For the most part, indeed, they have now all ceased to confound
the process of speculation _per se_ with the danger of inadequate
verification; and therefore the old ideal of natural history as
concerned merely with collecting species, classifying affinities, and,
in general, tabulating facts, has been well-nigh universally
superseded. But this great gain has been attended by some measure of
loss. For while not a few naturalists have since erred on the side of
insufficiently distinguishing between fully verified principles of
evolution and merely speculative deductions therefrom, a still larger
number have formed for themselves a Darwinian creed, and regard any
further theorizing on the subject of evolution as _ipso facto_
unorthodox.

Having occupied the best years of my life in closely studying the
literature of Darwinism, I shall endeavour throughout the following
pages to avoid both these extremes. No one in this generation is able to
imitate Darwin, either as an observer or a generalizer. But this does
not hinder that we should all so far endeavour to follow his _method_,
as always to draw a clear distinction, not merely between observation
and deduction, but also between degrees of verification. At all events,
my own aim will everywhere be to avoid dogmatism on the one hand, and
undue timidity as regards general reasoning on the other. For everything
that is said justification will be given; and, as far as prolonged
deliberation has enabled me to do so, the exact value of such
justification will be rendered by a statement of at least the main
grounds on which it rests. The somewhat extensive range of the present
treatise, however, will not admit of my rendering more than a small
percentage of the facts which in each case go to corroborate the
conclusion. But although a great deal must thus be necessarily lost on
the one side, I am disposed to think that more will be gained on the
other, by presenting, in a terser form than would otherwise be
possible, the whole theory of organic evolution as I believe that it
will eventually stand. My endeavour, therefore, will be to exhibit the
general structure of this theory in what I take to be its strictly
logical form, rather than to encumber any of its parts by a lengthy
citation of facts. Following this method, I shall in each case give only
what I consider the main facts for and against the positions which have
to be argued; and in most cases I shall arrange the facts in