It is important to distinguish certain well-marked sexual features in both males and females which are known as the Primary and Secondary Sexual Characters.
A man's primary sexual characters are represented by his genital organs, and centre round the production of spermatozoa and the function of impregnation; a woman's primary sexual characters centre round her genital organs, the production of ova, and the development and birth of the foetus.
The Primary Sexual Characters are, of course, those that pertain to the sexual organs themselves and to their functions, and naturally they are the most pronounced of all the sexual attributes.
As accessories to these leading sexual features are the Secondary Sexual Characters, which comprise all those attributes of body and mind not directly related to the sexual organs proper, but which nevertheless are distinctive and constitute notable differences between the sexes.
As Darwin has so well shown, these secondary sexual characters help the males to fight for, or to court the sexual favor of the females; for instance, the horns of the stag and the spurs of the cock are weapons which their owners use against male rivals of their own species, and the strongest gains the consent of the female, who, quietly awaiting the issue, bestows her favors on the victor. So also the function of the lion's mane is to serve partly as a weapon of defence for the protection of his neck, but chiefly as a mark of beauty to attract the female.
Male birds usually effect their conquests, as Darwin further shows, by peaceful means, such as the melodiousness of their singing and the gaudiness of their plumage; e.g., the canary cock's singing, the cock's comb, the tail of the bird of paradise, and the superior brilliancy of all male birds.
The secondary sexual characters do not appear in animals until they have arrived at an age when they are capable of reproduction; and as a rule the females are not gaudily and showily equipped, because of their greater necessity of protection from beasts of prey. At the rutting season, when the sexual vigor is at its maximum, the plumage is gaudiest, the fur the handsomest, the horns the largest, the voice the loudest, the scent glands the most odoriferous, and all the sexual characters the most pronounced.
Flowers and the songs of birds are the tokens of the reproductive transport of nature, flowers being the dress of love, and the songs of birds love-songs. Men find these very beautiful in themselves, and think of them as specially designed to gratify their senses. But is it not that they are beautiful, by secret sympathy of being, because they are expressions of the generative energy of nature in which men share? And most felt of beautiful in spring, when the sympathy of a common thrill is active.
Botanists tell us that cross-fertilization is necessary for the reproduction of plants, they having separate sexes like the animals. Some plants are wind-fertilized, and some are visited by insects, the object being in either case an assurance that the pollen, or male elements, shall be carried, either by the wind, or water, or insects, from the anthers of one plant to the stigma, or female structure, of another.
Flowers which are dependent on the agency of the wind for the scattering of their pollen are never gayly colored, and "beauty serves merely as a guide to birds and beasts, in order that the fruit may be devoured and the manured seeds disseminated".
Animals and plants have not been created beautiful in order to delight man, but for sexual reasons, in order to compel sexual conjunction, upon which the future of every species depends.