that varieties on the one hand, and what are called closely-related species
on the other, do not differ except in degree. Whenever this wider
difference separating the latter can be spanned by intermediate forms, as
it sometimes is, no botanist long resists the inevitable conclusion.
Whenever, therefore, this wider difference can be shown to be compatible
with community of origin, and explained through natural selection or in any
other way, we are ready to adopt the probable conclusion; and we see
beforehand how strikingly the actual geographical association of related
species favors the broader view. Whether we should continue to regard the
forms in question as distinct species, depends upon what meaning we shall
finally attach to that term; and that depends upon how far the doctrine of
derivation can be carried back and how well it can be supported.

In applying his principle of natural selection to the work in hand, Mr.
Darwin assumes, as we have seen: i. Some variability of animals and plants
in nature; 2. The absence of any definite distinction between slight
variations, and varieties of the highest grade; 3. The fact that
naturalists do not practically agree, and do not increasingly tend to
agree, as to what forms are species and what are strong varieties, thus
rendering it probable that there may be no essential and original
difference, or no possibility of ascertaining it, at least in many cases;
also, 4. That the most flourishing and dominant species of the larger
genera on an average vary most (a proposition which can be substantiated
only by extensive comparisons, the details of which are not given); and, 5.
That in large genera the species are apt to be closely but unequally allied
together, forming little clusters round certain species--just such clusters
as would be formed if we suppose their members once to have been satellites
or varieties of a central or parent species, but to have attained at length
a wider divergence and a specific character. The fact of such association
is undeniable; and the use which Mr. Darwin makes of it seems fair and