election and fecundal selection. Fecundal
selection is said by them to be constantly tending to increase the
reproductive rate, because fecundity is partly a matter of heredity, and
the fecund parents leave more offspring with the same characteristic.
Lethal selection, on the contrary, constantly asserts its power to
reduce the reproductive rate, because the reproductive demands on the
parents reduce their chances of life by interference with their natural
ability of self-protection. 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. Evidently, therefore, the make-up of
society must necessarily be changing from generation to generation. It
will be the object of the rest of this chapter to investigate the ways
in which it is changing, while in the latter half of the book we shall
point out some of the ways in which it might be changed to better
advantage than it is at present.

Sexual selection, or differential success in marrying, will be discussed
at some length in Chapter XI; here it may be pointed out that the number
who fail to marry is very