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.

T.H. HUXLEY.

HODESLEA, EASTBOURNE,

_April, 1894._



CONTENTS


I

ON A PIECE OF CHALK [1868]
(A Lecture delivered to the working men of Norwich during the meeting of
the British Association.)