Chapter VIII · THE ORIGIN OF THE MARINE FAUNA.

When we survey the distribution of living organisms over the surface of the globe, we cannot fail to be impressed with the enormous range in the characters of the physical conditions which are capable of supporting animal and vegetable life. Thus we find Birds flying in the sunlight of the cold and very light atmosphere of the mountain tops, and Fishes swimming in the chill darkness of the depths of the ocean, supporting a pressure of two tons to every square inch of their bodies. We find Algæ, which give the snow sometimes the name of “Red snow,” flourishing at temperatures below the freezing point of water, and we meet with Insect larvæ swimming freely in the water of the hot springs. Some sea-water animals can only be induced to live in the aquarium when the water is kept as pure as it is in the open sea, and languish and die as soon as any impurity occurs; on the other hand, several of the Crustaceans seem to flourish best in stinking and putrescent pools. The desert, the forest, the swamp, the lake, the river, as well as the surface and the bottom of the sea have each their characteristic set of animals and plants modified in structure and form to support life in their natural habitats.

There can be no doubt that at the time when animals and plants first made their appearance upon the earth, their distribution was far more limited than it is now, and that all the adaptations to life in special and extraordinary conditions have been acquired in the course of evolution by organisms which originally existed in one particular zone of the earth.

The reasons which have led scientific men to this opinion are manifold, but not the least important of them are those based upon the presence of organs or rudiments of organs of animals of the present time, which could only have been called into existence at a period when their ancestors had an altogether different habit of life.

For example, in the Birds and Reptiles, as well as in the Mammals, the presence of openings in the throat during the early stages of development, similar in their position, in their blood-vessels and in other respects to the openings of the gills in Fishes, indicates that their ancestors in remote periods lived in water and not on dry land.

Again the presence of rudimentary eyes in the Mole and other subterranean animals indicates that at one time its ancestors must have lived in the light of the day. The characters of the embryos of some of the land and fresh water Snails proves that they are derived from ancestors that lived in the sea.

When we collect together all the evidence of this kind and place it side by side with the facts revealed to us by Geology, the irresistible conclusion is arrived at that all animals are originally derived from ancestors that lived in the sea. And when we consult the botanists and find that they are agreed that all plants must have had a marine origin also, the case for the sea being the original home of living organisms may be said to be completed.

It is difficult to picture to ourselves the condition of the earth in those very distant times, when the dry land bore no forests nor grass, the air supported no Birds nor Butterflies, and in the rivers and lakes swam no Fish nor Frogs. It must have been “dry” land indeed, when there were no trees to attract the rain clouds and no herbs or mosses to retain the moisture on the ground. The rivers must have risen and fallen with great rapidity as they carried away the rain that fell in cloud-bursts on the mountain tops.

But speculation on the character of the land in those times is not within the scope of this work, and we must turn again to the sea to inquire where the primordial animals and plants lived in the days of a lifeless land.

We have seen that in the sea there are three possible habitats for animals and two for plants. The surface waters of the great oceans bear a characteristic population of animals and plants, the bottom of the sea supports a considerable number of animals but no plants, and lastly the shallow waters exhibit an immense variety of Sea-weeds, Fish, Worms, and other creatures. Which of these three was the original cradle of the great classes of animals and plants?

The early discovery of certain animals in very deep water which are closely allied to, if not identical with, some fossils of early geological strata, suggested the idea that a very primordial set of creatures might be found at the bottom of the sea when it was more thoroughly investigated; but as I have pointed out in a previous chapter the hopes of those who anticipated the discovery of a rich Fauna of “living fossils” were doomed to disappointment.

It is not probable, however, that the abyss of the oceans could have been the cradle of life, even if it had shown a more ancient Fauna than it actually does.

We cannot tell in what form life first appeared upon the earth. Whether the unstable living substance called Protoplasm was in the earliest conditions of the globe formed spontaneously by the chance combination of its elements, or whether some germ or other made a hazardous journey through space from another planet enwrapped in the casing of a meteorite, are questions upon which no light has yet been thrown by scientific observation or speculation; but this can be said, that at a very early period in the history of life upon the earth the simple green plants must have played an important part. It is on the substances that are formed by the activity of this green coloured substance that all plants and animals are directly or indirectly dependent for their food in the present-day economy of Nature, and we are forced to believe that, whatever may have been the form of the earliest living things, Chlorophyll—the green coloured substance of plants—must have had an extremely ancient origin.

Now, in the darkness of the ocean depths Chlorophyll does not and cannot exist; for it is one of its characteristic features that it is active only in the rays of direct sunlight; and, therefore, it is extremely improbable that the cradle of the marine Fauna could have been there. We are then left with two alternatives. It must have been either at the bottom of the shallow waters or on the surface of the seas.

Both of these sites have had their advocates, but the balance of opinion has now turned decidedly in favour of the first of them—the shallow waters. It is not easy to explain the reasons for this view without assuming a fairly complete knowledge on the part of the reader of the various forms of life that are found in the sea, but still a few words of explanation may be written to indicate that the view is a reasonable one.

In the first place we find, when we take a general survey of the animals that live in the surface water, that they are all specially modified in some way or another in structure or development in adaptation to the peculiar conditions of their life. The long spines of the Foraminifers and the Crustacean larvæ, the air-bladders of the Portuguese men-of-war, the oil drops of the Copepods, the raft of the Mollusc Janthina are, as we have seen, among the characters which distinguish this peculiar Fauna. Now, when we compare these surface-dwelling forms with their nearest relations in the shallow waters, the conclusion we come to is that these features have been acquired by the ancestors of the former, which may have been similar in some respects to those now living in shallow water.

Some of the Gastropods of the shore-waters have a simple cup-shaped shell like that of the common Limpet, but the great majority of them have a shell that is twisted up into a spiral form. This twisting of the shell is, of course, due to the twisting of the mantle or fold of skin which secretes the calcium carbonate of which the shell is mainly composed; and, when we study the internal anatomy of the animal we find that the shape of the mantle is associated with a loss of the organs of one side of the body. To put a long story into a few words, we may say that the Gastropods with twisted shells are lop-sided. Now when we examine the shells of the Gastropods that live in the surface waters of the ocean we notice that their shells are (with a few exceptions such as Janthina) perfectly symmetrical, and we might jump to the conclusion that this was due to a corresponding symmetry of the internal organs.

Such a conclusion would, however, be an erroneous one, for the results of the careful anatomical study of these Molluscs proves most definitely, that although a false symmetry of the organs is often shown, there is a general suppression of the organs of one side of the body. A study of the development of these animals also shows, that in the early stages of their life, the shell is not symmetrical like that of the adult, but twisted into a spiral like that of a Whelk or a Periwinkle. These facts indicate that the surface-swimming Molluscs have passed through a stage in their evolution when their bodies were twisted up into a spiral shell, and that the false symmetry, which they exhibit in the adult condition, is an adaptation to their peculiar habits of life. The study of the group of Gastropods alone then does not give us any evidence in favour of the view that the surface-swimming Fauna is primitive; in fact, it proves almost conclusively that its share in the Fauna has been contributed from the shallow water districts.

The group of the Tunicates affords similar evidence. There is no good reason for believing that the Salps and Pyrosoma which drift about in the surface waters are more primitive than the fixed Sea-squirts of the rocks and Sea-weeds, in fact, the view is gaining ground, as our knowledge increases, that all the free Tunicates must have passed through a sessile ancestry. The evidence afforded by the Cœlenterates is not so potent. Several naturalists believe that some free-swimming form of Jelly-fish was the ancestor, and that the fixed Zoophyte was a stage introduced into the life-history at a later period in the evolution of the group. Others believe that the Zoophyte-stage came first and that the Jelly-fish was introduced, for the purpose of distributing, over a wide area, the eggs of the species. My own researches lead me to incline towards the latter view, but I feel that it is still far from being proved.

A great deal more could be written upon this fascinating speculation about the origin of Life in the Sea. But it is still a speculation, and all that can be done at present is to weigh the evidence carefully and see in which way the scale seems to point. If I have succeeded in making clear to the general reader the nature of the evidence we can use in judging this question, and have indicated to him the direction in which it seems to me to point, my task has been accomplished.