In most cases we must be content with the testimony,
given somewhat later, and recorded after the new variety had the
opportunity of showing its greater merits.
If it now happens that such a variety of recorded origin should
occasionally revert to its parent-species, we have all we can wish for,
in the way of a thoroughly proved case of atavism. But such instances
are very rare, as the birth of most varieties has only been very
imperfectly controlled.
Next to this comes the systematic relation of a variety to its species.
The historic origin of the variety may be obscure, or may simply be
forgotten. But the distinguishing marks are of the order described in
our last lecture, either in the positive or in the negative direction,
and on this ground the rarer form is considered to be a variety of the
more wide-spread one. If [172] now the presumed variety sports and runs
over to the presumed type, the probability of the supposed relation is
evidently enhanced. But it is manifest that the explanation rests upon
the results of comparative studies, and not upon direct observations of
the phenomena themselves.
The nearer the relations between the two types in question, the less
exposed to doubt and criticism are the conclusions. But the domain of
atavism is not restricted to the cases described.
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