He argued that whenever any character possessed only by males conferred upon its possessor no direct advantages in the struggle for existence, but served merely to confer advantages upon one male in its struggle with other males to secure a mate, then we could not speak of natural selection, but of sexual selection. In some cases, as in polygamous birds, the two forms of selection might be in opposition, the gorgeous but cumbrous plumes fa voured by sexual selection being definitely disadvantageous in the struggle for existence.
Although much attacked, the theory survives, if in a somewhat altered form, and the evidence now available makes it clear that sexual selection has undoubtedly played a considerable part in moulding the form and behaviour of higher animals.
The most important types of character which have presumably thus originated are as follows:—(i) for pursuit and capture of one sex by the other; (2) offensive and defensive weapons em ployed by members of one sex in fighting for the other; (3) means for making known the presence of one sex to the other; (4) characters employed in stimulating sexual emotion in the other sex; i.e., organs employed in courtship of animals (q.v.).
As examples of these various types may be cited (I) the en larged prehensile antennae of various small Crustacea (Moina; many Copepods) or the enlarged limbs (gnathopods) with which male gammarid Crustacea seize and grip the female ; (2) (a) of fensive—the antlers of stags, the enlarged canines of boars and stallions; (b) defensive—the mane of the lion or of certain male baboons; (3) the chirping of male crickets and the croaking of male frogs, the drumming of certain woodpeckers, the flashing of fireflies; (4) the train of the peacock, the crest (present in both sexes) of the Crested Grebe, the bower of bower-birds.
Naturally these different categories grade into each other; courtship characters grade into recognition marks; weapons may serve also as adornments, and the same character, e.g., great strength, may serve both to fight enemies and to capture mates. It has been suggested with some plausibility that prominent weapons may secure advantage by deterring other males from at tempting combat; there seems no doubt that young stags with poorly-developed antlers avoid battle with large-antlered males. There are a number of prominent secondary sexual characters to which no 'function can yet be assigned; e.g., the enormous horns
of the males of the Goliath and other beetles.
The fact that organs for capturing the female may grade in sensibly into copulatory organs, and organs for stimulating the emotions into those facilitating the meeting of the sexes, suggests a valid criticism of the pure neo-Darwinian doctrine which main tains that sexual selection is something altogether apart from natural selection.
Certain organs and instincts are obviously necessary to a sexu ally-reproducing species if the race is to continue ; and in so far as selection has contributed to their origin, it will, as Darwin himself pointed out, have been natural selection.
Devices for securing efficient mating and fertilization will even tually become more elaborate as we ascend the animal scale, as internal supplants external fertilization, as locomotion becomes more rapid, as the brain becomes more complex. The two chief ways to secure mating will be material force (pursuit and cap ture; or fighting for possession), or stimulation of the opposite sex (advertisement of a "sexual situation" by means of recogni tional sounds, colours, actions or scents; or by emotionally stimu lating display). In all these cases, while natural selection will see to it that the devices are reasonably efficient, there may and usu ally will also exist Darwinian sexual selection, as a result of com petition between males, and this may intensify and exaggerate the characters. Since it will usually be impossible to disentangle the shares of natural and of Darwinian sexual selection, and since they both tend to produce characters of the same type, it is per haps better to re-define sexual selection. If, instead of making competition between members of one sex the criterion, we adopt selection with reference to successful and efficient mating as our definition, the situation clears up considerably. For sexual selec tion in the original sense we may then substitute the term infra sexual selection.
In all recognitional and courtship characters selection is exerted via the brain and mind of the opposite sex, and may thus, with Lloyd Morgan, be called psychical sexual selection (as opposed to psychical selection of a non-sexual nature, such as must have operated in the genesis of conspicuous flowers by psychical selec tion via insects).