XX INTRODUCTION. 



arches into vascular channels, in order that it should be able to maintain a slow circulation 

 as a terrestrial animal, when encased in its cocoon of mud. I see nothing improbable in the 

 supposition that in Protopterus we have a living v/itness of a fish in a transition stage towards 

 becoming, in course of time, under favourable conditions, a true amphibian ; and I do believe 

 that amphibia are altered forms of fish, to which in some cases, they bear a considerable 

 resemblance ; and I think it probable that one of the steps In the transition — and a most 

 interesting and important step it is — is made by the gradual conversion of the swim-bladder 

 and pneumatic duct, into a lung, trachea, and glottis. 



Generally speaking, the function of a fish's swim-bladder is no doubt merely a mechanical 

 one; but this organ cannot be of much importance, nor does it exhibit "such a plain and 

 direct instance of mechanical contrivance" as was maintained by Paley, Roget, Cuvier, and 

 others. For how is it that in different genera of fishes, of precisely similar habits, some have 

 an air-bladder, others have not ? One can at once understand why such an organ should 

 be absent in the PleuroncdidcB (Soles, Turbots, Flounders, etc.), whose habits confine them to 

 the bottom of the water, and which do not, therefore, require the mechanical upward lift 

 afforded by an air-bladder; but when we find that one surface-swimming Mackerel {Scomber 

 colias) has a swim-bladder, and another (5. vulgaris), of precisely similar habits, is devoid of 

 one, it is obvious, notwithstanding the general function of the organ when present, that it is 

 by no means an essential adjunct to swimming. Many of the Silurida possess a large and 

 sometimes complex swim-bladder, but genera occur in which there is no swim-bladder at all. 

 The air-bladder in the Sihcrida, and in some of the Cyprinidce, communicates with the organ 

 of hearing by means of the ear-bones, or auditory ossicles, and doubtless serves to intensify 

 the sound. Fish that keep to the bottom, are, as a rule, devoid of this organ, but in the 

 mud-loving Eel we meet with swim-bladder and pneumatic duct. 



With respect to the reproductive system, fishes are generally oviparous. In osseous 

 fishes, as a rule, the ova are deposited and impregnated by the milt of the male externally. 

 This is the case with all our fresh-water species. Rare instances are met with, as in the 

 marine viviparous Blenny, in which the females produce offspring already somewhat advanced 

 in growth. In such cases impregnation must occur internally, though no structural peculiarity 

 is to be detected in the male or female organs. In the cartilaginous Sharks and Dogfish the 

 generative apparatus is different, and approximates to a certain extent that type of structure 

 observable in reptiles and birds. The ova of the female fish is familiarly known as the 

 "roe;" the milt of the male is spoken of as the "soft roe." 



In the construction of their nervous and cerebral system fishes stand the lowest in the 

 vertebrate scale. I cannot do better than quote the remarks of Cuvier on the general 

 attributes of fishes, and their relative position in the animal scale. 



"Breathing by the medium of water, that is to say, only profiting by the small quantity 

 of oxygen contained in the air mixed with the water, their blood remains cold ; their vitality, 

 the energy of their senses and movements, are less than in mammalia and birds. Thus 

 their brain, although similar in composition, is proportionally much smaller, and their external 

 organs of sense not calculated to impress upon it powerful sensations. Fishes are in fact, 

 of all the vertebrata, those which give the least apparent evidence of sensibility. Having 

 no elastic air at their disposal, they are dumb, or nearly so, and to all the sentiments which 

 voice awakens or entertains they are strangers. Their eyes are, as it were, motionless, their 

 face bony and fixed, their limbs incapable of flexion and moving as one piece, leaving no 

 play to their physiognomy, no expression to their feelings. Their ear, enclosed entirely 

 in the cranium, without external concha, or internal cochlea, composed only of some sacs and 

 membranous canals, can hardly suffice to distinguish the most striking sounds, and, moreover, 

 they have little use for the sense of hearing, condemned to live in the empire of silence, 

 where everything around is mute. 



