I had been deeply impressed by discovering in the Pampean
formation great fossil animals covered with armour like that on
the existing armadillos; secondly, by the manner in which closely
allied animals replace one another in proceeding southwards over
the Continent; and thirdly, by the South American character of
most of the productions of the Galapagos archipelago, and more
especially by the manner in which they differ slightly on each
island of the group; none of the islands appearing to be very
ancient in a geological sense.

It was evident that such facts as these, as well as many others,
could only be explained on the supposition that species gradually
become modified; and the subject haunted me. But it was equally
evident that neither the action of the surrounding conditions,
nor the will of the organisms (especially in the case of plants)
could account for the innumerable cases in which organisms of
every kind are beautifully adapted to their habits of life--for
instance, a woodpecker or a tree-frog to climb trees, or a seed
for dispersal by hooks or plumes. I had always been much struck
by such adaptations, and until these could be explained it seemed
to me almost useless to endeavour to prove by indirect evidence
that species have been modified.

After my return to England it appeared to me that by following
the example of Lyell in Geology, and by collecting all facts
which bore in any way on the variation of animals and plants
under domestication and nature, some light might perhaps be
thrown on the whole subject. My first note-book was opened in
July 1837. I worked on true Baconian principles, and without any
theory collected facts on a wholesale scale, more especially with