throw unexpected light upon those exceptional means of dispersal which,
because they are exceptional, are often of paramount importance in
leading to the production of new species and in thus determining the
nature of insular floras and faunas.

Having no knowledge of scientific botany, it needed some courage, or, as
some may think, presumption, to deal with this aspect of the problem;
but ... I had long been excessively fond of plants, and ... interested
in their distribution. The subject, too, was easier to deal with, on
account of the much more complete knowledge of the detailed distribution
of plants than of animals, and also because their classification was in
a more advanced and stable condition. Again, some of the most
interesting islands of the globe had been carefully studied botanically
by such eminent botanists as Sir Joseph Hooker for the Galapagos, New
Zealand, Tasmania, and the Antarctic islands; Mr. H.C. Watson for the
Azores; Mr. J.G. Baker for Mauritius and other Mascarene islands; while
there were floras by competent botanists of the Sandwich Islands,
Bermuda and St. Helena....

But I also found it necessary to deal with a totally distinct branch of
science--recent changes of climate as dependent on changes of the
earth's surface, including the causes and effects of the glacial epoch,
since these were among the most powerful agents in causing the dispersal
of all kinds of organisms, and thus bringing about the actual
distribution that now prevails. This led me to a careful study of Mr.
James Croll's remarkable works on the subject of the astronomical causes
of the glacial and interglacial periods.... While differing on certain
details, I adopted the main features of his theory, combining with it
the effects of changes in height and extent of land which form an
important adjunct to the meteorological agents....