Of course, in all species where each individual carries only one of the two kinds of sexual organs, there must of necessity be some slight difference between the individuals that carry the male organ, which performs one function, and those that carry the female organ, which performs another function. But in many species the sexes display differences which have no direct connection with the generative organs—for example, the deer, where the stag alone has horns. Those characters which differ with the sex, but are not directly connected with the organs of reproduction, are known as secondary sexual characters. [Illustration: QUEEN WHYDAH] Theory of Sexual Selection In nearly all species where the male and female differ in beauty, it is the male who surpasses the female. Natural selection is, in many cases, not able to explain the origin of these differences, or why, when they occur, the male should be more beautiful than the female. This Darwin saw. In order to account for the phenomena of sexual dimorphism, he formulated the theory of sexual selection. This hypothesis is based on the assumption that there is, in all species of animals, a competition among the males to secure females as mates. It is not difficult to understand how this competition arises in polygamous species. Assuming that approximately equal numbers of males and females are born (an assumption which appears to be justified as regards the majority of species), it is clear that for every male who secures more than one wife, at least one male will be obliged to live in a state of single blessedness. But how can there be competition in the case of monogamous species? The sexes being approximately equal in number, there are sufficient females to allow of a mate for every male. The Law of Battle Such is the nature of things, said Darwin, that, even under these circumstances, there is competition among the males for females.