Advancement in general in living nature depends on progressive
evolution. In different parts of the vegetable kingdom, and even in
different families this progression takes place on different lines. By
this means it results in an ever increasing divergency between the
several groups. Every step is an advance, and many a step must have been
taken to produce flowering plants from the simplest unicellular algae.
But related to, and very intimately connected with this advancement is
the retrogressive [222] evolution. It is equally universal, perhaps
never failing. No great changes have been attained, without acquiring
new qualities on one side, and reducing others to latency. Everywhere
such retrogressions may be seen. The polypetalous genera _Pyrola_,
_Ledum_, and _Monotropa_ among the sympetalous heaths, are a remarkable
instance of this. The whole evolution of the monocotyledons from the
lowest orders of dicotyledons implies the seeming loss of cambial growth
and many other qualities. In the order of aroids, from the calamus-root
or sweet flag, with its small but complete flowers, up to the reduced
duckweeds (_Lemna_), almost an unbroken line of intermediate steps may
be traced showing everywhere the concurrence of progressive and
retrogressive evolution.
Degressive evolution is not so common by far, and is not so easy to
recognize, but no doubt it occurs very frequently.
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