They are only reputed to be such, and the presumption rests [632] upon
the systematic affinity between the derivative species and its nearest
probable allies. Such reversions are now to be examined at some length
and may be adequately treated under the head of systematic atavism. To
this form of atavism pertain, on the basis of our definition, those
phenomena by which species assume one or more characters of allies, from
which they are understood to have descended by the loss of the character
under discussion. The phenomena themselves consist in the production of
anomalies and varieties, and as the genetic relation of the latter is
often hardly beyond doubt, the anomalies seem to afford the best
instances for the study of systematic atavism. This study has for its
chief aim the demonstration of the presence of the latent characters,
and to show that they return to activity suddenly and not by a slow and
gradual recovery of the former features. It supports the assertion that
the visible elementary characters are essentially an external display of
qualities carried by the bearers of heredity, and that these bearers are
separate entities, which may be mingled together, but are not fused into
a chaotic primitive life-substance. Systematic atavism by this means
leads us to a closer examination of the internal and concealed causes,
which rule the affinities and divergencies of [633] allied species.
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