The earliest studies of the sea from this special standpoint by Edward Forbes showed that in the temperate zone the sea floor could be divided into regions on a basis of depth, and that each of these zones was inhabited by a characteristic group of animals. Forbes's zones, the littoral, laminarian, continental shelf, conti nental slope and abyssal regions, have been further subdivided by later work, and on them has been superposed a subsidiary grouping according to animal communities, which depends ulti mately on the nature of the bottom. But in all these investigations the character of the sea water itself has been supposed to be con stant. In fact, it varies in nature from season to season, and in some regions from day to day, even locally from hour to hour.
The more important qualities of the sea from a biological stand point are its mean temperature and the daily and seasonal range in temperature ; the amount of suspended matter and its chemi cal nature and physical state; the concentration of salts in solu tion, the amount and relative proportions of the ions of calcium, magnesium, potassium and sodium present; the concentration of hydrogen ions and the extent to which changes of such concen tration is hindered by buffering; the absolute amounts of phos phates, nitrates, silica, and perhaps other substances present; the amount and the character of absorption of light which it effects; its viscosity and the amount of oxygen and carbon-dioxide in solution. All these qualities can be determined by physical and chemical measurements and definitely recorded in figures. They have been so studied, intensively and at regular intervals in those areas such as the seas off the west European coast and some of those off Canada and the United States where fisheries are impor tant, and more generally by the great series of expeditions, of which the first and greatest was that of H.M.S. "Challenger," devoted to the study of the ocean and its life all over the world.
Variations in these qualities may affect animals either directly or through the plants, diatoms and peridinians, which form the basis of all marine life. Plants require for their growth carbon dioxide, water, phosphates and nitrates, in addition to other ele ments which are always present in the sea in adequate amount; from these materials they can form the living protoplasm of their bodies and build up reserve stores of food. This process involves an expenditure of energy which plants, and plants alone, can obtain from sunlight. Thus variation in light intensity and dura tion will directly alter the amount of plant growth and thus deter mine the amount of animal life which can exist in any body of sea. But in British waters the total amount of plant life is deter mined, not by an insufficiency of daylight, but by the exhaustion of the stock of phosphates and nitrates in solution in the sea.
The penetration of light through sea water is so poor that no effective growth of plants is possible at a greater depth than some 40 fathoms. Thus all animal life below this depth is dependent on food which is carried down from the surface layers of the ocean. This transference is effected either by animals which migrate through a definite belt of water or by the slow descent of dead animals and plants.
But variations in the quality of sea water may affect ani mals much more directly. Temperature seems to be the limiting
factor for very many animals; it may act in many ways. A high temperature may produce irreversible changes in the physical state of the protoplasm, either of some one organ or of an animal as a whole, or it may disorganize its controlling mechanism. Low temperatures may slow down the rate of living to such an extent that growth becomes impossible. These effects may be exerted on any part of the life history; reproduction may become impos sible at a temperature at which the adult lives successfully.
Any change of the concentration of salts in the sea, by alter ing the osmotic pressure it exerts, directly affects all animals whose surface has the properties of a semi-permeable membrane, because changes in it lead either to abstraction of water from the animal or absorption of water by it at a rate which may become too great to be coped with. The efficiency of such changes as a bar to dispersion is shown by the very small fauna of brackish water. Variations in the concentration of metallic ions may affect ani mals by destroying the physico-chemical mechanism on which the maintenance of its surface depends, or by rendering the processes of fertilization and cleavage impossible. The hydrogen ion con centration produces similar, though more intense effects. Varia tions in the amount of oxygen available clearly modify the activi ties of all animals, because the whole of their metabolism is based on a series of oxidizations and reductions. The real nature of the relationship between any animal and its environment can only be discovered by experiment. The factors involved are so numerous that it is evident that the problem can never be solved by observa tion of the natural occurrence of the species. Only by varying the conditions of the medium, one at a time, and observing the effects of these changes, not only on the whole animal at each stage of its life history, but also on its isolated organs, will it become possi ble to appreciate the real nature of the correlation which un doubtedly exists between the organism and its surroundings. One of the most fundamental parts of this relationship is that which concerns food. Although every animal is ultimately de pendent on plants for its nutrition, it may feed on other animals and be connected only by a food-chain with the basal source. The first step is thus to trace as fully as possible the series of forms involved in this chain, which can only be done by long-continued observation. The next is to work out the action of the feeding mechanism, so as to determine to what extent the animal is re stricted to food of definite size or nature. Then it becomes impor tant to investigate how far the food which actually enters the mouth can be made useful, which involves a study of the diges tive enzymes, and of the conditions under which they will act. Finally, the problems of absorption and transport of food must be considered. The next stage is to add precision to the facts so learnt by determining the amount of food which can be collected and digested under definite conditions, and, so far as is possible, the mode of its utilization, whether for maintenance, general activities or for growth.