This may be called the ordinary type of latency.
Besides this there is another form of latent characters, in which this
awakening power is extremely limited, or wholly absent. It is the
systematic latency, which may be said to belong to species and varieties
in the same way as the ordinary latency belongs to individuals. As this
individual latency may show itself from time to time during the life of
a given plant, the first may only become active from time to time during
the whole existence of the variety or the species. It has no regular
period of activity, nor may it be incited by artificial stimulation.
[220] It emerges from concealment only very rarely and only on its own
initiative. Such instances of atavism have been described in previous
lectures, and their existence has been proved beyond doubt.
Systematic latency explains the innumerable instances in which species
are seen to lack definite characteristics which ordinarily do not fail,
either in plants at large, or in the group or family to which the plant
belongs. If we take for instance the broom-rape or _Orobanche_, or some
other pale parasite, we explain their occurrence in families of plants
with green leaves, by the loss of the leaves and of the green color. But
evidently this loss is not a true one, but only the latency of those
characters.
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