It is generally
called atavism, or better, systematic atavism, and the clearest cases
are those in which a quality which is latent in the greater part of a
family or group, becomes manifest in one of its members. Bracts in the
inflorescence of crucifers are ordinarily wanting, but may be seen in
some genera, _Erucastrum pollichii_ being perhaps the [223] most widely
known instance, although other cases might easily be cited.
For our special purpose we may take up only the more simple cases that
may be available for experimental work. The great lines of evolution of
whole families and even of genera and of many larger species obviously
lie outside the limits of experimental observation. They are the outcome
of the history of the ancestors of the present types, and a repetition
of their history is far beyond human powers. We must limit ourselves to
the most recent steps, to the consideration of the smallest differences.
But it is obvious that these may be included under the same heads as the
larger and older ones. For the larger movements are manifestly to be
considered only as groups of smaller steps, going in the same direction.
Hence we conclude, that even the smallest steps in the evolution of
plants which we are able to observe, may be divided into progressive,
retrogressive and degressive ones.
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