CHAPTER VI
THE DISTRIBUTION OF ANIMAL LIFE
In the last chapter we looked at a few of the interesting generalisations which have emerged of late years from the study of plant distribution. An enormous amount of detailed investigation had been done before these generalisations were arrived at, and though still much remains to be done, yet the broad lines of a science of plant distribution may now be said to be established. The scientific study of animal distribution has not yet reached a corresponding stage of advancement, partly no doubt because the dependence of the more highly organised and active animal upon the physical conditions is less close than that of the stationary plant, so that the subject is more difficult. Facts are accumulating on all sides, but the subject is still rather at the level of collecting information than at that of laying down broad generalisations. There are, however, indications of progress in many directions, and an attempt will be made here to suggest some of the lines along which research is especially busy at the present time.
In speaking of plants we confined our attention exclusively to land plants, for the reason that aquatic plants are usually small in size, relatively simple in structure, of somewhat limited vertical distribution, owing to their dependence upon light, and of little direct importance to man. In considering animals, on the other hand, we cannot exclude the aquatic forms, which are often of great human importance. In many regions man depends largely, sometimes even exclusively, on the animals of the sea for his food. We shall, then, begin with some account of aquatic animals, considering the subject, as before, especially from the point of view of the inhabitants of Europe and North America.
Beginning with the sea we find that the scientific study of marine animals received an enormous impetus from the work of the Challenger expedition. The results of that expedition appeared in many large volumes, which form a conspicuous feature in any complete scientific library and contain a mass of useful material. The Challenger expedition was followed by many others, European and American, and the result is that we now know a great deal about marine animals and their distribution. Further, the Fishery Boards of various Governments carry on continuous observations on the conditions of life in the seas near their coasts, which have added and are adding enormously to our knowledge.
We cannot here consider in detail the various facts brought to light by these means. Only a few general points can be touched upon. One interesting generalisation is that the life of the ocean can be divided into three groups: the life of the littoral or shore zone, the life of the open ocean (pelagic fauna), and the life of the great ocean depths (abyssal fauna). The last, though of great zoological interest, is so remote from human life that we need not consider it. The pelagic forms include both the small delicate organisms which float passively with the ocean currents, and also powerful swimmers like many fish, and aquatic mammals such as whales and seals. The littoral forms live in the region which is within the reach of land influences, that is, from low-tide mark to the edge of the Continental Shelf (cf. [p. 27]). Among forms directly important to man they include many fish; crustaceans such as crabs and lobsters; shell-fish such as oysters, mussels, clams, etc.; less important forms such as sea-urchins, which are extensively eaten in the Mediterranean; sponges, an important article of commerce; the various corals, especially the precious coral, and so on.
Of the useful marine animals, those which are most readily captured are the littoral forms, many of which, on shores where the tides are well marked, are exposed, or at least brought within easy reach, by the daily ebb and flow of the tide, and can be obtained with the minimum of apparatus. The extensive shell-mounds found on many shores, e. g. on those of Denmark, show at how early a date man availed himself of the abundant food supply to be obtained on the shore rocks. All edible animals found in the sea are “fish” to maritime populations, but fish in the restricted sense are usually more active, and require more skill for their capture than the less intelligent molluscs or crustaceans, and were probably not used at so early a date. They are by no means equally distributed in all seas, and their distribution shows many points of interest.
We must notice, in the first instance, that the waste of the land is of great importance in feeding marine forms, whether directly or indirectly. Marine animals, therefore, occur most abundantly over the Continental Shelf, where they are within reach of the food brought down by the rivers from the land. Again, many fish, or the organisms upon which fish feed, depend largely upon those minute plants called diatoms which float in the upper layers of the waters of the ocean. These are especially abundant in the colder seas, which doubtless helps to explain the abundance of fish in high latitudes. These diatoms, like many other small organisms in the sea, are swept about by the ocean currents, whose course greatly influences the movements of fish.
We saw in the case of forests that hot climates conduce to a great variety of species, while in colder climates the species are few, but the number of individuals very great. Something of the same sort seems to occur with fishes. In warm seas the number of species is very great, while in colder seas there are fewer species, but those which do occur are sometimes found in vast numbers. Fortunately for man these prolific northern species are often edible, whereas in warm seas poisonous or inedible forms are common. The valuable cod family is found chiefly in high latitudes.
The consequence of the facts just described is that valuable fisheries tend to occur in cool or cold climates rather than hot ones, and because of the dependence of so many forms on the Continental Shelf, they occur in the northern or land hemisphere rather than in the southern or oceanic one.