With respect to plants, this
subject has been discussed by Nageli in an admirable essay. He admits that
natural selection has effected much, but he insists that the families of
plants differ chiefly from each other in morphological characters, which
appear to be quite unimportant for the welfare of the species. He
consequently believes in an innate tendency towards progressive and more
perfect development. He specifies the arrangement of the cells in the
tissues, and of the leaves on the axis, as cases in which natural selection
could not have acted. To these may be added the numerical divisions in the
parts of the flower, the position of the ovules, the shape of the seed,
when not of any use for dissemination, etc.
There is much force in the above objection. Nevertheless, we ought, in the
first place, to be extremely cautious in pretending to decide what
structures now are, or have formerly been, of use to each species. In the
second place, it should always be borne in mind that when one part is
modified, so will be other parts, through certain dimly seen causes, such
as an increased or diminished flow of nutriment to a part, mutual pressure,
an early developed part affecting one subsequently developed, and so forth
--as well as through other causes which lead to the many mysterious cases
of correlation, which we do not in the least understand. These agencies
may be all grouped together, for the sake of brevity, under the expression
of the laws of growth.
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