ses the
patches of darker colour in the two upper petals; and that when this
occurs, the adherent nectary is quite aborted; when the colour is
absent from only one of the two upper petals, the nectary is only much
shortened.

With respect to the difference in the corolla of the central and
exterior flowers of a head or umbel, I do not feel at all sure that C.
C. Sprengel's idea that the ray-florets serve to attract insects, whose
agency is highly advantageous in the fertilisation of plants of these
two orders, is so far-fetched, as it may at first appear: and if it be
advantageous, natural selection may have come into play. But in regard
to the differences both in the internal and external structure of the
seeds, which are not always correlated with any differences in the
flowers, it seems impossible that they can be in any way advantageous
to the plant: yet in the Umbelliferae these differences are of such
apparent importance--the seeds being in some cases, according to Tausch,
orthospermous in the exterior flowers and coelospermous in the central
flowers,--that the elder De Candolle founded his main divisions of
the order on analogous differences. Hence we see that modifications of
structure, viewed by systematists as of high value, may be wholly due to
unknown laws of correlated growth, and without being, as far as we can
see, of the slightest service to the species.

We may often falsely attribute to correlation of growth, structures
which are common to whole groups of species, and which in truth are
simply due to inheritance; for an ancient progenitor may have acquired
through natural selection some one modification in structure, and, after
thousands of generations, some other and independent modification; and
these two modifications, having been transmitted to a whole group
of descendants with diverse habits, would naturally be thought to be
correlated in some necessary manner. So, again, I do not doubt that some
apparent correlations, occurring throughout whole orders, are entirely
due to the manner alone in which natural selection can act. For
instance, Alph. De Candolle has remarked that winged seeds are never
found in fruits which do not open: I should explain the rule by the fact
that seeds could not gradually become winged through natural selection,
except in fruits which opened; so that the individual plants producing
seeds which were a little better fitted to be wafted further, might get
an advantage ove