long fed on distinct plants, it cannot be expected that
intermediate links connecting the several forms should now be found.
The naturalist thus loses his best guide in determining whether to rank
doubtful forms as varieties or species. This likewise necessarily occurs
with closely allied organisms, which inhabit distinct continents or
islands. When, on the other hand, an animal or plant ranges over the
same continent, or inhabits many islands in the same archipelago, and
presents different forms in the different areas, there is always a
good chance that intermediate forms will be discovered which will link
together the extreme states; and these are then degraded to the rank of
varieties.

Some few naturalists maintain that animals never present varieties; but
then these same naturalists rank the slightest difference as of specific
value; and when the same identical form is met with in two distant
countries, or in two geological formations, they believe that two
distinct species are hidden under the same dress. The term species thus
comes to be a mere useless abstraction, implying and assuming a separate
act of creation. It is certain that many forms, considered by highly
competent judges to be varieties, resemble species so completely in
character that they have been thus ranked by other highly competent
judges. But to discuss whether they ought to be called species or
varieties, before any definition of these terms has been generally
accepted, is vainly to beat the air.

Many of the cases of strongly marked varieties or doubtful species well
deserve consideration; for several interesting lines of argument, from
geographical distribution, analogical variation, hybridism, etc., have
been brought to bear in the attempt to determine their rank; but space
does not here permit me to discuss them. Close investigation, in many
cases, will no doubt bring naturalists to agree how to rank doubtful
forms. Yet it must be confessed that it is in the best known countries
that we find the greatest number of them. I have been struck with the
fact that if any animal or plant in a state of nature be highly useful
to man, or from any cause closely attracts his attention, varieties of
it will almost universally be found recorded. These varieties, moreover,
will often be ranked by some authors as species. Look at the common oak,
how closely it has been studied; yet a German author makes more than a
dozen species out of forms, which are al