geology,
entomology, chemistry and mineralogy, added to his sincere and
attractive personality, well-balanced mind and excellent judgment,
formed a strong and effective bias in the direction Darwin was destined
to follow.

Apart, however, from the strong personal influence of Henslow, Sedgwick
and others with whom he came much in contact, two books which he read at
this time aroused his "burning zeal to add the most humble contribution
to the noble structure of Natural Science"; these were Sir J. Herschel's
"Introduction to the Study of Natural Philosophy," and Humboldt's
"Personal Narrative." Indeed, so fascinated was he by the description
given of Teneriffe in the latter that he at once set about a plan
whereby he might spend a holiday, with Henslow, in that locality, a
holiday which was, indeed, to form part of his famous voyage.

By means of his explorations in the neighbourhood of Cambridge, and one
or two visits to North Wales, Darwin's experimental knowledge of geology
and allied sciences was considerably increased. In his zeal for
collecting beetles he employed a labourer to "scrape the moss off old
trees in winter, and place it in a bag, and likewise to collect the
rubbish at the bottom of the barges in which reeds were brought from the
fens, and thus ... got some very rare species."

During the summer vacation of 1831, at the personal request of Henslow,
he accompanied Professor Sedgwick on a geological tour in North Wales.
In order, no doubt, to give him some independent experience, Sedgwick
sent Darwin on a line parallel with his own, telling him to bring back
specimens of the rocks and to mark the stratification on a map. In later
years Darwin was amazed to find how much both of them had failed to
observe, "yet these phenomena were so conspicuous that ... a house burnt
down by fire could not tell its story more plainly than did the valley
of Cwm Idwal."

This tour was the introduction to a momentous change in his life. On
returning to Shrewsbury he found a letter awaiting him which contained
the offer of a voyage in H.M.S. _Beagle_. But owing to several
objections raised by Dr. Darwin, he wrote and declined the offer; and if
it had not been for the immediate intervention of his uncle, Mr. Josiah
Wedgwood (to whose house he went the following day to begin the shooting
season), who took quite a different view of the proposition, the
"Journal of Researches during the Voyage of H.M.S. _Beagle_," by Charles