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. For the alternate discharge and re-entrance
of heat and the inflow of air are necessary if we are to live. Here we
have at once a necessity in the former of the two senses. But the
alternation of heat and refrigeration produces of necessity an
alternate admission and discharge of the outer air, and this is a
necessity of the second kind.
In the foregoing we have an example of the method which we must
adopt, and also an example of the kind of phenomena, the causes of
which we have to investigate.
2
Some writers propose to reach the definitions of the ultimate
forms of animal life by bipartite division. But this method is often
difficult, and often impracticable.
Sometimes the final differentia of the subdivision is sufficient
by itself, and the antecedent differentiae are mere surplusage. Thus
in the series Footed, Two-footed, Cleft-footed, the last term is
all-expressive by itself, and to append the higher terms is only an
idle iteration. Again it is not permissible to break up a natural
group, Birds for instance, by putting its members under different
bifurcations, as is done in the published dichotomies, where some
birds are ranked with animals of the water, and others placed in a
different class. The group Birds and the group Fishes happen to be
named, while other natural groups have no popular names; for instance,
the groups that we may call Sanguineous and Bloodless are not known
popularly by any designations
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