wo-thirds are provisional
species, that is, are not known strictly to fulfil the definition above
given of a true species. It should be added that De Candolle no longer
believes that species are immutable creations, but concludes that the
derivative theory is the most natural one, "and the most accordant with
the known facts in palaeontology, geographical botany and zoology, of
anatomical structure and classification."

When a young naturalist commences the study of a group of organisms
quite unknown to him he is at first much perplexed in determining what
differences to consider as specific and what as varietal; for he knows
nothing of the amount and kind of variation to which the group is
subject; and this shows, at least, how very generally there is some
variation. But if he confine his attention to one class within one
country he will soon make up his mind how to rank most of the doubtful
forms. His general tendency will be to make many species, for he will
become impressed, just like the pigeon or poultry fancier before alluded
to, with the amount of difference in the forms which he is continually
studying; and he has little general knowledge of analogical variation
in other groups and in other countries by which to correct his first
impressions. As he extends the range of his observations he will meet
with more cases of difficulty; for he will encounter a greater number of
closely-allied forms. But if his observations be widely extended he
will in the end generally be able to make up his own mind; but he will
succeed in this at the expense of admitting much variation, and the
truth of this admission will often be disputed by other naturalists.
When he comes to study allied forms brought from countries not now
continuous, in which case he cannot hope to find intermediate links,
he will be compelled to trust almost entirely to analogy, and his
difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn between
species and sub-species--that is, the forms which in the opinion of some
naturalists come very near to, but do not quite arrive at, the rank of
species; or, again, between sub-species and well-marked varieties, or
between lesser varieties and individual differences. These differences
blend into each other by an insensible series; and a series impresses
the mind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to the
s