which, I hope, in most cases will suffice. No one can feel more sensible
than I do of the necessity of hereafter publishing in detail all the facts,
with references, on which my conclusions have been grounded; and I hope in
a future work to do this. For I am well aware that scarcely a single point
is discussed in this volume on which facts cannot be adduced, often
apparently leading to conclusions directly opposite to those at which I
have arrived. A fair result can be obtained only by fully stating and
balancing the facts and arguments on both sides of each question; and this
is here impossible.

I much regret that want of space prevents my having the satisfaction of
acknowledging the generous assistance which I have received from very many
naturalists, some of them personally unknown to me. I cannot, however, let
this opportunity pass without expressing my deep obligations to Dr. Hooker,
who, for the last fifteen years, has aided me in every possible way by his
large stores of knowledge and his excellent judgment.

In considering the origin of species, it is quite conceivable that a
naturalist, reflecting on the mutual affinities of organic beings, on their
embryological relations, their geographical distribution, geological
succession, and other such facts, might come to the conclusion that species
had not been independently created, but had descended, like varieties, from
other species. Nevertheless, such a conclusion, even if well founded,
would be unsatisfactory, until it could be shown how the innumerable
species, inhabiting this world have been modified, so as to acquire that
perfection of structure and coadaptation which justly excites our
admiration. Naturalists continually refer to external conditions, such as
climate, food, etc., as the only possible cause of variation. In one
limited sense, as we shall hereafter see, this may be true; but it is
preposterous to attribute to mere external conditions, the structure, for