrestrial changes in
the organic or the inorganic world ever proceed in a spasmodic manner.
Here and there, and from time to time, local revolutions of a violent
nature undoubtedly occur, but, so far as we may judge from the aspect
of the present or the records of the past, these accidents are
strictly local; the earth has gone forward in its changes much as it
is now advancing. Its revolutions have been those of order rather than
those of accident.

The first duty of the naturalist is to take Nature as he finds it. He
must avoid supposing any methods of action which are not clearly
indicated in the facts that he observes. The history of his own and of
all other sciences clearly shows that danger is always incurred where
suppositions as to peculiar methods of action are introduced into the
interpretation. It required many centuries of labour before the
students of the earth came to adopt the principle of explaining the
problems with which they had to deal by the evidence that the earth
submitted to them. Wherever they trusted to their imaginations for
guidance, they fell into error. Those who endeavour to abbreviate our
conception of geologic time by supposing that in the olden days the
order of events was other than that we now behold are going counter to
the best traditions of the science.

Although the aspect of the record of life since the beginning of the
Cambrian time indicates a period of at least a hundred million years,
it must not be supposed that this is the limit of the time required
for the development of the organic series. All the important types of
animals were already in existence in that ancient period with the
exception of the vertebrates, the remains of which have apparently now
been traced down to near the Cambrian level. In other words, at the
stage where we first find evidence of living beings the series to
which they belong had already climbed very far above the level of
lifeless matter. Few naturalists will question t