others; for instance, an individual with a tendency to produce more
males than females would not succeed better in the battle for life than
an individual with an opposite tendency; and therefore a tendency of
this kind could not be gained through natural selection. Nevertheless,
there are certain animals (for instance, fishes and cirripedes) in
which two or more males appear to be necessary for the fertilisation of
the female; and the males accordingly largely preponderate, but it is
by no means obvious how this male-producing tendency could have been
acquired. I formerly thought that when a tendency to produce the two
sexes in equal numbers was advantageous to the species, it would follow
from natural selection, but I now see that the whole problem is so
intricate that it is safer to leave its solution for the future.
CHAPTER IX.
SECONDARY SEXUAL CHARACTERS IN THE LOWER CLASSES OF THE ANIMAL KINGDOM.
These characters absent in the lowest classes Brilliant
colours Mollusca Annelids Crustacea, secondary sexual characters
strongly developed; dimorphism; colour; characters not acquired before
maturity Spiders, sexual colours of; stridulation by the
males Myriapoda.
With animals belonging to the lower classes, the two sexes are not
rarely united in the same individual, and therefore secondary sexual
characters cannot be developed. In many cases where the sexes are
separate, both are permanently attached to some support, and the one
cannot search or struggle for the other. Moreover it is almost certain
that these animals have too imperfect senses and much too low mental
powers to appreciate each other s beauty or other attractions, or to
feel rivalry.
Hence in these classes or sub-kingdoms, such as the Protozoa,
Coelenterata, Echinodermata, Scolecida, secondary sexual characters, of
the kind which we have to consider, do not occur: and this fact agrees
with the belief that such characters in the higher classes have been
acquired through sexual selection, which depends on the will, desire,
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