A plausible explanation of the occurrence of peculiarities of structure and coloration in the males of many animals, both vertebrate and invertebrate, was first given by Darwin in his theory of sexual selection. Objections have been raised, however, to that portion of the theory which concerned ornamental peculiarities on the ground that the existence of an esthetic sense sufficiently acute to account for the minute and complex details of coloration is in many cases improbable, and from time to time other theories have been advanced which sought to avoid this difficulty. Among these may be mentioned the theory of Wallace, to the effect that the greater ornamentation of the male is due to his 'surplus of vitality'; that of von Kennel, which is essentially a negative statement of Wallace's, holding that the drain upon the vitality of the female in the production of ova prevents the full fruition of development seen in the males; and the sacrificial theory of Jäger and Stolzmann, according to which the brilliancy or excessive development of the male conduces to his destruction whereby the female is indirectly protected and the male is removed from competition with her in the struggle for nourishment.
But all these theories are open to the criticism either that they are too general or that they fail to explain the origin of the sexual peculiarities, and, recently, Professor J. T. Cunningham, in his 'Sexual Dimorphism in the Animal Kingdom' (A. and C. Black, 1900), has endeavored to avoid both these objections by discarding natural selection in all its forms and relying upon the Lamarckian hypothesis. He regards secondary sexual characters as being due to the inheritance of the effects of definite mechanical and physiological irritations, and endeavors to correlate the peculiarities of various species with their habits. Thus he explains the mane of the lion not as a protection, but as the result of the local irritation of the skin due to the habit possessed by fighting lions of seizing one another by the nape of the neck; the coloration and furrowing of the cheek of the mandrill are due to the inheritance of irritations and injuries inflicted by the males in combat; the plumes of the birds of paradise have reached their extensive development by having been erected during sexual excitement and so stimulated to special growth through many generations, and the naked head and neck of the turkey-cock and his wattles represent 'the inherited scars of a long line of pugnacious ancestors.' These examples, and they are by no means extreme, may serve to indicate the general tenor of Professor Cunningham's argument, and it seems more than doubtful if he has strengthened the Lamarckian position or provided valid evidence for the overthrowal of the theory of sexual selection.