The structures as yet referred to are permanent in the male after he has arrived at maturity; but with some Blennies and in another allied genus[18] a crest is developed on the head of the male only during the breeding-season, and their bodies at the same time become more brightly-coloured. There can be little doubt that this crest serves as a temporary sexual ornament, for the female does not exhibit a trace of it. In other species of the same genus both sexes possess a crest, and in at least one species neither sex is thus provided. In this case and in that of the Monacanthus, we have good instances to how great an extent the sexual characters of closely-allied forms may differ. In many of the Chromidæ, for instance in Geophagus and especially in Cichla, the males, as I hear from Professor Agassiz,[19] have a conspicuous protuberance on the forehead, which is wholly wanting in the females and in the young males. Professor Agassiz adds, “I have often observed these fishes at the time of spawning when the protuberance is largest, and at other seasons when it is totally wanting and the two sexes shew no difference whatever in the outline of the profile of the head. I never could ascertain that it subserves any special function, and the Indians on the Amazon know nothing about its use.” These protuberances in their periodical appearance resemble the fleshy caruncles on the heads of certain birds; but whether they serve as ornaments must remain at present doubtful. The males of those fishes, which differ permanently in colour from the females, often become more brilliant, as I hear from Professor Agassiz and Dr. Günther, during the breeding-season. This is likewise the case with a multitude of fishes, the sexes of which at all other seasons of the year are identical in colour. The tench, roach, and perch may be given as instances. The male salmon at this season is “marked on the cheeks with orange-coloured stripes, which give it the appearance of a Labrus, and the body partakes of a golden-orange tinge. The females are dark in colour, and are commonly called black-fish.”[20] An analogous and even greater change takes place with the _Salmo eriox_ or bull-trout; the males of the char (_S. umbla_) are likewise at this season rather brighter in colour than the females.[21] The colours of the pike (_Esox reticulatus_) of the United States, especially of the male, become, during the breeding-season, exceedingly intense, brilliant, and iridescent.[22] Another striking instance out of many is afforded by the male stickleback (_Gasterosteus leiurus_), which is described by Mr. Warington,[23] as being then “beautiful beyond description.” The back and eyes of the female are simply brown, and the belly white. The eyes of the male, on the other hand, are “of the most splendid green, having a metallic lustre like the green feathers of some humming-birds. The throat and belly are of a bright crimson, the back of