Darwin proposed the theory of sexual selection to explain the origin of ostentatious plumage in certain bird species, maintaining that the ornate features of males are a consequence of female mate selection based on an Line abstract aesthetic sense, not unlike the process of animal breeders producing fancy-male varieties of pigeons by conscious artificial selection. Wallace suggested an alternative explanation: through greater physical energy the most highly adorned males are able to win the competition with rival males. Meanwhile Huxley pointed out that male adornment is instrumental in establishing dominance relationships among males: adornment reduces the(10) physical activity necessary to intimidate rivals.
     However, Jacobs later examined the process of female choice, concluding that what appeared to be choice of an adorned male by a female was really a mutual attraction to a certain reproductive site. Mate selection requires an awareness of features characteristic of a suitable breeding site, which might be(15) mirrored in the ornamentation of the male, and thus mate selection is related directly to adaptive niche specialization. From this insight, Austin proceeded to develop a food-courtship theory of mate selection: the population most efficient in use of the energy available in a particular niche will be the fittest to survive there. Through natural selection, organisms will tend to become specialized to(20) form isolated populations, each adapted to utilize the energy most efficiently that is available in a particular niche and this process of segregation and specialization of populations is facilitated by employing in the mating process samples of the food available in the preferred niche. In particular cases, the male may display the food to the female or feed it to her in the courtship(25) ceremony, maybe bearing permanent representations of specific foods on his plumage, and the female may be attracted to the male for these representations of the territorial foods.
     Austin’s theory may be applied to the case for mate choice among peafowls, whose males’ "eyespotted" tail feathers bear a striking resemblance(30) to blue berries. According to the food-courtship theory, it is because their plumage bears representations of food that peacocks attract peahens, which may explain why males with the most "eyespots" on their tail have the greatest mating success. Not inconsistent with a possible role of the "eyespots" in reproductive competition among males and in aesthetic selection, this(35) explanation seems more plausible than the suggestion that by selecting mates according to the perfection of their tail-feather "eyespots", peahens are able to identify mates with the greatest "fitness". This process, bringing together males and females of similar tastes and physiologies, may lead to speciation. Some of the male display features may come to be involved in species(40) identification, and it has also been noted that male adornment could have a dual function, repelling rival males as well as attracting females.

Which of the following illustrative devices is NOT used in the passage（）