Now, the greater the amount of food any animal gets, and the less the
amount of expenditure it performs in muscular action, the greater will
be the surplus it has left over for the purposes of reproduction. Eggs
or young, in fact, represent the amount thus left over after all the
wants of the body have been provided for. But in the rose-aphis the
wants of the body, when once the insect has reached its full growth, are
absolutely nothing; and it therefore then begins to bud out new
generations in rapid succession as fast as ever it can produce them.
This is strictly analogous to what we see every day taking place in all
the plants around us. New leaves are produced one after another, as fast
as material can be supplied for their nutrition, and each of these new
leaves is known to be a separate individual, just as much as the
individual aphis. At last, however, a time comes when the reproductive
power of the plant begins to fail, and then it produces flowers, that is
to say stamens (male) and pistils (female), whose union results in
fertilisation and the subsequent outgrowth of fruit and seeds. Thus a
year's cycle of the plant-lice exactly answers to the life-history of an
ordinary annual. The eggs correspond to the seeds; the various
generations of aphides budding out from one another by parthenogenesis
correspond to the leaves budded out by one another throughout the
summer; and the final brood of perfect males and females answers to the
flower with its stamen and pistils, producing the seeds, as they produce
the eggs, for setting up afresh the next year's cycle.
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