The same holds good in many other cases, and systematists have often
become uncertain [238] as to the true value of some form, by its
relationship to the allied types in the way of retrogressive
modification. Color-differences are so showy, that they easily
overshadow other characters. The white and the blue thorn-apple, the
white and the red campion (_Lychnis vespertina_ and _diurna_) and many
other illustrative cases could be given, in which two forms are
specifically separated by some authors, but combined by others on the
ground of the retrograde nature of some differentiating mark.
Hitherto we have dealt with negative characters and tried to prove that
the conception of latency of the opposite positive characteristics is a
more natural explanation of the phenomenon than the idea of a complete
loss. We have now to consider the positive varieties, and to show that
it is quite improbable that here the species have struck out for
themselves a wholly new character. In some instances such may have been
the case, but then I should prefer to treat these rather as elementary
species. But in the main we will have to assume the latency of the
character in the species and its reassumption by the variety when
originating, as the most probable explanation.
Great stress is laid upon this conception by the fact, that positive
varieties are so excessively rare when compared with the common
occurrence [239] of negative ones.
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