If now, as we have thus attempted to prove, no marine animal, be its species or nature what it may, has yet been fairly examined for this purpose without being found pos sessed of phosphorescence ; and if, as is probable, it is therefore extended to the whole of these uncountable races of beings, while it is also under the control of their wills, it is worth our while to inquire what purpose is served by this very singular provision, or what, in ancient phra seology, is the final cause of this arrangement. That it is destined for wise ends, we have no reason to doubt ; nor do we think it difficult to show what these are. If we have taken a correct view of them, we cannot suffi ciently admire the resources of nature, who, in depriving so large a portion of her dependent creatures of the light of the sun, has compensated for that loss by a power de rived from their own internal resources.
While the property of emitting light is, among the land animals, confined to a very few insects ; in the ocean, it is not only extended to very many, probably to every in dividual that exists, but in the latter, the superiority in numbers of the different races is such that while, in a few climates, the twinkle of an insect is occasionally seen, the nocturnal darkness of the immense ocean is illuminated by its inhabitants. Though, among the former, we have not yet succeeded in discovering the uses of this provision, we may be sure that they are very partial ; while in the latter they must be such as are alike interesting to the whole; and without which the business of life could not be carried on. These ends are mutual communication, as far as the wants of these animals are concerned ; this light is a substitute for that of the sun ; and as their great and hourly wants are mutual self-preservation, or, alter nately, prey and defence, so by these lights they are guided to each other for attack ; while, by their power of obscuring them, they are also furnished with the means of resisting it. Betraying their existence by their light, they become the object of pursuit to their enemies ; while nature, pro viding them at the same time with the means of quench ing it, has given to them those compensating powers, which, in the article of defences, she has in some man ner or other bestowed on all her creation.
The great business of fishes, as it has sometimes jest ingly been said, is to eat each other, and a great part of it must be carried on in the dark, or, at least, without the light of the sun. From the experiments of Monsieur Bouguer it has been deduced, that the transmission of light through sea-water is diminished in a ratio so rapid, that at the depth of 723 feet it ceases to be transmitted any longer. Now, we do not think that the method adopt ed by this philosopher was a correct one, or capable of determining this question. But as the general principle
is unquestionable, we are willing to allow 1000 feet instead of 723 ; and it will immediately be seen that the main purpose of our argument will not be affected, though we should adopt a still higher limit. At some depths, there fore, and that probably not very great, there is absolute and perpetual darkness. But fishes are not thus limited to the surface, or near it, or even to depths of a thousand fathoms, much less to one of as many feet. There does not indeed seem to be any limit of depth for the habitable ocean. Innumerable fishes are known to reside, to breed, and to prey, in regions to which light can never penetrate. This is the case in particular with the pelagic fish, which form in themselves numerous tribes. It would indeed be a strange supposition, were we to imagine that the dark regions of the sea were uninhabited. In Captain Ross's .voyage, shrimps were brought up by the sounding line from depths of 1300 feet ; and other animals of va rious kinds were found in the same manner at 6000. It is difficult to prove this fact in many particular instances, because soundings seldom extend very deep, nor do fishermen fish at great depths. The greater number of our own seas are shallow ; and it is the character of some of our principal fish to frequent the banks, or shoaler parts, where woods grow, and where they probably find their prey. Besides this, it is tedious and expensive to fish in deep water. But we can quote one positive fact to this purpose. It is the habit of the ling to frequent the deep valleys of the sea, while the cod, like many others, resides on the hills, or banks, as they are com monly called. In the Shetland seas, one of the most pro ductive spots for this fish, is a valley about 1200 feet deep, bounded on each side by hills that must be nearly preci pitous, since, in sounding, the water suddenly deepens from 20 and 30 to 200 fathoms. In this place, as well as in others, where this kind of fishery is carried on, it is found that the hest fishing exists at the greatest depths ; nor is it unusual to sink the long lines in water of 250 fathoms deep. But the time required for setting and drawing up from this depth, the enormous length of line that is used is so great, as to prevent the fishermen from making any attempts in deeper water ; but they are all of opinion that this fish abounds most in the deepest places, and might advantageously be fished for at much greater depths. Thus the ling, for one, is proved to reside in places which must be perpetually dark, although we were to double M. Bouguer's estimate of the point of non transmission.