nothing more than the gratuitous assumption of a theory.

* * * * *

I will now sum up the various considerations which have occupied us
during the present chapter.

First of all we must take note that the classification of plants and
animals in groups subordinate to groups is not merely arbitrary, or
undertaken only for a matter of convenience and nomenclature--such, for
instance, as the classification of stars in constellations. On the
contrary, the classification of a naturalist differs from that of an
astronomer, in that the objects which he has to classify present
structural resemblances and structural differences in numberless
degrees; and it is the object of his classification to present a tabular
statement of these facts. Now, long before the theory of evolution was
entertained, naturalists became fully aware that these facts of
structural resemblances running through groups subordinate to groups
were really facts of nature, and not merely poetic imaginations of the
mind. No one could dissect a number of fishes without perceiving that
they were all constructed on one anatomical pattern, which differed
considerably from the equally uniform pattern on which all mammals were
constructed, even although some mammals bore an extraordinary
resemblance to fish in external form and habits of life. And similarly
with all the smaller divisions of the animal and vegetable kingdoms.
Everywhere investigation revealed the bonds of close structural
resemblances between species of the same genus, resemblance less close
between genera of the same family, resemblance still less close between
families of the same order, resemblance yet more remote between orders
of the same class, and resemblance only in fundamental features between
classes of the same sub-kingdom, beyond which limit all anatomical
resemblance was found to disappear--the different sub-kingdoms being
formed on wholly different patterns. Furthermore, in tracing all these
grades of structural relationship, naturalists were slowly led to
recognise that the form which a natural classification must eventually
assume would be that of a tree, wherein the constituent branches would
display a progressive advance of organization from below upwards.

Now we have seen that although this tree-like arrangement of natural
groups was as suggestive as anything could well be of all the forms oL
life being bound together by the ties of genetic relati