amount
of variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility
is a special endowment and sign of creation. The belief that species
were immutable productions was almost unavoidable as long as the history
of the world was thought to be of short duration; and now that we have
acquired some idea of the lapse of time, we are too apt to assume,
without proof, that the geological record is so perfect that it would
have afforded us plain evidence of the mutation of species, if they had
undergone mutation.

But the chief cause of our natural unwillingness to admit that one
species has given birth to other and distinct species, is that we are
always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs
had been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the
term of a hundred million years; it cannot add up and perceive the full
effects of many slight variations, accumulated during an almost infinite
number of generations.

Although I am fully convinced of the truth of the views given in this
volume under the form of an abstract, I by no means expect to convince
experienced naturalists whose minds are stocked with a multitude of
facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under
such expressions as the "plan of creation," "unity of design," etc., and
to think that we give an explanation when we only restate a fact. Any
one whose disposition leads him to attach more weight to unexplained
difficulties than to the explanation of a certain number of facts
will certainly reject my theory. A few naturalists, endowed with
much flexibility of mind, and who have already begun to doubt on the
immutability of species, may be influenced by this volume; but I look
with confidence to the future, to young and rising naturalists, who will
be able to view both sides of the question with impartiality. Whoever
is led to believe that species are mutable will do good service by
conscientiously expressing hi