It would be interesting, and profitable as well, to know as much of any one of our sea fish as we now know of the salmon, but so little progress is being made in observing the natural history of fish that we cannot expect for some time to know much more than we do at present; everything in the fish world seems so much to be taken for granted that we are still inclined rather to revive the old traditions than to study or search out new facts. Naturalists are so ignorant of how the work of growth is carried on in the fish world—in fact, it is so difficult to investigate points of natural history in the depths of the sea—that we cannot wonder at less being known about marine animals than about any other class of living beings. It is the want of precise information about the growth of the fish that has of late been telling heavily against our fisheries, for in the meantime all is fish that comes to the fisherman’s net, no matter of what size the animals may be, or whether or not they have been allowed time to perpetuate their kind. No person, either naturalist or fisherman, knows how long a period elapses from the date of its birth before a turbot or codfish becomes reproductive. It is now well known, in consequence of the repeated experiments made with that fish, that the salmon grows with immense rapidity, a consequence in some degree of its quick digestive power. The codfish, again—and I reason from the analogy of its greatly slower power of digesting its food and from other corroborative circumstances—must be correspondingly slow in its growth; but people must not, in consequence of this slow power of digestion, believe all they hear about the miscellaneous articles often said to be found in the stomach of a codfish, as a large number of the curiosities found in the intestinal regions of his codship are often placed there by fishermen, either by way of joke or in order to increase the weight and so enhance the price of the animal. As regards the natural history of one of our best-known food fishes, I have taken the pains to compile a brief _precis_ of its life from the best account of it that is known, keeping in the background at present any knowledge or speculation of my own regarding it. I allude to the mackerel; and the following facts are from an evidently well-studied chapter of Mr. Jonathan Couch’s _Fishes of the British Islands_, by which it will be at once seen that our knowledge of the growing power of this well-known fish is very defective.