n the length and curvature of the style as well as in the
size and shape of the corolla; and the two forms are evidently becoming
each adapted to special conditions, since in some districts the one, in
other districts the other is most abundant.[159]

These examples show us that the kind of change suggested above is
actually going on, and has presumably always been going on in nature
throughout the long geological epochs during which the development of
flowers has been progressing. The two great modes of gaining increased
vigour and fertility--intercrossing and dispersal over wider areas--have
been resorted to again and again, under the pressure of a constant
struggle for existence and the need for adaptation to ever-changing
conditions. During all the modifications that ensued, useless parts were
reduced or suppressed, owing to the absence of selection and the
principle of economy of growth; and thus at each fresh adaptation some
rudiments of old structures were re-developed, but not unfrequently in
a different form and for a distinct purpose.

The chief types of flowering plants have existed during the millions of
ages of the whole tertiary period, and during this enormous lapse of
time many of them may have been modified in the direction of insect
fertilisation, and again into that of self-fertilisation, not once or
twice only, but perhaps scores or even hundreds of times; and at each
such modification a difference in the environment may have led to a
distinct line of development. At one epoch the highest specialisation of
structure in adaptation to a single species or group of insects may have
saved a plant from extinction; while, at other times, the simplest mode
of self-fertilisation, combined with greater powers of dispersal and a
constitution capable of supporting diverse physical conditions, may have
led to a similar result. With some groups the tendency seems to have
been almost continuously to greater and greater specialisation, while
with others a tendency to simplification and degradation has resulted in
such plants as the grasses and sedges.

We are now enabled dimly to perceive how the curious anomaly of very
simple and very complex methods of securing cross-fertilisation--both
equally effective--may have been brought about. The simple modes may be
the result of a comparatively direct modification from the more
primitive types of flowers, which were occasionally, and, as it were,
accidentally visited an