The moiety A becomes the body of the adult and, sooner or
later, perishes, while portions of the moiety B are detached and, as
offspring, continue the life of the species. Thus, if we trace back
an organism along the direct line of descent from its remotest
ancestor, B, as a whole, has never suffered death; portions of it,
only, have been cast off and died in each individual offspring.
Everybody is familiar with the way in which the "suckers" of a
strawberry plant behave. A thin cylinder of living tissue keeps on
growing at its free end, until it attains a considerable length. At
[88] successive intervals, it develops buds which grow into strawberry
plants; and these become independent by the death of the parts of the
sucker which connect them. The rest of the sucker, however, may go on
living and growing indefinitely, and, circumstances remaining
favourable, there is no obvious reason why it should ever die. The
living substance B, in a manner, answers to the sucker. If we could
restore the continuity which was once possessed by the portions of B,
contained in all the individuals of a direct line of descent, they
would form a sucker, or stolon, on which these individuals would be
strung, and which would never have wholly died.
A species remains unchanged so long as the potentiality of development
resident in B remains unaltered; so long, e.
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