Germinal Variations—Fishes and mate—hunting—Some remarkable Sexual differences displayed by the Teeth of Rays—The Double-eyed Fish—The Coloration of the Dragonet—Some curious facts about Salmon—The strange use of the kidneys in the Stickle-back—The Stickle-back and parental duties—Siamese Fighting-fish. Mammals, Birds, Reptiles and Amphibia, as has already been shown, all exhibit practically the same line of conduct in regard to their mate-hunting instincts; all use like modes of expression. And this is a very significant fact. It becomes more so when we turn to the fishes, for here again we meet with the same behaviour, and here again we meet with the same rules in “secondary sexual characters.”
An instance or two of the latter distinction between the sexes should suffice. As a rule, among fishes, the males are smaller than the females: commonly there is no other external distinguishing feature between them. In many cases, however, the males are more or less strikingly different, thereby showing a departure in the nature of a higher degree of complexity, or “specialization,” just as obtains among the birds. And the same 176sequence to this also obtains. That is to say, as has already been remarked in the case of the Mammals and the Birds, the new features first appear in the males, leaving the females and young of both sexes unmodified. A singular illustration of this is afforded by some of the Rays, or Skates, as they are often called. In the Thorn-backed Ray (Raia clavata) the teeth of the adult male are sharp-pointed and directed backwards, while those of the female are broad and flat, forming a sort of mosaic or pavement. The young male agrees with the female in this respect. In the Common Blue Skate (Raia batis) the teeth are pointed in both sexes, though more so in the adult male. In the Spotted Skate (Raia maculata) the teeth are fully pointed in both sexes. Here, then, the normal course in the evolution of new characters is followed, but it is remarkable that the teeth, which are so intimately related to the capture of food, should be thus affected. Whether the change of teeth is associated with a change of food, or whether neither pointed nor pavement teeth affect the feeding, is unknown.
Still more remarkable is the case of the Double-eyed Fish (Anableps). In this fish there is an intromittent organ in the shape of a tube which is formed by a continuation of the urinogenital ducts down the front of the anal fin. In the hinder half of this organ a bend is made either to the right or left. Out of seventeen males, this bend was to the right in eleven, to the left in six. Further, there is a small fleshy tubercle at the side of the anal fin-ray, at the middle of its length. When this prominence is on the left side, the organ bends to the right; when it is on the 177right, the bend is to the left. In the females the genital opening is covered by a special scale, which is free on one side, left or right, and not on the other. Thus copulation is possible only from the side, and a left-sided male can only conjugate with a right-sided female, and vice versa. Here is one of the most extraordinary cases of specialized secondary sexual characters known. How do the sexes distinguish their complemental mates? It is important that they should, for unions are otherwise impossible.
In the Dragonet (Callionymus lyra) the male differs conspicuously from the female in being much the larger—an exception to the rule—and in having the fin-rays enormously elongated. Further he wears a conspicuously resplendent livery, but this is strictly a “nuptial” livery, the colours waning as soon as the period of sexual activity is past. That these colours play the same part as with the birds is clear from the observations of the late Saville Kent. “The male,” he says, “resplendent in his bridal livery, swims leisurely round the female, who is reclining quietly on the sand, his opercula distended, his glittering dorsal fins erect and his every effort being concentrated upon the endeavour to attract the attention of his mate.... The female, at first indifferent, becomes at length evidently dazzled by his resplendent attire and the persistency of his wooing. She rises to meet him, the pair so—far as is practicable with fishes—rush into each other’s arms, and with their ventral areas closely applied, ascend perpendicularly towards the surface of the water.” In the course of this ascent the ova and sperms are shed, and fertilization takes place.
The difficulties in the way of the study of the behaviour of fishes during the critical period of mate-hunting are many and obvious. Something may be inferred from the 178nature of the secondary sexual characters which they exhibit, and more definite information can be obtained from such species as can be kept in aquariums. From these two sources enough has been gleaned to show that these cold-blooded creatures, in many cases, exhibit the same emotions and the same means for their fulfilment as the higher vertebrates. And it is significant that wherever anything approaching what may be called “Courtship” obtains, the males commonly exhibit secondary sexual characters, whether in the form of ornament or of armature; while among species which consort in shoals during the breeding season no such distinctions are present. The ova and milt are shed and fertilization takes place as they escape.
Comment is frequently made in works on Natural History on the fact that among fishes the males are commonly smaller, often conspicuously so, than the females. Among mammals the males are the larger; but among birds this is by no means always the case. It is somewhat surprising to find this discrepancy among the birds of prey, where, as in the case of the Sparrow-hawk, the male is little more than half the size of his mate; commonly, however, there is little or no difference. Among the fishes the differences are often much more marked, as for example in the Conger-eel, wherein the male never exceeds a length of two feet six inches or a weight of one pound; females, on the other hand, may exceed eight feet in length and attain a weight of one hundred and twenty-eight pounds, though such giantesses are rare, but specimens of fifty pounds and upwards are frequently met with. The explanation of this may lie in the fact that among fishes it is no uncommon thing to 179and males becoming sexually mature long before they have attained their full stature. With the Salmon, for instance, ripe spermatozoa have been found in individuals of not more than a few inches in length, and in this species also the male is the smaller. Ova take longer to attain maturity, for in addition to the germ-plasm they must be provided with a more or less extensive amount of food-material in the shape of yolk. The formation of this is inhibited until the demands on the system for the building up of the body have begun at least to lessen.
Mate-hunger among the fishes seems generally to find peaceable modes of expression, either in “display” or in consorting in vast shoals, though, so far, the factors which govern their conduct in this matter are as yet unknown. But here, as with the higher vertebrates, there are some species which adopt more violent methods. A good illustration of such conduct is furnished by the Salmon, which, during the period of sexual activity, develops a curious modification of the lower jaw, which is produced forwards and upwards to form a hook-shaped projection of fibrous tissue. When the mouth is closed this hook is received into a cavity formed within the fore-part of the roof of the mouth. It has been described as a weapon of offence. But this it can hardly be. On the other hand it has been suggested that it serves to protect the jaws when charging a rival, for the shock on such occasions is considerable. It answers, in short, like to the fibrous mass of tissue which protects the fore-part of the head in Whales like the Black Whale (Globicepha............