either by an increase or decrease of the black, red, yellow, etc., pigment-cells, or _chromatophors_, in the skin of the fish; or by the rapid contraction or expansion of the chromatophors which happen to be developed. The former change is gradual, like every kind of growth or development; the latter rapid, owing to the great sensitiveness of the cells, but certainly involuntary. In many bright-shining fishes--as Mackerels, Mullets--the colours appear to be brightest in the time intervening between the capture of the fish and its death: a phenomenon clearly due to the pressure of the convulsively-contracted muscles on the chromatophors. External irritation readily excites the chromatophors to expand--a fact unconsciously utilised by fishermen, who, by scaling the Red Mullet immediately before its death, produce the desired intensity of the red colour of the skin, without which the fish would not be saleable. However, it does not require such strong measures to prove the sensitiveness of the chromatophors to external irritation, the mere change of darkness into light is sufficient to induce them to contract, the fish appearing paler, and _vice versa_. In Trout which are kept or live in dark places, the black chromatophors are expanded, and, consequently, such specimens are very dark-coloured; when removed to the light they become paler almost instantaneously. Total absence of chromatophors in the skin, or _Albinism_, is very rare among fishes; much more common is _incipient Albinism_, in which the dark chromatophors are changed into cells with a more or less intense yellow pigment. Fishes in a state of domestication, like the Crucian Carp of China, the Carp, Tench, and the Ide, are particularly subject to this abnormal coloration, and are known as the common Gold-fish, the Gold-Tench, and the Gold-Orfe. But it occurs also not rarely in fishes living in a wild state, and has been observed in the Haddock, Flounder, Plaice, Carp, Roach, and Eel. It will be evident, from the foregoing remarks, that the amount of variation within the limits of the same species--either due to the natural growth and development, or to external physical conditions, or to abnormal accidental circumstances--is greater in fishes than in any of the higher classes of Vertebrates. The amount of variation is greater in certain genera or families than in others, and it is much greater in Teleosteans and Ganoids than in Chondropterygians. Naturally, it is greatest in the few species which have been domesticated, and which we shall mention in the succeeding chapter. [Illustration: Fig. 96.--Chimæra colliei ♂, west coast of North America. A. Front view of head. B. Palate. _a_, Peritoneal aperture; _b_, Nostrils; _c_, Vomerine teeth; _d_, Mandibular teeth; _e_, Palatine teeth; _f_, Claspers.] CHAPTER XIV.