castes are connected together by finely graduated varieties. So it is, as
I have myself observed, with some dimorphic plants. It certainly at first
appears a highly remarkable fact that the same female butterfly should have
the power of producing at the same time three distinct female forms and a
male; and that an hermaphrodite plant should produce from the same seed-
capsule three distinct hermaphrodite forms, bearing three different kinds
of females and three or even six different kinds of males. Nevertheless
these cases are only exaggerations of the common fact that the female
produces offspring of two sexes which sometimes differ from each other in a
wonderful manner.

DOUBTFUL SPECIES.

The forms which possess in some considerable degree the character of
species, but which are so closely similar to other forms, or are so closely
linked to them by intermediate gradations, that naturalists do not like to
rank them as distinct species, are in several respects the most important
for us. We have every reason to believe that many of these doubtful and
closely allied forms have permanently retained their characters for a long
time; for as long, as far as we know, as have good and true species.
Practically, when a naturalist can unite by means of intermediate links any
two forms, he treats the one as a variety of the other, ranking the most
common, but sometimes the one first described as the species, and the other
as the variety. But cases of great difficulty, which I will not here
enumerate, sometimes arise in deciding whether or not to rank one form as a
variety of another, even when they are closely connected by intermediate
links; nor will the commonly assumed hybrid nature of the intermediate
forms always remove the difficulty. In very many cases, however, one form
is ranked as a variety of another, not because the intermediate links have
actually been found, but because analogy leads the observer to suppose