From a
systematic point of view these leaves indicate the origin of the
water-parsnips from ordinary umbellifers, which generally have bi- and
tripinnate leaves.
Similar cases of double adaptation, dependent on external conditions at
different periods of the evolution of the plant are very numerous. They
are most marked among leguminous plants, as shown by the trifoliolate
leaves of the thorn-broom and allies, which in the adult state have
green twigs destitute of leaves.
As an additional instance of dimorphism and probable double adaptation
to unrecognized external [458] conditions I might point to the genus
_Acacia_. As we have seen in a previous lecture some of the numerous
species of this genus bear bi-pinnate leaves, while others have only
flattened leaf-stalks. According to the prevailing systematic
conceptions, the last must have been derived from the first by the loss
of the blades and the corresponding increase of size and superficial
extension of the stalk. In proof of this view they exhibit, as we have
described, the ancestral characters in the young plantlets, and this
production of bi-pinnate leaves has probably been retained at the period
of the corresponding negative mutations, because of some distinct,
though still unknown use.
Summarizing the results of this discussion, we may state that useful
dimorphism, or double adaptation, is a substitution of characters quite
analogous to the useless dimorphism of cultivated ever-sporting
varieties and the stray occurrence of hereditary monstrosities.
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