664 ICHTHYOLOGY [VARIATION. The majority of Teleostei are mixogamous that is, the males and females congregate on the spawning-beds, and, the number of the former being in excess, several males attend to the same female, frequently changing from one to another. The same habit has been observed in Lepi- dosteus. Gastrosteus is truly polygamous, several females depositing their ova in the same nest, guarded by one male only. Some Teleostei (Ophiocephalus), and probably all Chondropterygians, are monogamous ; and it is asserted that the connexion between the pair is not merely tem porary, but lasts until they are separated by accident. All those Teleosteans also are probably monogamous which bring forth living young. Hybridism is another source of changes and variations within the limits of a species, and is by no means so rare as has been hitherto believed ; it is apparently of excep tional occurrence, merely because the life of fishes is more withdrawn from our direct observation than that of ter restrial animals. It has been observed among species of Serranus, Pleuronectidce, Cyprinidce, Clupeidce, and especi ally Salmonidce. As with other animals, the more certain kinds of fishes are brought under domestication, the more readily do they interbreed with other allied species. It is characteristic of hybrids that their characters are very vari able, the degrees of affinity to the one or the other of the parents being inconstant; and, as these hybrids are known readily to breed with either of the parent race, the variations of form, structure, and colour are infinite. Of internal organs the teeth, the gill-rakers, and the pyloric appen dages are those particularly affected by such mixture of species. Some fishes are known to grow rapidly (in the course of from one to three years) and regularly to a certain size, growth being definitely arrested after the standard has been attained. Such fishes may be called "full-grown," in the sense in which the term is applied to warm-blooded verte brates ; the sticklebacks, most Cyprinodonts, and many Clupeoids (herring, sprat, pilchard) are examples of this regular kind of growth. 1 But in the majority of fishes the rate of growth is extremely irregular, and it is hardly possible to know when growth is actually and definitely arrested. All seems to depend on the amount of food and the more or less favourable circumstances under which the individual grows up. Fishes which rapidly grow to a de finite size are short-lived, whilst those, Teleosteans as well as Chondropterygians, which steadily and slowly increase in size attain to a great age. Carp and pike have been ascertained to live beyond a hundred years. Abundance or scarcity of food, and other circumstances connected with the localities inhabited by fishes, affect considerably the colour of their muscles and integuments ; the periodical changes of colour in connexion with their sexual functions have been referred to above. The flesh of many Teleostei is colourless, or but slightly tinged by the blood ; that of Scombridce, and most Ganoids and Chon dropterygians, is more or less red ; but in badly-fed fishes, as well as in very young ones, the flesh is invariably white (anaemic). Many fishes, like the Salmonidce, feed at times exclusively on crustaceans, and the colouring substance of these invertebrates, which by boiling and by the stomachic secretion turns red, seems to pass into their flesh, impart ing to it the well-known " salmon " colour. The colora- 1 This applies only to individuals growing up under normal condi tions. Dr H. A. Meyer has made observations on young herrings. Individuals living in the sea had attained at the end of the third month a length of 45 to 50 millimetres, whilst those reared from artificially impregnated ova were only from 30 to 35 millimetres long. When the latter had been supplied with more abundant food, they grew pro portionally more rapidly in the following months, so that at the end of the fifth month they had reached the same length as their brethren in the sea, viz., 65 to 70 millimetres. tion of the integuments of many marine fishes, again, is dependent on the nature of their surroundings. In those which habitually hide themselves on the bottom, in sand, between stones, or among seaweed, the colours of the body readily assimilate to those of the vicinity, and are thus an important element in the economy of their life. The changes from one set or tinge of colours to another may be rapid and temporary, or more or less permanent; in some fishes as in the Pediculati, of which the sea-devil, or Lophius, and Antennarius are members scarcely two individuals are found exactly alike in coloration, and such differences are only too frequently mistaken for specific characters. The changes of colour are produced in two ways, either by an increase or decrease of the pig ment-cells, or chromatophors (black, red, yellow, &c.), 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, owing to the great sensitiveness of the cells, is rapid, 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 pheno menon clearly due to the pressure of the convulsively- contracted muscles on the chromatophors. External irritation readily excites the chromatophors to expand a fact unconsciously utilized 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. It does not, how ever, require such strong measures to prove the sensitiveness of the chromatophors to external irritation, the mere change from 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, the 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 due either to 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 that have been domesticated, which we shall men tion in the following section. DOMESTICATION, TENACITY OF LIFE, HIBERNATION, &c. Only a few fishes are thoroughly domesticated that is, Dom bred in captivity, and capable of transportation within catic certain climatic limits viz., the carp, crucian carp (European and Chinese varieties), tench, orfe or ide, and goramy. The first two have accompanied civilized man almost to every part of the globe where he has effected a
permanent settlement.
