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 sequence 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 right, 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 nature 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 and 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 (Globicephalus) and the Bottle-nose Whale (Hyperoodon), serving as a battering-ram. In the Pacific Salmon (Onchorhynchus) both jaws are hooked, so that when the mouth is closed the hooks cross one another as in the beak of the Crossbill. In this Salmon, too, the front teeth attain a considerable length, while the body becomes laterally compressed and a hump forms at the shoulders. Little, however, seems to be known as to the nature of their battles.

The combats of the Salmon of our own islands, however, are evidently severe, and this has long been known, for Darwin speaks of as many as three hundred, all with one exception males, being found dead in the Tyne during the month of June, killed by fighting. Such battles are fought, it is to be noticed, not so much for the possession of females—for it is a polygamous fish—as for the privilege of fertilizing the eggs as they are shed. The absence of a “display” here is a noticeable feature, and it is on this account, probably, that the reproductive period is not associated with the appearance of any form of resplendent livery. On the contrary, the marvellous silvery sheen which adorned both sexes on their arrival at the spawning ground from the sea has entirely vanished by the time that the consummation of the journey has been attained, and in its place is naught but a slimy, dingy copper-coloured hue. But no sooner has the reproductive period passed than the silver lustre makes its appearance once more.

These facts are the more interesting when contrasted with what obtains among other fighting species which must woo the females. Take the case of the common freshwater Stickle-back. In this species the body is invested with an armature of bony plates and spines in place of scales, while the males are arrayed in vivid hues of red and blue. Any survey, however, of the reproductive activities of this little fish must take into account certain quite remarkable prenuptial actions and instincts. Briefly, before the male commences his search for a mate he constructs a nest of fine fragments of aquatic weeds, which are held together, not by interweaving as with birds’ nests, but by a sticky and copious secretion from the kidneys. According to some authorities, this secretion is to be regarded as a pathological product caused by the undue pressure of the ripening testes. It is difficult to accept this interpretation, for it might with as much reason be argued that the copious secretions of the salivary glands of the edible Swift—which builds a nest constructed entirely of hardened saliva—are also pathological in character. But be this as it may, the nest completed, the male seeks a mate, or mates, for polygamy is the rule of his tribe. In his search for these he has constantly to do battle with other males, whom he endeavours to disembowl by swift rushes contrived to t rip open his rival as he passes, by means of one or other of the erectile spines which project from his back and belly. With the females whom he desires he uses the arts of peaceful persuasion, swimming backwards and forwards before her in his endeavour to excite her amorous instincts. At last he persuades her to enter his bower and deposit a few eggs, fertilizing them immediately they are laid. The first to enter leaves by forcing a passage through the opposite wall of the nest, a happy contrivance, for thereby a current of water can be constantly driven through, leaving fresh oxygen to the developing eggs. One female after another is inveigled into the bower, until the complement of eggs is complete. These, singularly enough, are now taken charge of by the male. He it is who creates life-sustaining currents which bathe the eggs, by the rapid vibrations of his breast-fins, and he it is who protects them from their most persistent enemies—the females who laid them. As soon as the fry appear the duties of the male are still further increased. He must guard them from their mothers, and other foes, and he must prevent their too extensive wanderings. Such as stray too far afield are sucked into the mouth and brought back again to the nursery, where they are set at liberty by a reversal of the sucking action. That the male of a polygamous species, and with all the attributes of a polygamous species—pugnacity and vivid coloration—should take upon himself the duties which under like circumstances among the higher vertebrates are undertaken by the female is a very remarkable and puzzling feature. In this species, in short the male plays successively a polygamous and a polyandrous rÔle.

Strange as these facts are, they are not apparently without parallel among fishes, for certain of the labyrinth-gilled fish present many features in common, though as yet proof seems to be wanting. Thus the small Siamese “Fighting Fish” (Betta pugnax) is endowed with so ferocious a nature that it is kept, as the Malays keep fighting cocks, for the amusement of native sportsmen, two fish being pitted against one another and large bets being made on the result. In a state of quiescence it presents no very remarkable coloration, but if two be brought together, or if one sees its image in a looking-glass, it becomes thrown into a paroxysm of rage, the fins are raised and the whole body becomes irradiated with metallic colours of dazzling beauty. There can be no doubt but that a like play of colour occurs during moments of sexual excitement; it is highly probable that it is polygamous. Of its breeding habits, however, little or nothing seems to be known. Not so, however, in the case of a closely-related species, less pugnacious in disposition, but almost as vividly coloured, in so far as the male is concerned. Now in this species a nest of froth is made and the eggs, after deposition therein, are jealously guarded by the male. Hence, on these facts, we may assume with a fair amount of certainty that the closely-related “Fighting Fish” displays like habits.

That the Reptiles, Amphibia and Fishes have much in common with one another, and with the higher vertebrates, in the manner of their love-making is indisputable. We find no evidence anywhere that the first faint throbbings of the sexual pulse in the female are quickened to fever beats after the efforts of several successive wooers, each more demonstrative than the last, to arouse this state—the conditions required by the Sexual Selection theory. But successful mating depends, in each year, on the sexual fitness of the male himself, and the mate, or mates, which for that year he has taken “for better or worse.” It is possible, of course, that a male, ambitious but impotent, will be forsaken by his mate; it is possible that a female of low sexual vitality may fail to respond to the most impassioned displays; in either case no offspring result, and thus the failures are eliminated. It is possible that here, as with the higher vertebrates, coition may by no means always be immediately preceded by display. But the “display” has done its work. It has stimulated the sexual appetite, as the sight of tempting food stimulated the bodily appetite.

But both the Amphibia and the Fishes reveal a lower plane of the sexual instincts, when the sexes, dominated by some imperious instinct, gather in hordes, commingling to shed their precious germs into the surrounding water, there to effect the work of fertilization and the achievement of new birth. The all-important union of these germs is no mere work of chance, as it might seem, but the sperms seek the ova with unerring surety, guided, in this case, by that very efficient substitute for instinct, chemotaxis, or the attraction which certain chemical substances have for lowly organized living bodies. In this case the allurement is furnished by the ova. It is surely no unreasonable surmise that here we have the beginnings of the complex phenomena which the earlier chapters have revealed. On this lower plane we are probably confronted by instinct alone, but from this level upwards intelligence plays an increasingly important part.