This is quite true, but the analysis is
incomplete, for an increased number of progeny not only decreases the
life chances of the parents, but also of the young, by reducing the
amount of care they receive.
In short, lethal selection and reproductive selection accomplish the
same end--a change in the constitution of the species--by different
means; but they are so closely linked together and balanced that any
change in the operation of one is likely to cause a change in the
operation of the other. This will be clearer when the effect of
reproductive selection is studied in man.
Recalling the truism that most human characters have a hereditary basis,
it is evident that the constitution of society will remain stable from
generation to generation, only if each section of society is reproducing
at the same rate as every other (and assuming, for the moment, that the
death-rate remains constant). Then if the birth-rate of one part of the
population is altered, if it is decreased, for example, the next
generation will contain proportionately fewer representatives of this
class, the succeeding generation fewer still, and so on
indefinitely--unless a selective death-rate is operating at the same
time. It is well known not only that the death-rate varies widely in
different parts of the population, as was pointed out in the earlier
part of this chapter, but that the birth-rate is rarely the same in any
two sections of the population.
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