Quite on the
contrary, they compel us to confine our study to the most recently
acquired, or youngest characters, which constitute the differentiating
marks between nearly allied forms.
Obviously this is especially the case in the realm of the hybrids, since
only nearly related forms are able to give hybrid offspring. In dealing
with this subject we must leave aside all questions concerning more
remote relationships.
It is not my purpose to treat of the doctrine of hybridization at any
length. Experience is so rapidly increasing both in a practical and in a
purely scientific direction that it would take an entire volume to give
only a brief survey of the facts and of all the proposed theories.
[251] For our present purposes we are to deal with hybrids only in so
far as they afford the means of a still better distinction between
elementary species and varieties. I will try to show that these two
contrasting groups behave in quite a different manner, when subjected to
crossing experiments, and that the hope is justified that some day
crosses may become the means of deciding in any given instance, what is
to be called a species, and what a variety, on physiologic grounds. It
is readily granted that the labor required for such experiments, is
perhaps too great for the results to be attained, but then it may be
possible to deduce rules from a small series of experiments, which may
lead us to a decision in wider ranges of cases.
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