ABSTRACT
Animals which have their sexes separated, the males necessarily differ from the females in their organs of reproduction. These are the primary sexual characters. But the sexes often differ in what J. Hunter has called secondary sexual characters, which are not directly connected with the act of reproduction. The female often differs from the male in having organs for the nourishment or protection of her young, such as the mammary glands of mammals, and the abdominal sacks of the marsupials. Sexual selection will also be largely dominated by natural selection tending towards the general welfare of the species. Hence the manner in which the individuals of either or both sexes have been affected through sexual selection cannot fail to be complex in the highest degree. As variations occurring later in life, and transmitted to one sex alone, have incessantly been taken advantage of and accumulated through sexual selection in relation to the reproduction of the species.
