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organs fully developed and functional In addition to this, higher
animals have one or more sexual cycles within their breeding seasons,
and only at one phase of the cycle are they prepared to mate. In
general, either a sexual season or a sexual cycle, or both, operates to
restrict mating.
In man, however, neither of these factors is at work. There appears
to be indications of a breeding season in some primitive peoples like
the Eskimo, but even there they are but relics, Similarly, while there
still exist physiological differences in sexual desire at different phases
of the female sexual cycle, these are purely quantitative, and may
readily be overridden by psychological factors. Man, to put it briefly,
is continuously sexed: animals are discontinuously sexed. If we try
to imagine what a human society would be like in which the sexes
were interested in each other only during the summer, as in song-
birds, or, as in female dogs, experienced sexual desire only once every
few months, or even only once in a lifetime, as in ants, we can realize
what this peculiarity has meant. In this, as in his slow growth and
prolonged period of dependence, man is not abruptly marked off
from all other animals, but represents the culmination of a process
that can be clearly traced among other Primates. What the bio-
logical meaning of this evolutionary trend may be is difficult to under**
stand. One suggestion is that it may be associated with the rise of
mind to dominance. The bodily functions, in lower mammals
rigidly determined by physiological mechanisms, come gradually
under the more plastic control of the brain. But this, for what it is
worth, is a mere speculation.
Another of the purely biological characters in which man is unique
is his reproductive variability. In a given species of animals, the
maximum litter-size may, on occasions, reach perhaps double the
minimum, according to circumstances of food and temperature, or
even perhaps threefold. But during a period of years, these varia-
tions will be largely equalized within a range of perhaps fifty per cent*
either way from the average, and the percentage of wholly infertile
adults is very low. In man, on the other hand, the range of positive
fertility is enormous—from one to over a dozen, and in exceptional
cases to over twenty; and the number of wholly infertile adults is
considerable. This fact, in addition to providing a great diversity of
patterns of family life, has important bearings on evolution. It means
that in the human species differential fertility is more important as a
basis for selection than is differential mortality; and it provides the
possibility of much more rapid selective change than that found in
wild animal species. Such rapidity of evolution would, of course* be