the missing links in the evidence from his own knowledge, discovers fresh
proof of the singular thoroughness with which all difficulties have been
considered and all unjustifiable suppositions avoided, at every reperusal
of Mr. Darwin's pregnant paragraphs, the novice in biology is apt to
complain of the frequency of what he fancies is gratuitous assumption.

Thus while it may be doubted if, for some years, any one is likely to be
competent to pronounce judgment on all the issues raised by Mr. Darwin,
there is assuredly abundant room for him, who, assuming the humbler, though
perhaps as useful, office of an interpreter between the "Origin of Species"
and the public, contents himself with endeavouring to point out the nature
of the problems which it discusses; to distinguish between the ascertained
facts and the theoretical views which it contains; and finally, to show the
extent to which the explanation it offers satisfies the requirements of
scientific logic. At any rate, it is this office which we purpose to
undertake in the following pages.

It may be safely assumed that our readers have a general conception of the
nature of the objects to which the word "species" is applied; but it has,
perhaps, occurred to a few, even to those who are naturalists _ex
professo_, to reflect, that, as commonly employed, the term has a double
sense and denotes two very different orders of relations. When we call a
group of animals, or of plants, a species, we may imply thereby, either
that all these animals or plants have some common peculiarity of form or
structure; or, we may mean that they possess some common functional
character. That part of biological science which deals with form and
structure is called Morphology--that which concerns itself with function,
Physiology--so that we may conveniently speak of these two senses, or
aspects, of "species"--the one as morphological, the other as
physiological. Regarded from the former point of view, a species is nothing