Contributions to the theory of natural selection. A series of essays ... By Alfred Russel Wallace ...

ITDEFINITELY FROM THE ORIGINAL TYPE 33
the weakest —the very young, the aged, and the
diseased —while those that prolong their existence
can only be the most perfect in health and vigourthose who are best able to obtain food regularly, and
avoid their numerous enemies. It is, as we commenced by remarking, " a struggle for existence," in
which the weakest and least perfectly organized
must always succumb.
The Ablcndance or Rarity of a Species dependent upon
its more or less perfect Adaptation to the Conditions of Existence.
It seems evident that what takes place among the
individuals of a species must also occur among the
several allied species of a group,-viz., that those
which are best adapted to obtain a regular supply
of food, and to defend themselves against the attacks
of their enemies and the vicissitudes of the seasons,
must necessarily obtain and preserve a superiority
in population; while those species which fromn some
defect of power or organization are the least capable of counteracting the vicissitudes of food-supply,
&c., must diminish in numbers, and, in extreme
cases, become altogether extinct. Between these extremes the species will present various degrees of
capacity for er;uring the means of preserving life;
and it is thus we account for the abundance or rarity
of species. Our ignorance will generally prevent us
from accurately tracing the effects to their causes;
but could we become perfectly acquainted with the
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