species, namely Q. pedunculata, sessiliflora and pubescens. The forms
which connect these three sub-species are comparatively rare; and, as Asa
Gray again remarks, if these connecting forms which are now rare were to
become totally extinct the three sub-species would hold exactly the same
relation to each other as do the four or five provisionally admitted
species which closely surround the typical Quercus robur. Finally, De
Candolle admits that out of the 300 species, which will be enumerated in
his Prodromus as belonging to the oak family, at least two-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