3 .   From The Sea and Its Living Things
by H. Stafford Hatfield

In the book from which this third example is taken, no use is made of my of the science lists or of international science words, and the lenses of the small number of working words which are used in addition to the 850 are given as they come in. There are, however, a great number of names of sea-plants and sea-animals of which it is hard d give the sense in a small number of words, and which have, for this reason, been made clear in a different way -- by picturing the plants and animals and, in addition, by giving their Latin names in footnotes. The pictures have not been printed here, but notes pointed to them have been kept to let the reader see the system of the book.
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FOOD OF SEA PLANTS AND ANIMALS

      The first business of all living things is to get food. The great Division between plants and animals is dependent on this food question. Plants are able to make use, as food, of very simple chemical substances. In theory, they are not dependent on animals for their existence. In what is named 'tank culture,'¹ common food-plants are produced from seed put into wood dust (of no use to them as food) wetted with water having in it certain simple chemical salts. The plant has the power of building up the chemically complex substances out of which it is made from the element carbon (C) (a small amount of which is present in the air as carbon dioxide, C02), from ammonia (NH3) or other substances having nitrogen (N) in them, and from salts present in almost all water. For these chemical reactions the plant makes use of light; our chemical knowledge is; so far, quite unequal to producing them in this or any other way.
      However, plants are in fact dependent for their existence on animals, because but for these, the carbon of the air would long have been used up. Plants have, from far back in the past been building up great stores of carbon, stores which now have the form of coal, and which, till we get the coal out of the earth for burning, are no longer of use for plants or animals. We get heat and power by burning this coal, and in the process the carbon gets back into the air as carbon dioxide. This same process goes on in the bodies of all animals, but what is 'burned' by them is not coal but the substance of plants taken by them as food, or that of the bodies of other animals, themselves living on plant food. In making use of these substances, animals give off carbon dioxide into the air, as we ourselves do in breathing. This same exchange of materials between plants and animals takes place in the sea and in fresh water. If fish kept in an aquarium' (a glass vessel in which observation of them is possible) are to be healthy, it is necessary to have in the water enough plants of the right sort, and to put the aquarium where it will get enough light. The plants then take in carbon dioxide and give off oxygen; the animals make use of the oxygen for breathing, and give off carbon dioxide. If this exchange between plants and animals is not possible, it is necessary to have a current of air going through the water all the time.
      On land we have a great number of all sorts of plants, forming the food of some animals which themselves are food for others. Very important is the fact that the waste material from the bodies of animals, and these bodies themselves after death, give back to the earth a number of chemical substances necessary to plants, such as sulphur (S), phosphorus (P), and nitrogen, these substances, naturally, being equally necessary to animals. Animals get them as part of the complex food material of plants, the plants themselves get them from the substances fanning the earth, or from the air, in addition to what they get from animal waste. In the sea, we have in some places a strong growth of plants used as food by animals, but it is clear that this is far from equal to supporting the very great number of animals present in the sea, most of them a long way from the shores where these sea plants, or 'seaweeds,'² are. The fact is that the plants of the sea on which much the greater part of its animals are dependent are very small. The only sign of their existence is the fact that the water is not quite clear.
      In some places, specially in the warmer seas, sea-water is very dear; we are able to see to a depth of ten or even more meters. In others, it is not so; the water seems dirty yellow or green, not a clear blue. It is in such water that fish are present in great numbers, the reason being that in it there is a very great number of these small plants, and of small animals living on them as food. This mass of living things is the necessary base for the existence of all greater sea animals, in the same way as, on land, the grass and other plants with the insects living on them, and other small and simple living things, are the necessary food for the greater and higher animals.
      This mass of small food is named the 'plankton,' and it is generally made up of a great number of different sorts and sizes of plants and animals³ By using a net of silk, such as a stocking, anyone may get enough of these little things in a minute or two
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      to keep him at work for a long time with a strong glass, or better, a microscope,⁴ making out their strange and interesting forms.
      Chief of the planktons are the diatoms,⁵ very small glass-like boxes, having all sorts of beautiful forms, and full of green plant substance. The green coloring of plants is responsible for their power of using light for their chemical work. There are thousands of sorts of diatoms, and we commonly come across a great number together in any one place. Theft beautiful little shells, falling to the bed of the sea on their death, make a thick cover of soft mud over great stretches of its deepest parts.
      Again, there are certain 'periclinians' (Peridiniales) which have the power of motion, having two whip-like tails. These take in solid food, and so may possibly be looked on as animals, but some have green coloring, and the power of living as plants do.
      These and other very small plants are the food of a great number of plankton animals of different sizes -- a plankton animal being one which, though it may have the power of motion, is so small and slow that it is transported from place to place by the water currents, and not by the operation of swimming or walking, as other animals are. Among the most important animals are the 'copepods,' a division of the greater division of land and sea animals named Crustacea. Some sorts of copepods are as much as a centimeter or more long, but most of them are very much smaller. The commonest copepod in the colder waters of the north is Calcznus finrnarchicus, which, though only about a millimeter long, is one of the most important animals in the sea, being the chief food of the herring,⁶ the commonest and cheapest of all food fish, and of a number of other important food fish. The copepods are quite complex animals, with heads and legs and eyes, but other much simpler animals, the 'radiolarians' (Radiolaria) and 'globigerinas' (Globigerinidae) are very important as food for greater animals. The greater 'crustaceans,' such as shrimps,⁷ prawns^,8 lobsters,⁹ and crabs,¹⁰ are used as food by man.
      The animals of which we have been talking are the most important of those living from birth to death in the plankton. But a great number of other water animals, even of the fishes, go through an early stage of living in the plankton. When they come out of the egg they are very small, and quite unable to make any headway against tides and currents. They come up near the surface and are there transported by the motion of the water, possibly to places where there is more room and food for them. While undergoing growth, they have food enough in the plankton, though, naturally, they themselves may come to an early end as food for others. A great number of sea animals, such as shell-fish (a 'shell' is a hard outer cover), are fixed to the sea-bed when their growth is complete, and others get about only very slowly. The young animals in the plankton are quite different in structure, as a rule, from their fathers and mothers; in fact, such young animals have not infrequently at first been taken for separate sorts of animals, their relation to the older animal being a later discovery. The common crab (Cancer pagurus), for example, goes through two separate stages, in which it has quite a different structure, before it is ready for our tables.
      Though men make use of animals for food, the land animals in question are, almost all, those themselves living on plants. Their meat has a better taste; and anyhow, such animals are common everywhere, even in natural conditions unchanged by man. On the other hand, almost all the fish used for food are those living on other smaller fish; only one or two, such as the lobster, take anything they are able to get, like rats on land. Some good-sized fish get their food from seaweeds, but these in turn have masses of small animals living on them, which are more important to the fish than the weed itself; they are the butter on its bread.
      One of the strangest facts about the sea is the distribution of the plankton. As we have seen, its starting-point is plants, which, as on land, are building up simple substances, with the help of light, into the complex materials necessary for animal food. Now, on land, the growth of plants is greatest where there is most sun; this is of use for giving not only light, but heat. Plant growth on land is helped by heat; this is why glass-houses are used. But in the sea, the plankton is present in much greater amounts in the colder parts. So in the warmest parts of the earth the land (if there is enough rain) is thickly covered with plants and trees of all sorts, but the sea is very poor in fish. In the north and south seas, on the other hand, we have very great numbers of fish of all sizes, but the land is poor in plants. The Mediterranean is badly off in plankton, and so in fish. The North Atlantic and the North Sea are well stored with plankton, and so with fish.
      In the English Channel, the amount of plankton diatoms has been roughly measured. Strangely enough, the weight of the fishes' plant food produced in one square kilometer of sea is a little greater, if anything, than the weight of our chief plant food, the potato, produced on an equal measure of good land. The fishes have to get on with potatoes only a small part of a millimeter in size, but then, only very little fish make use of this uninteresting food. The surprising thing is that the only sign we get of this great mass of produce, on which our fish food is in the end dependent, is the fact that the water round our islands is not clear. The range down into the earth of potatoes in a field is only about half a meter; that of the plankton down into the sea is about 50 meters. However, plankton plants, needing light for growth, have to keep quite near the surface. For the same reason, seaweeds are present only in shallow water. There are no plants on the bed of the deep seas.
      There is no limit to the number of different and beautiful buns to be seen among plankton plants and animals. The delicate structure of diatoms is used as a test for our microscopes. An account of this side of sea science is, however, only possible with the help of a great number of pictures. Though it has little to do with the everyday work of the fisherman, it is very important when it comes to a discussion of the reasons for the changes in the amount and position of our food fish. The fisherman has need of direction as to where good fishing is to be had, and when. There are changes from year to year, and it would be a great help if he had early news of what changes are probable. The reader will see that the plankton may well be a guide of much value, when we have enough knowledge to say, from its properties and amount, what it is doing in the way of growth and so on.
      For this reason tests of the plankton at a number of different places are now made regularly, and it is to be hoped that in time the number of these regular observations of the condition of the sea will be greatly increased. As with the weather, the greater the number of observation stations, the more certain the answers which science is able to give to questions which are important not only to science but to industry and trade. It is, however, not a simple business to make a true test of the plankton at a given place; what is needed is a knowledge not only of the sorts, but of the numbers, of the different plants and animals present Some of these are so small that they will get through any net which readily lets water through; and it is necessary to get them all out of a measured amount of water, representative of the sea at the point under observation to a depth of about a hundred meters or more. At present, no simple way of effecting this has been worked out. In addition, a very expert eye is needed for looking at the mass of material taken by a net or other apparatus, there being hundreds and thousands of possible forms to be noted.
      It is the smallest plants and animals which give us the greatest trouble. They are the food for other and greater animals, some very much greater. How do these get them out of the water? One of the commonest structures for this purpose makes use of 'cilia.' Cilia are short, very thin, hair-like bodies with the power of waving in such a way as to put any liquid round them in motion, driving it in the desired direction. They are used by almost every animal in one form or another; some of the pipe-like structures in our bodies have their inner sides covered with them. A sponge, which is a network of pipes in which an animal is living, is all the time sending a current of water through them with the help of the cilia coating their in-sides, and so getting its food from whatever bits of solid substance may be in the water. In some animals these solid bits are gripped by a sticky substance on the skin to which the cilia are fixed; in others, they are taken in a net of some sort, with very small holes, which is got clear by the operation of special cilia. Two common shell-fish, oysters¹¹ and mussels, make use of the first system, the structure being commonly named the 'beard,' because it is somewhat like the beard, or growth of hair, on a man's chin. The food is sorted out; naturally, not all the solid material in the water is of use to the animal, and some of it has to be let go again from time to time.
      One of the plankton animals, 0ikopleura, has an even stranger way of getting food. It has the power of building round itself a 'house' of jelly-like material, with a quite complex structure. By waving its tail, it sends water through the house, taking it in through openings covered with a network stopping all but the very smallest bits of solid. These bits are then kept back by a net with much smaller holes inside the house, and are pulled off this from time to time by the help of cilia. This house is not of use for long; the nets get stopped up in an hour or two, but the animal is able to make a new house in half an hour.
      Some of the smaller Crustacea have their legs covered with hairs, acting as a net when the leg is moved through the water. Their food is kicked into their mouths. In others, such as the common copepods, the legs in swimming are all the time producing a current of water between them; this goes through a sort of net by which the food is kept back to be pushed into the mouth.
      The sea-cucumber ¹³ has a simpler system. It sends out long, sticky feelers, waving them about till bits of food become fixed to them, and then, pushing them into its mouth, makes a meal in much the same way as some cats do by letting their tails down into a bottle of milk and then taking it off with their tongues.
      As on land, a great number of the smaller animals in the sea get their food by attacking other animals. There is no doubt about the use a fish makes of its mouth when this is armed with strong, sharp teeth, but nothing is more surprising than the power of attacking other animals seen in some of the most beautiful and delicate living things in the sea, for example, the anemones¹⁴ and jellyfish¹⁵ These have long delicate feelers, every one armed with sharp needles and little bags of poison; an animal gripped by such a feeler has no chance of getting away, because power of motion is taken away by the poison. The starfish¹⁶ is able to get the shell of an oyster open by gripping it and pulling; it is not strong enough to overcome the oyster's muscle straight away, but the oyster gets tired first. The shell then comes open, and though the starfish is unable to get the oyster into its mouth, and so into its stomach, by pushing out its stomach it gets it round the body of the oyster. After doing its work of digestion, the stomach goes back into place ready for another oyster. The 'bivalves,' animals which, like the oyster, have two shells shutting together, are a food as much desired by other sea animals as by man. We get them open with a knife; in the sea, one animal has a way of cutting a hole in the shell, another makes the shell soft by putting on it a strong acid, a third, waiting quietly by till the animal inside is opening the shell, gets the edge of its shell between the two halves of the other by a sudden quick motion. Others, again, get the edges of the bivalve's covering broken by forcing them together.
      On land, all living things have a fight for existence, but, in a general way, conditions are simpler. A great number of animals, among which are some of the highest, make use only of plant food -- we ourselves are able to do without animal food --but the number of animals living only on animal food is not important. In the sea, on the other hand, much the greater number of animals take no food but other animals, so that between some of them and the diatoms, the chief plant food, there may be quite a long chain. The porpoise,¹⁷an air-breathing animal living only in the sea, and only on fish, is dependent on such as are of a good size; the fishermen have no love for him. A great part of this fish food is, however, dependent on other, smaller fish, and on crustaceans, mollusks,¹⁸ worms, most of which, again, make use of animals smaller than themselves. A number of fish, however, such as the herring, mackerel,¹⁹ and shad,²⁰ have 'filters' in their gills -- networks such as we gave an account of in connection with smaller and simpler animals. Some of these are small enough to keep back diatoms; the sardine,²¹ for example, takes in this way plant and animal food. Strangest of all, certain whales, the greatest of all sea animals, get their food in the same way, using a filter made of 'whalebone; an elastic, horn-like substance which is of value to man for a number of purposes needing thin, strong springs. One of its uses is for 'boning' parts of women's dress to make them stiff; unlike steel, whalebone is not attacked by water. But women's dress (and behavior!) is much less stiff than in past times, so whalebone is no longer so much in request.
      However, as we have said, a very great number of animals in the sea are far from getting their food in this quiet sort of way; they get it by going after and attacking others, and there is no end to the different sorts of apparatus with which they are armed for this purpose. Naturally, together with this development of apparatus for attack, there has been a development of ways of meeting attack, of 'defense.' In the sea, as on land, one of the commonest forms of defense is the property of being, or becoming at need, so like the things round -- stones, plants, sand, and so on -- that the animal is not readily seen by its attacker. On land, this natural copying of other things -- the leaves or stems of plants, the color of the earth, and so on -- is common, specially among insects, but generally the animal has no power of changing its looks. A great number of animals are so marked that, at a distance, it is hard to see them against the plants among which they are living. But they are, as a rule, unable to make any change in themselves when they go from one place to another where the plants are different in form and color. Some fish -- in fact, a great number of different sorts -- are able to do this, however. We will give a more detailed account of this very interesting process later.
      Almost everyone will at times have seen the sea giving out a bright light wherever its surface is cut through by a ship, a stick, or the hand. This 'phosphorescence' is produced by certain of the plankton animals. Now, light-giving animals are quite uncommon on land; fire-flies and glow-worms ²² are the only ones
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frequently seen. But in the sea, this light-giving power has undergone much greater development. It is, no doubt, based on much the same chemical reactions, though the range of color produced is greater. We are completely at a loss to say how it is done. As with most other chemical powers of living things, the best which science is able to do is very poor in comparison. Much the greater part of the electric or other power used in lighting our streets and houses is wasted. Our eyes are acted on by only a very short range of waves, but we are quite unable to make apparatus limited to producing these only; we get in addition a great range of waves, of the same sort as light having no effect on the eyes, but producing only unnecessary heat. The fire-fly or glow-worm, on the other hand, is producing no light-waves other than those acting most strongly on the eye; it is wasting no power. No doubt this is true of fish and all other animals which are light-givers. Strangest of all, these animals make their light by burning a substance, that is, by a chemical reaction between it and the oxygen of the air. We, however, are able to do this only with oil or wax or other like substances, which give out much more heat than light in burning. The animal makes a special substance, 'luciferin,' which in burning gives out only light. That it is burning, in the chemical sense of uniting with oxygen, is made clear by the fact that no light is given out when air is kept away from the animal.
      Some of the simplest of all living things, bacteria,²³ are able to give out light it is a common experience that bits of dead fish do so, and this is the effect of the growth of bacteria on them. Sometimes growth of bacteria may take place on living animals. It would be of the greatest value if we were able to get enough of them living on our bodies to be of help to us in getting about in the dark. The phosphorescence in the sea is commonly caused by equally simple plankton animals. We will have more to say further on about the structures producing light. Here we are interested in the use of this property to the animal, and this is a very complex question.
      One way of getting fish out of the sea is by using a line and hook, on which is a 'bait,' that is, something, having an attraction for the sort of fish we are after -- most commonly, food, but sometimes simply a bright, or brightly colored, bit of other substance. Now, this idea is much older than man; it is used in a number of ways by certain fish. These angler fish ²⁴ -- so named from the 'angler,' or fisherman, we see waiting for long hours with his rod over the river for the fish which do not come -- have a 'baited' rod like the fisherman's, with something interesting-looking, and sometimes a light, on the end of it. The rod is quite short, about ten or twenty centimeters, and the strange thing is that the fish itself has a great mouth armed with sharp teeth, which it keeps open all the time. No fish is more cruel-looking; it might well put fear into the strongest man. But the little fish seem to see nothing but the bait. it is clear that little fish have little sense.
      By some angler fish, then, light is used as a bait. Strong light has an attraction for fish; in some places, the fishermen take with them to sea high-powered lights. In other fish, apparatus for producing light clearly has the purpose of helping them, in the deep waters where there is no daylight, to the discovery of their food. But it is hard to see what use this light-producing power is to some of the simpler animals. Probably not any. This sort of question comes up in connection with a great number of animals. For example, there are fish of the most beautiful forms and colors, looking as if they had been designed by man. For what purpose? There is here no question of sex selection, which is said to be the reason for the beautiful colors of male birds and insects.
      In the sea, as on land, some animals have taken to a very unpleasing way of supporting themselves, by becoming 'parasites' living on or in, and getting their food from, the bodies of other animals. Some make their living-places on the outside, others on the inside, of their 'host,' their hotel-keeper. Some of them have kept the structure and powers of independent animals, moving about freely but using the blood of the host as their food; others have given up all attempt at working for their living, and become fixed to the body of the host. They are then able to do without the process of digestion; in fact, there may be a complete loss of structure but for the parts necessary for producing offspring. One strange example of such a parasite is Socculina, living on crabs, and looking at full growth like a little bag with roots going into the body of the unhappy host. But from its eggs, which it sends out into the sea, comes a small animal, named a 'nauplius,' forming part of the plankton, and swimming freely with its six legs. It is, in fact, like the early stage of a crab. After a time, a change of form gives it no less than twelve legs, which have to be used quickly for the discovery of a crab host. It gets into the blood of the crab, letting its legs go in the process. Then it puts its roots out, and, last of all, when the crab is changing its shell for a new and greater one (as crabs have to do to make room for growth), it takes up a position on the outside of the new shell and from there the eggs are sent out.
      As on land, there is in the sea 'symbiosis,' the living together of animals, or animals and plants, which are necessary to one another. This is not at all the same relation as that of parasite and host. As a stage on the way to symbiosis itself, in which the two beings are dependent on one another, we have examples of animals living with others which are a help in keeping off danger, and which in exchange are helped to food by the friend profiting by their power of defense. Crabs make use of anemones in this way. The anemone, an animal with long feelers which give it the look of a flower, is dependent on food coming to it, being itself fixed to a stone. Its feelers, however, are poisoned as a defense. A crab, on the other hand, gets it food broken up by its strong 'claws,' and naturally, some of it is wasted, specially the smallest bits. Some crabs have taken to living with anemones. The poison feelers of the anemone are a defense to the crab, and the anemone gets the little bits of food from the crab's meal. Another crab gets anemones Led to its claws, pushing them forward when attacked, and taking out of them, for its use, the food they get a grip of. This may seem a loss to the anemone, but probably it does better when transported from place to place by the crab than it would have done hanging on to one stone all the time, and waiting for food to come to it.
      The commonest form of symbiosis, common on land and in the sea, is that of the green algae,²⁵ a very simple plant, and an animal. Here we have the most complete example of that necessary adjustment of plants to animals and animals to plants.
      . . . The algae make complex materials from the simplest chemical substances, so producing food for the animal, which in turn lets them have back the waste produced by its living processes, to be worked up again. The eggs of some animals take little bits of alga with them when the female lets them go, so that the offspring is quite certain of not being without its necessary friend. The offspring of other animals are dependent on chance for meeting algae, but these are so common that there is no danger of not doing so under natural conditions. Such an animal is the worm Convoluta, living in the sand on the shore of Brittany and the Channel Islands. It is seen in the form of bright green masses on the yellow shore, suddenly coming up out of the sand after the tide has gone out, and going down into it again a short time before the tide comes back. The green color is that of the algae with it. Though when young it is able, like other worms, to make use of smaller animals for food, it quickly becomes completely dependent on the algae, and its power of digestion, being no longer used, is then a thing of the past. The alga has need of daylight for its work as a plant, so the worm has to come out from its safe cover in the sand, taking the chance of destruction by other animals. In the end, the worm gets tired of living in this uninteresting way, and makes a meal of the algae themselves, going white from the tail up in the process. Death is the only possible outcome of this foolish behavior, but the worm makes good use of this last meal by producing eggs in great numbers, so the family goes on. The offspring, never having seen their mother, have naturally no idea of how she came to her early death, and so go on, in time, to make the same error themselves.
      As we have said, symbiosis is only a special example of the general fact that living beings are dependent on one another. Over a given range of natural conditions in the sea or on the land, we have a most complex network of relations between the animals and plants, by which, among other things, the number of them is limited. If any one sort undergoes a great change in numbers, the natural balance of the organization may get so much out of order that all sorts of other changes take place. Examples of this are more readily seen on land than in the sea, where we have much less chance of getting a complete knowledge of what is going on. One point is of special interest. In symbiosis, we have two living things helping one another. Against this, we have everywhere living things attacking one another. It might seem that any animal would be better off without its attackers, its 'enemies.' But this is not necessarily so. In a certain wide stretch of country, covered with woods, there were living a number of animals kept in more or less natural conditions for purposes of sport. The sportsmen went after them with guns, and the payment made to the owner of the land for this amusement was naturally dependent on the number of animals of the desired sort, chiefly deer.²⁶ Now these deer were not troubled by sportsmen only, but by a number of animal enemies of no value for this sort of sport, specially the fox²⁷ and it seemed to the owners of the land that it would be a good thing to put an end to all the foxes and other enemies of the deer, so that the valued animals would be safer. This was done, and naturally, there was at first a great increase in the numbers of the deer. But in a short time it became clear that these animals had undergone a great loss of quality, and from year to year they became poorer and feebler. The reason was simple. The foxes, in the past, had kept them moving, which had the effect of making them stronger and quicker; in addition, the feebler ones were put to death by these enemies, so that offspring were produced chiefly by the stronger, those able to get away from the foxes. We see from this example how its enemies may be of value to an animal or plant by keeping its quality high.
      Though the number of different living forms seems almost without end, we keep on meeting with the same sort of structure or process again and again, in a lower or higher stage of development. An eye may be anything from a point on the body simply having the power of reaction to light and dark, to a very complex apparatus such as we have in our heads. There are good reasons for the belief that the complex structures of plants and animals are the outcome of the slow development, through thousands or millions of years, of simpler structures, which have been of value to their owners.
      It is for this reason surprising that, of all living things on land or in the sea, only two fish are able to give electric shocks strong enough to be a danger to small animals, and even to man. The apparatus in the fish named the torpedo or electric ray,²⁸ by which electric shocks are produced, is naturally very complex. To be of we, it has to give out a current of some hundreds of units ('volts'), and though very feeble electric currents are produced by muscles and nerves when acting normally, only the most delicate instruments give us any sign of them. They would have to be a thousand times stronger to be a danger even to the smallest animal, and we have never come across any simple
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form of shock-producing apparatus, such as might have been an earlier stage in the development of the one present in the electric ray. It is, in fact, hard to see of what use a low-powered apparatus would be. Electric shocks are only a danger to animals with a very complex nerve-system, and these are of such a size that a strong shock is necessary.
      The last word has not been said on the question of how these 'inventions' with which living beings are armed come into existence. Some are like our inventions, ways of using, or even making things out of, natural materials. Others are the development of parts of the body in such a way as to get an effect which is of value to the plant or animal. The present theory is that such inventions come about by 'natural selection.' Any small change in the structure of a plant or animal which is of use to it gives it a better chance of living on and producing offspring. These offspring will themselves probably have the same point of structure; some may have it in an even better form than before. For example, those fish with bodies best formed for moving through the water are the quickest swimmers, and so have the best chance of getting food by overtaking other fish, and of keeping out of danger from greater fish. Now, of the same sort of fish, some of those living at any one time will have bodies a little better formed than others; no two living things are ever completely the same in every way. Of the young fish produced at this time, probably much the greater number will come to an early end in the stomachs of other fish; it is the quickest swimmers which will have the best chance of living on and producing offspring. So a natural selection of quick swimmers takes place, like the selection made by man in producing horses, when he takes care that the quickest runners become the fathers and mothers of the young horses to be trained later as runners. The effect of man's selection has been the beautiful lines seen in our best horses, and there is little doubt that the operation of natural selection has had much to do with the development of those structures and processes which are of value to different sorts of plants and animals. But we frequently come across forms, as in the electric ray, which seem to have no earlier stages, and where it is hard to see how development by degrees has been possible.


1. A 'tank' is a box-like or basin-like vessel, frequently of great size, and 'culture' is 'producing, causing growth of' (plants or animals).
2. See Picture II.
3. See Picture III.
4. Apparatus for looking at very small things, through which they are seen as greatly increased in size.
5. Sorts of algae of the order Bacillarlales.
6. Clupea harengus (Picture IV).
7. Crago
8. Peneus and like groups (Picture V).
9. Homnarus (Picture V).
10. Cancer and other groups of Hrachyura (Picture V).
11. Ostrea (Picture V).
12. Mytilus.
13. Name given to animals of the division Holothurioidea, specially the group Gucumaria (see Picture II).
14. Flower-like animals of the order Actiniaria, branch Coelenterata (Picture II).
15. Animals with jelly-like bodies, frequently umbrella-like in form, of the same branch as the anemones, but free-swimming (Picture II).
16. Any of a number of different annuals, roughly starlike in outline, forming the division Asteroidea, of the branch Echinodetmata (Picture II).
17. Phocoàa
18. Mollusca, see Picture V.
19. Scombé, scombna, an important food fish in America and Europe.
20. Aiosa sapidissima, etc. an American food fish.
21. The young of Sardinella pilchardus, the tinning of which in oil when very small is a great industry in Europe.
22 The first name is given to a number of winged insects, chiefly of the family Lampyridae, which give out light; the second to the light-giving unwinged females of certain sorts.
23. Schizotnucetes. the simplest of plants, seen only under a microscope.
24. Fish of the order Pediculatl, specially Lophius piscatorius, and the deep-sea angler fishes (Ceratioidea). which have a lighted bait (Picture VI).
25. Algae, the general name for thousands of different sorts of very simple plants of the branch Thiallophyta.
26 Animals of the family Cervidae, noted for their powers of running and for the fact that the males have branching horns which come off every year.
27. Sorts of dog-like animal of the group Vulpes, having a sharp-pointed nose and a long thick tail.
28. Torpedo torpedo, a great flat fish of the order Hypotremata (Picture VI).
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