The essay on Geological Reform unfortunately brought me, I will not say
into collision, but into a position of critical remonstrance with regard
to some charges of physical heterodoxy, brought by my distinguished
friend Lord Kelvin, against British Geology. As President of the
Geological Society of London at that time (1869), I thought I might
venture to plead that we were not such heretics as we seemed to be;
and that, even if we were, recantation would not affect the question of
evolution.
I am glad to see that Lord Kelvin has just reprinted his reply to my
plea, and I refer the reader to it. I shall not presume to question
anything, that on such ripe consideration, Lord Kelvin has to say upon
the physical problems involved. But I may remark that no one can have
asserted more strongly than I have done, the necessity of looking to
physics and mathematics, for help in regard to the earliest history of
the globe.
And I take the opportunity of repeating the opinion that, whether what
we call geological time has the lower limit assigned to it by Lord
Kelvin, or the higher assumed by other philosophers; whether the germs
of all living things have originated in the globe itself, or whether
they have been imported on, or in, meteorites from without, the problem
of the origin of those successive Faunae and Florae of the earth, the
existence of which is fully demonstrated by palaeontology, remains
exactly where it was.
For I think it will be admitted, that the germs brought to us by
meteorites, if any, were not ova of elephants, nor of crocodiles; not
cocoa-nuts nor acorns; not even eggs of shell-fish and corals; but only
those of the lowest forms of animal and vegetable life. Therefore, since
it is proved that, from a very remote epoch of geological time, the
earth has been peopled by a continual succession of the higher forms of
animals and plants, these either must have been created, or they have
arisen by evolution. And in respect of certain groups of animals, the
well-established facts of palaeontology leave no rational doubt that
they arose by the latter method.
In the second place, there are no data whatever, which justify the
biologist in assigning any, even approximately definite, period of time,
either long or short, to the evolution of one species from another
by the process of variation and selection. In the essay on Geological
Contemporaneity and Persistent Types of Life I have taken pains to
prove that the change of animals has gone on at very different rates in
different groups of living beings; that some types have persisted with
little change from the palaeozoic epoch till now, while others have
changed rapidly within the limits of an epoch. In 1862 (see Coll. Ess
viii pp. 303,304) in 1863 (vol ii., p 461) and again in 1864 (ibid.,
pp. 89-91) I argued, not as a matter of speculation, but from
palaeontological facts, the bearing of which I believe, up to that
time, had not been shown, that any adequate hypothesis of the causes
of evolution must be consistent with progression, stationariness and
retrogression, of the same type at different epochs; of different
types in the same epoch; and that Darwin's hypothesis fulfilled these
conditions.
According to that hypothesis, two factors are at work, variation and
selection. Next to nothing is known of the causes of the former process;
nothing whatever of the time required for the production of a certain
amount of deviation from the existing type. And, as respects selection,
which operates by extinguishing all but a small minority of variations,
we have not the slightest means of estimating the rapidity with which it
does its work. All that we are justified in saying is that the rate
at which it takes place may vary almost indefinitely. If the famous
paint-root of Florida, which kills white pigs but not black ones, were
abundant and certain in its action, black pigs might be substituted for
white in the course of two or three years. If, on the other hand, it
was rare and uncertain in action, the white pigs might linger on for
centuries.