expression must all have had a gradual and natural origin.
During the summer of the following year, 1840, I read Sir C.
Bell's admirable work on expression, and this greatly increased
the interest which I felt in the subject, though I could not at
all agree with his belief that various muscles had been specially
created for the sake of expression. From this time forward I
occasionally attended to the subject, both with respect to man
and our domesticated animals. My book sold largely; 5267 copies
having been disposed of on the day of publication.

In the summer of 1860 I was idling and resting near Hartfield,
where two species of Drosera abound; and I noticed that numerous
insects had been entrapped by the leaves. I carried home some
plants, and on giving them insects saw the movements of the
tentacles, and this made me think it probable that the insects
were caught for some special purpose. Fortunately a crucial test
occurred to me, that of placing a large number of leaves in
various nitrogenous and non-nitrogenous fluids of equal density;
and as soon as I found that the former alone excited energetic
movements, it was obvious that here was a fine new field for
investigation.

During subsequent years, whenever I had leisure, I pursued my
experiments, and my book on 'Insectivorous Plants' was published
in July 1875--that is, sixteen years after my first observations.
The delay in this case, as with all my other books, has been a
great advantage to me; for a man after a long interval can
criticise his own work, almost as well as if it were that of
another person. The fact that a plant should secrete, when
properly excited, a fluid containing an acid and ferment, closely