LECTURE IX

ORGANIC PARTNERSHIPS OR SYMBIOSIS

Hermit-crabs and sea-anemones—Hermit-crabs and hydroid polyps—Fishes and sea-anemones—Green fresh-water polyps—Green Amœba—Sea-anemones and yellow Algæ—Cecropia trees and ants—Lichens—Root fungi—Origin of Symbiosis—Nostoc and Azolla apparently contradict the origin through natural selection.

Wehave already seen, by means of many examples, to what a great degree animals and plants are able to adapt themselves to new conditions of life; how animals imitate their surroundings in colour and form, how instincts have varied in all directions, how plants have made use of the chance of frequent contact with little animals to obtain nourishment from them, and have developed contrivances adapted for bringing as many of these as possible into their power and causing them to yield them the largest possible amount of food. A great many of these could only be interpreted in terms of natural selection, and in others it seemed at least very probable that selection was one of the factors in bringing them about.

Particularly clear proof of the reality of natural selection is afforded by those cases where one form of life associates itself with a very different one so intimately that they are dependent on one another and cannot live without one another—at least in extreme cases—and that new organs, and, indeed, new dual organisms, are sometimes produced by this interdependence of life. This phenomenon—so-called 'Symbiosis'—was discovered by two sharp-sighted botanists, Anton de Bary and Schwendener. But Symbiosis occurs not only between plants; it occurs also between plants and animals and between two species of animal, and we understand by it a life of partnership depending on mutual benefits, so that each of the two species affords some advantage to the other, and makes existence easier for it. In this respect Symbiosis differs from Parasitism, in which one species is simply preyed upon by another without receiving any benefit from it in return, and also from the more innocent Commensalism of Van Beneden, the table-companionship in which one species depends for its existence on the richly-spread table of another. Symbiosis is particularly interesting, because, in addition to extreme cases with marked adaptations, many occur which are ofgreat simplicity, and which seem to have brought about almost no change in the two associated species.

We shall take our first examples from the Animal Kingdom.

The partnership between certain sea-anemones (Actiniæ) and hermit-crabs (Paguridæ) had been noticed long before any particular attention was devoted to it. Many species of hermit-crab frequently carry a large sea-anemone about with them on the mollusc shell which they use as a protecting-house; indeed, two or three of these beautiful many-tentacled polyps are often attached to them, and this is not at all a matter of chance, but depends upon instinct on the part of both animals; they have the feeling of belonging to each other. If the sea-anemone be taken away from the hermit-crab and put in a distant part of the aquarium, the crab seeks about till it finds it, then seizes it with its claws and sets it on its house again. The instinct to cover itself with Actiniæ is so strong within it that it loads itself with as many of these friends as it can procure, sometimes with more than there is room for on the shell. The sea-anemone on its part calmly submits to the crab's manipulations—a fact very surprising to any one who is aware of the anemone's ordinarily extreme sensitiveness to contact, and knows how it immediately draws itself together on any attempt to detach it from the ground, and will often let itself be torn in pieces rather than give way. The mutual instincts of the two creatures are thus adapted to each other; but it does not at first sight seem as if any structural changes had taken place in favour of the partnership. This is true, indeed, as regards the hermit-crab, but not as regards the sea-anemone, although the nature of the adaptation on the sea-anemone's part only becomes apparent when the two animals are closely observed in their life together.

We owe our understanding of this adaptive change in the sea-anemone, and, indeed, our knowledge of this whole case of Symbiosis, to the beautiful observations of Eisig. Starting from the hypothesis that the mutual relations could only be the outcome of natural selection, Eisig pointed out that this partnership must offer some advantage not to one partner only, but to both; otherwise it could not have arisen through selection. The advantage to the sea-anemone is obvious enough; since of itself it can only move very slowly, and is usually firmly fixed in one place, it is easy to see that it would be useful to it to be carried about on the floor of the sea by the hermit-crab, and to get its share of the hermit-crab's food. But the service yielded to the hermit-crab by the sea-anemone in return is not nearly so apparent. Eisig made an observation in the Zoological Station at Naples which solved this riddle. He saw an octopus attack thehermit-crab and attempt to draw it out of its shell with the point of one of its eight arms. But before this had succeeded there sprang from the body of the sea-anemone a large number of thin worm-like threads which spread over the arm of the robber, who immediately let go his hold of the crustacean and troubled himself no further with it. These threads, called acontia, are thickly beset with stinging-cells, which must at least cause a violent smarting on the soft skin of the octopus. Thus we see that the Actinia instinctively defends its partner from attacks, and does it so effectively that we need not wonder how the instinct to provide itself with Actiniæ could have arisen in the hermit-crab. But the acontia seem to have been greatly strengthened in the course of the sea-anemone's association with hermit-crabs, for they do not occur in all forms, and they are most highly developed in those which live in Symbiosis with crustaceans.

Fig. 34.Hermit-crab (E), within a Gasteropod shell, on which a colony ofPodocoryne carneahas established itself. From the common root-work (which is not clearly shown) there arise numerous nutritive polyps with tentacles (np), among which are smaller 'blastostyle' polyps with a circle of medusoid buds (mk), spine-like personæ (stp), and on the margin of the mollusc shell a row of defensive individuals (wp).F, antennæ.Au, eyes of the hermit-crab; slightly enlarged.

In this case the structural change, the transformation of the mesenteric filaments that occur in all Actiniæ into projectile acontia, is comparatively slight, but in another partnership between hermit-crabs and polyps the latter have undergone a much more marked adaptation. At NaplesEupagurus prideauxiiis one of the commonest hermit-crabs. It lives at a depth of about a hundred feet, and is often brought to the Zoological Station by the fishermen in large quantities. Its borrowed mollusc shell often bears a little polyp,Podocoryne carnea(Fig. 34), which forms colonies of oftenseveral hundred individuals, arising from a common root-work of stolons which covers the shell. The polyp colony is composed of different kinds of individuals or personæ, illustrating the principle of division of labour: it includes (1) nutritive persons (np) which possess a proboscis, mouth, and tentacles on their club-shaped bodies; (2) much smaller blastostyles (bl), that is to say, polyps with degenerate mouth and tentacles, which are wholly given over to the production of buds (mk), which then develop into sexual animals, little free-swimming medusoids; and (3) protective personæ in the form of hard spines (stp), beneath the shelter of which the soft polyps withdraw when the mollusc shell is rocked about on the sea-floor by the rolling of the waves. In addition to these three different kinds of individuals or personæ there are also (4) defensive polyps (wp) of long, thread-like shape, thickly set with stinging-cells, but possessing neither mouth nor tentacles. It might at first be thought that these are for the defence of the colony, but this is not so; the fact is that they rather serve for the direct defence of the hermit-crab. This is indicated by the position they occupy in the colony; they are not regularly distributed over the surface, but are ranged round the edge, and, indeed, only on the edge which surrounds the opening of the mollusc shell. Here these defensive polyps stand in close array, sometimes spirally contracted, sometimes hanging loosely down over the hermit-crab like a fringe. Their function, like the acontia of Actiniæ, is to defend the crab when an enemy tries to follow it within the shelter of its domicile. This can easily be demonstrated by drawing out the hermit-crab from the Gasteropod shell, and, when the colony has settled down again, seizing the shell with the forceps and drawing it slowly through the water. The water-stream which then flows upon the shell mimics the attack of an enemy, and immediately all the defensive polyps, as at a given signal, strike from above downwards, and repeat this three or four times; they are scaring off the supposed enemy.

In this species of polyp a special form of individual has developed with a quite definite position in the colony, and furnished with a special instinct or reflex mechanism which is directly useful only to the crab, and has therefore, in a sense, arisen for its advantage. This can quite well be explained through natural selection, for indirectly these polyps are also of use to the colony, inasmuch as they protect their valuable partner, and thus render it possible for the hydroid colony to make the partnership of use to the hermit-crab as well as to itself.

This mutual arrangement thus satisfies the requirement which,from the selectionist point of view, must be made in regard to all that is new—that it must be useful to its possessor.

If it be asked what service the hermit-crab renders to the polyp colony in return, the answer is that, as in the symbiosis with sea-anemones, the hermit-crab carries the polyps to their food, which is also its own. Hermit-crabs eat all sorts of animal food, living or dead, which they find on the sea-floor, and the remains of their meal fall to the share of the polyps. Once, without special intention, I laid a hermit-crab with its polyp colony in a flat vessel of sea-water beside a bright green living sponge. After some time the majority of the polyps had become bright green; they had filled themselves with the green cells of the sponge.

I do not know how else we should picture to ourselves the origin of symbiotic instincts in such lowly animals except through the transmission and augmentation of variations in the instincts of the two partners—variations which made their possessors more capable of survival. Mollusc shells, ever since there were any, must have served as a foundation and point of attachment for polyp colonies; as a matter of fact, we find to-day on mollusc shells many kinds of polyp colonies which show no special adaptation to a life of partnership with hermit-crabs. From such indifferent associations a symbiotic one must gradually have been evolved in some instances, through the preservation and augmentation of every useful variation, both of instincts and reflex actions, as well as of form and structure. I shall not attempt to trace the course of this evolution in detail, but it is obvious that the development of defensive polyps, and of their instinct to defend the crustacean, can be interpreted neither as due to any direct influence nor as due to the effect of use, but only to the utility of this arrangement, the beginnings of which—polyps with stinging-cells—were already present. Their augmentation and perfecting must be referred entirely to natural selection. It is the same with adaptations which do not refer directly to the crustacean partner, but rather to the disposition of the polyps on the shell. The spinous personæ which protect the softer polyps from being crushed by being rolled about on the pebbles by the waves cannot possibly be regarded as the direct result of this crushing. But it is obvious that some such colonies must have had among their members some with a stronger external skeleton, and therefore less easily crushed than the rest, and this would lead to their more frequent survival.

No adaptation seems to have taken place in the hermit-crab in this case, but that is probably only apparently the case; the probability is that it would not tolerate the presence of the polyp-colony on theshell unless its instinct compelled it thereto, just as its instinct impels it to cover itself with sea-anemones, and fearlessly to grasp the dangerous animal, which, however, only shows its partner its softer side. Truly, such transformations of instinct are wonderful enough, but that they should have come about through intelligence is here quite inconceivable; there remains nothing but natural selection.

A case in which no apparent corporeal adaptations have occurred, but which depends altogether on slight modifications of the instincts, is afforded by the well-known relations between ants and aphides. These two groups of insects live in a kind of symbiosis, although they are by no means inseparably connected with each other. Wherever strong colonies of aphides cover the young shoots of a plant, such as a stinging-nettle, a rose, or an elder, we almost always find ants which walk cautiously about among the plant-lice, often in great numbers, stopping now and again to stroke them with their antennæ, and then licking up the sweet juice from the intestine which they now give forth. Darwin showed by experiment that the aphides retain this juice if no ants are on the spot, and only give it off when ants are put beside them. Herein lies the proof that we have again to do with a case of modification of instincts. This juice is, of course, not the secretion of special glands, as it was still believed to be in Darwin's time, and it does not come from the so-called 'honey-tubes' situated on the back of the abdomen of the aphides; it is simply their excrement, which is liquid like their food, and the voiding of it has become instinctively connected with the presence of the friendly ants.

That the aphides are not in any way afraid of the ants implies, in itself, a modification of their instinct, for these poisonous insects, prone to biting, are otherwise much dreaded in the insect world. Moreover, the aphides, harmless as they seem, are not quite without means of defence, although these are never used against the ants. Other animals which approach them they bespatter with the sticky, oily secretion prepared in the so-called 'honey-tubes' already noted, squirting it especially into the eyes of an assailant, so that the attack is abandoned.

Of course the aphides have no idea wherein the utility of their friendship for the ants consists, but it is not difficult for us to discover it, since the ants, by their mere presence in the aphid colony, frighten and keep off their enemies. We see, then, that the conditions for a process of natural selection are here afforded: the instinct to be friendly to the ants is thoroughly useful, and the instinct of the ants to seek out the aphides, and, instead of devouringthem, to 'milk' them, is also advantageous; it must be an old acquisition, an instinct early developed, for in several species it has gone so far that the aphides are carried into the ants' nest, and are there (as one might say) kept and tended as domesticated animals.

A pretty case of symbiosis between two animals is reported by Sluiter, and I mention it because it concerns a vertebrate animal, and intelligence has something to do with it. In the neighbourhood of Batavia there are frequently to be found on the coral reefs large yellow sea-anemones, with very numerous and comparatively long tentacles, and a little brightly-coloured fish, of the genusTrachichthys, makes use of these forests, beset with stinging-cells, to find security from its enemies. These appear to be numerous, for in an aquarium, at any rate, the little fish very soon falls a victim to one or other of them, unless he is supplied with the protective sea-anemones. When this is the case it swims blithely about among the tentacles, and the sea-anemone does not sting it; for there has been a modification of instinct on its part as well as on that of the fish. The advantage it gains from the fish is, that the latter brings large morsels of food—in the aquarium, pieces of meat—into the anemone's mouth. In doing so it tears away fibres for itself, and even if the Actinia has swallowed pieces too quickly, the fish pulls them half out of the gullet again, and only relinquishes them to be consumed by its partner when it has satisfied its own appetite. In this case, again, the modification of the instinct is the only adaptation which has been brought about by the symbiosis, and its origin seems difficult to understand. How can the fish have first formed the habit of putting its prey into the mouth of the anemone instead of eating it directly? Although in many cases it is difficult to guess at the beginnings of a process of selection, because they are scarcely discoverable in the subsequently accumulated variations, yet in this instance we may perhaps picture them to ourselves in this way: The fish was in the habit of letting fall pieces of food which could not be swallowed whole, and of diving down upon them repeatedly, to tear off a fragment each time. As the sea-floor in flat places is often covered with sea-anemones, these pieces would often sink down upon one, which would welcome it as a dainty, and set about swallowing it, slowly in its own fashion. The fish must then have found by experience that it could tear off little bits much more easily from a piece that was held firmly by the anemone than from one that was lying loose upon the ground, and this may have caused it to do intentionally what was at first done by chance. But the sea-anemone, suffering no harm from the fish—indeed, its association ofideas, if I may use the expression, must rather have been little fishes and unexpected food—had no cause to shoot its microscopic arrows at it, and did not do so even when the fish concealed itself among the tentacles. This latter habit on the part of the fish would be developed into an instinct through natural selection, since the individuals that most frequently exhibited it would be the best protected, and therefore, on an average, the most likely to survive. Whether the benevolent attitude of the anemone towards the fish is to be regarded as the expression of an instinct is open to dispute, for it is quite conceivable that each individual sea-anemone is disposed to gentleness by the behaviour of the fish, and so the development of a special hereditary instinct was unnecessary, because without it each anemone reacted in the manner most likely to secure its own advantage[7].

[7]Since the above was written Plate has observed several similar cases in the Red Sea. A little fish lives along with the anemone,Crambactis aurantiaca, a foot in size, and not only conceals itself among its tentacles, but remains among them when the anemone draws them in. These fishes, therefore, must be immune against the stinging-cells of the sea-anemone; and in the same way another species of fish appears to be immune from the strong poison secreted by sea-urchins of the genusDiademafrom the points of their spines, among which the fishes live. This relation certainly seems more like a one-sided adaptation on the part of the fishes than a true symbiosis, but in the cases observed by Sluiter the return service of the fishes seems to be regularly rendered. Here, as everywhere else in nature, there are transition stages, and a one-sided protective relation may gradually, under favourable circumstances, be transformed into a symbiosis.

The same may be true of the fish as far as laying its booty in the mouth of the anemone is concerned; there may be no inherited instinct in this; it may be an intelligent action, which is learnt anew in the lifetime of each individual.

It might of course be objected to this interpretation that the beginning of the process, namely, the assumption that chance fragments from the food of the fish falling just on the anemone is very improbable; but I once observed that flat rocks washed over by the sea on the Mediterranean coast (not far from Ajaccio) were so thickly covered with green anemones that at first I took the green growth for some strange sea-grass new to me until I had pulled up a little tuft of the supposed plants and identified them as the soft tentacles ofAnthea cereus. Anemones must be equally abundant in the tropical seas of Java, and a sinking fragment must often alight on the mouth of one of them.

Much attention and keen discussion have in the last few decades been focussed on cases of symbiosis between unicellular Algæ and simple animals. A good example is our green fresh-water polyp,Hydra viridis(Fig. 35,A). Its beautiful colour is due to chlorophyll, and it was long a matter of surprise that animals should producechlorophyll, which is a characteristic and fundamental important substance of assimilating plants, until Geza Entz and M. Braun demonstrated that the green did not belong to the animal at all, but to unicellular green Algæ, so-called Zoochlorellæ, which are embedded in the endoderm cells of the polyps in great numbers (Fig. 35,zchl). As these algoid cells assimilate, and thus liberate oxygen, their presence is of advantage to the polyp. That—as was at first believed—they also yield nourishment to the polyp I consider very probable, notwithstanding the apparently opposed results of the experiments of so acute an observer as von Graff, for I have seen a large number of these animals thrive for months, and multiply rapidly by budding in pure water which contained no food of any kind. In favour of this view, too, are some observations, to be cited presently, on unicellular animals, in regard to whose nourishment by the zoochlorellæ living within them there can be no doubt at all.

Fig. 35.Hydra viridis, the Green Fresh-water Polyp.A, the entire animal, greatly enlarged.M, the mouth.t, tentacles.sp, testis.ov, ovary, both in the ectoderm.Ei, a ripe ovum, already green, in process of being extruded. After Leuckart and Nitsche.B, section of the body-wall, about the position of the ovary inA.Eiz, the ovum lying in the ectoderm (ect), in which zoochlorellæ (zchl), belonging to the endoderm (ent), have already migrated through the supporting middle lamella (st).eik, nucleus of ovum. After Hamann.

The little algæ on their part find a peaceful and relatively secure abode within the polyp, and they apparently do not occur outside of it, at least they do not now migrate from outside into the animal, butare carried over as a heritable possession of the polyps from one generation to another, and in a very interesting manner, namely, by means of the eggs, and by these alone. As Hamann has shown, the zoochlorellæ migrate at the time when an egg is formed in the outer layer of the body of the polyp (Fig. 35) from the inner layer outwards, piercing through the supporting layer between them (st) and penetrating into the egg (B,Eiz). They make their way only into the egg, not into the sperm-cells, which in any case are too small to include them. Thus they are absent from no young polyp of this species, and it is easy to understand why earlier experimental attempts to rear colourless polyps from eggs could never succeed even in the purest water.

Fig. 36.A,Amœba viridis.k, thenucleus.cv, contractile vacuole.zchl,the zoochlorellæ.B, a single zoochlorellaunder high power. AfterA. Gruber.

Quite similar green algæ live in symbiosis with unicellular animals, as, for instance, with an amœba (Fig. 36) and with an Infusorian of the genusBursaria. In the Zoological Institute in Freiburg there is a living colony of a green amœba and a greenBursaria, both of which came from America, sent to us some years ago by Professor Wilder, of Chicago, inside a letter with driedSphagnum, or bog-moss. The plants came from stagnant water in the Connecticut valley in Massachusetts. That in this case the zoochlorellæ are of use to the animals within which they live, not only by giving off oxygen, but also by yielding food-stuff, has been proved by A. Gruber, who bred the two green species for seven years in pure water which contained no trace of any kind of organic food for them. Nevertheless, they multiplied rapidly, and still form a green scum on the walls of the glass in which they are kept. They only die away when they are kept in the dark, where the algæ are unable to assimilate; then one green cell after another wanes and disappears, and, in consequence, their hosts also die from the double cause of lack of oxygen and lack of food.

Even in this case the symbiotically united organisms have not remained unaltered. The algæ at least differ from others of their kind in their power of resistance to living animal protoplasm. They are not digested by it, and we may infer from this that they possess some sort of protective adaptation against the dissolving power ofanimal digestive juices; they must, therefore, have undergone some variation, and adapted themselves to the new situation. Probably their cell-membrane has become impenetrable to the stuffs which would naturally digest them, an adaptation which could not be referred to direct effect or to use, but only to the accumulation of useful variations which cropped up—in other words, to natural selection. That any adaptive variation has taken place on the part of the host, whether polyp, amœba, or Infusorian, cannot be made out. None of these have altered their original mode of life; they do not depend on the nourishment afforded by the algæ, but feed on other animals, if these come in their way, and they live in water rich in oxygen like other species allied to them, and therefore are not altogether dependent on the algæ in this connexion; but they can no more help having their partners than the pig can help having Trichinæ in its muscles.

Similar plant-cells, not green however, but yellow, called zooxanthellæ, live in great numbers in the endoderm of various sea-anemones and in the soft plasmic substance of many Radiolarians. In both these cases we must look for the benefit they confer on their host in the oxygen they give off, for, like the green zoochlorellæ, they break up carbonic acid gas in the light, and give off oxygen; they no longer occur, as far as is known, in a free state, but are always associated with the host, and they must therefore have altered in constitution, and have adapted themselves to the conditions of the symbiosis.

Higher plants, too, sometimes have symbiotic relations with animals; the most remarkable and best-known example is the relation between ants and certain trees, in which the ants protect trees which afford them in return both a dwelling-place and food. We owe our knowledge of these cases to Thomas Belt and Fritz Müller, and more recently it has been materially increased by Schimper's researches.

In the forests of South America there grow 'Imbauba,' or candelabra-trees, species of the genusCecropia, which well deserve their name, for their bare branches stretch out like candelabra, and bear little bunches of leaves only at their tips. These leaves are menaced by the leaf-cutting ants of the genusŒcodoma, which attack numerous species of plants in these regions, often in tens of thousands, biting off the leaves, cutting them in pieces on the ground, and carrying them on their backs piece by piece to their nests. There they use them to make a kind of compost heap, on which fungi, to which the ants are very partial, readily grow. The candelabra-tree protects itself from these dangerous robbers, inasmuch as it has established an association with another ant (Azteca instabilis), which finds a safedwelling-place in its hollow, chambered stem (Fig. 37,A), and feeds on a brown sap which oozes from the inside. On the stem there are even little pits regularly arranged in definite places (E), through which the female ofAztecacan easily bore her way into the interior. There she lays her eggs, and soon the whole interior of the trunk teems with ants, which come trooping out whenever the tree is shaken.

Fig. 37.A, a piece of a twig of anImbauba-tree (Cecropia adenopus), withthe leaves cut off. At the leaf-bases arethe hair-cushions (P).E, the openingfor the associated ant (Azteca instabilis).B, a piece of the hair-cushion with theegg-shaped nutritive corpuscles (nk).After Schimper.

This alone would not suffice to protect the tree against the leaf-cutting ants, for how should the Aztec ants living inside notice the presence of the lightly climbing leaf-cutters? But that is provided for, for the Aztecs also frequent the outside of the trunk, and just where attack would be most disastrous, namely, at the stalks of the young leaves. At these places there is a peculiar velvet-like cushion of hair (P), from which grow little stalked white papillæ (Fig. 37,B), which are rich in nourishment, and are not only eaten by the ants, but are harvested by them, being carried off into the ants' dwellings, presumably to feed their larvæ. In this case, then, a particular organ, offering special attraction to ants, has been developed by the plant at the places more especially threatened; while, as regards the ants, it is probable that only the instincts of feeding and habitat require to be modified, since courage and thirst for battle are present in all ants, almost any species being ready at any time to throw itself on any other which intrudes into its domain.

It should be noted that not all the candelabra-trees live in symbiosis with ants, and so secure a means of defence against the leaf-cutters. Schimper found in the primitive forests of South America several species ofCecropiawhich never had ants in the chambers of their hollow stem. But these species did not exhibit the nutritive cushions at the base of the leaf-stalk; these contrivances for attracting and retaining the presence of partner ants were altogether absent. Indeed, only one species,Cecropia peltata, has produced thesepeculiar structures, and, as they are of nodirectuse to the tree, we must say that it has produced them only for the ants. Here, again, natural selection must have gradually brought about the development of these nutritive cushions, though as yet we do not know what the beginnings of the process may have been. In no case can the origin of these cushions be referred to any direct influence of the environmental conditions.

We may now pass to the association of two species of plants, of which the lichens furnish the best-known and probably most complete illustration. Till about twenty years ago the lichens, which in so many diverse forms clothe the bark of trees, the stones, and the rocks, were regarded as simple plants like the flowering plants, the ferns, or the mosses; and many lichenologists occupied themselves with the exact systematic distinction of about a thousand species, each of which could be as well and exactly classified, according to form, colour, habitat, and minute structure, as any other kind of plant. Then De Bary and Schwendener discovered that the lichens were made up of two kinds of plants, fungi and algæ, so intimately associated with and adapted to one another, that on coming together they always assume the same specific form.

Fig. 38.A fragment of a Lichen (Ephebe kerneri), magnified 450 times.a, the green alga-cells.P, the fungoid filaments. After Kerner.

The framework, and therefore the largest part, and the one which determines the form of a lichen, is due to the fungus (Fig. 38). Colourless threads of fungus ramify in a definite manner according to the species of fungus, and in the network of spaces left by this ramification green alga-cells (a) lie singly, or in rows, or groups. The fungus is propagated by multitudes of minute spores, which itproduces periodically, and these are disseminated in the air by the bursting of the sporangia and are carried away by the wind in the form of fine dust; the alga multiplies simply by continual division into two, but it also, like the whole lichen, can survive desiccation, and, after falling to pieces, is likewise carried through the air as microscopic dust.

The partnership of the two plants rests on a basis of mutual benefit; the fungus, like all fungi, is without chlorophyll, and cannot therefore decompose carbonic acid gas or elaborate its own organic food-stuffs; it receives these from the alga. The alga has in the network of the fungus a safe shelter and basis of attachment, for the fungus is able to bore into the bark of trees and even into stones; besides which it absorbs water and salts, and supplies these to the partner alga. We here see the mutual advantage derived from the partnership, which is really an extremely intimate one. Fungus spores, sown by themselves, spring up and develop some branchings of fungoid hyphæ, a so-called mycelium, but without the requisite partner alga these remain weak and soon die away. The alga, on the other hand, can, in some cases, though not in all, survive without the fungus if the necessary conditions of its life be supplied to it, but it grows differently and more luxuriantly in association with the fungus.

The same species of alga may be found associated with different species of fungi, and then each partnership forms a distinct species of lichen of definite and characteristic appearance; Stahl even succeeded in making new species of lichen artificially by bringing the spores of a lichen-forming fungus into contact with alga-cells, with which they had never been associated in free nature.

The most remarkable feature of this remarkable association seems to me to be the formation of common reproductive bodies—an adaptation in face of which all doubt as to the theory of selection must disappear. Periodically there are developed in the substance of the lichen small corpuscles, the so-called soredia, each of which consists of one or more alga-cells surrounded and kept together by threads of the fungus. When they are developed in large numbers they form a floury dust over the maternal lichen, which 'breaks up' and leaves them, like the spores of the fungus, to be carried away by the wind. If these alight on favourable soil nothing more is needed than the external conditions of development, light, warmth, and water, to enable the lichen to spring up anew. The great advantage to the preservation of 'species' is obvious, for, when multiplication by the ordinary method occurs among lichens, the spores of the fungus,even if they have fallen on good ground, can only develop into a new lichen if chance bring to them the proper partner alga.

Obviously there must be, in the formation of the soredia, great advantage for the species, or rather 'for the two species,' for the fungi as well as the algæ benefit by the arrangement, which ensures the continuance of the partnership. It was not without reason, however, that the dual organism was so long regarded as a simple species in the natural history sense,for that is what it really is, although it has arisen in a manner quite different from the usual origin of species. As we know species which consist only of single cells, and others which consist of many cells, differentiated in different ways, and forming a cell-community or 'person,' and, finally, others which consist of a community of diversely differentiated personæ, making up a 'stock'; so in the lichens we see that even different species may combine to form a new physiological whole, a vital unit, an individual of the highest order. When, at the outset of these lectures, I said that the theory of evolution was now no longer a mere hypothesis, and that its general truth could no longer be doubted by any one acquainted with the facts available, I had in my mind, among other facts, especially that of symbiosis, and above all the case of the lichens.

There are many other interesting cases of symbiosis between two different kinds of plants, and one side of the partnership is represented by fungi in a relatively large number of instances. The reason is not far to seek: fungi must always be dependent on other plants for their food; they must be parasitic, because they cannot themselves produce the organic substances they require. They must therefore associate themselves in some way with other organisms, living or dead, and as a general rule they simply prey upon their associate, sucking up its juices and killing it. But in not a few cases they can render services in return, and, as we have seen in the case of the lichens, symbiosis may then occur. Fungi in general have the power of discovering and absorbing the least trace of water in the soil, and with it they absorb the salts necessary to the plant, and in this, apparently, consists the service which they are able to render even to large plants fixed deep in the earth, such as shrubs and trees. The roots of many of our forest trees, e.g. beech, oak, fir, silver poplar, and bushes like broom, heaths, and rhododendrons, are thickly wrapped round with a network of fungoid threads, and the mutual relations just indicated exist between these and the plants in question (Fig. 39,AandB). The plants give to the fungi some contribution from the superfluity of their food-stuffs, and receive in return waterand salts, which are of value especially in times of drought. Perhaps there is some connexion between this and the fact that limes wither and lose their leaves so quickly during great summer-heat; these and many other of our trees possess no root-fungi or mycorhizæ.

It is easy to understand, therefore, that genuine 'symbiosis' may have arisen from parasitism. But that this is not the only path that leads to symbiosis is shown by the cases of animal symbiosis we have already discussed.

Fig. 39.A, fragment of a Silver Poplar root, with an envelope of symbiotic fungoid filaments (mycelium); after Kerner.B, apex of a Beech root, with the closely enveloping mantle of mycelium; enlarged 480 times.

The partnership between polyps and hermit-crabs may have arisen from a one-sided commensalism, since polyps establishing themselves on mollusc shells which were often made use of by hermit-crabs would be better fed than those which settled down on stones. There are still species which make use of both modes of settlement. Then followed the adaptation of the crustacean to the polyp, for, first, those hermit-crabs would thrive best which tolerated the presence of the polyp; then those which sought its presence, that is to say, which gave a preference to shells covered with polyps; and, finally, those which would take no others, and even themselves fixed the sea-anemone upon it, if it chanced to be removed. Intelligence need not be taken into account in the matter at all, not even in the hermit-crab's case. We have only to recall the complex instincts, exercised only once in a lifetime, which compel the silkworm and the emperor moth to elaborate their effective cocoons. The elaboration of the spinning-instinct can only be due to natural selection, for the insect can have had no idea of the utility of its performance, and the same is true in the case of the sea-anemones orthe hydroid polyps and the hermit-crab. The sea-anemone is quite unconscious that it is defending its partner, the hermit-crab, when it lashes out its stinging acontia on any disturbance, and the hermit-crab is equally unaware that the sea-anemone is contributing to its safety; both animals act quite unconsciously, purely instinctively, and the origin of these instincts, on which the symbiosis is based, must be due, not to intelligent activities which have become habitual, but only to the survival of the fittest.

According to the principle of natural selection nothing can arise but that which is of use directly or indirectly to its possessor. Nevertheless, there are cases in which it appears as if something had arisen, which was of no use to the species in which the variation appeared, but only to the species protected by it. This is the case in the remarkable symbiosis between algæ of the family Nostocaceæ and the floating, moss-like water-fernAzolla. This plant, in external appearance almost like duckweed, has on the under surface of its leaves a minute opening, leading into a relatively roomy hair-lined cavity, and in this cavity there is always, enclosed in jelly, a bluish green unicellular alga,Anabæna. The cavity is present in every leaf, and the alga is present in every cavity, making its way in from a deposit of alga-cells which is found on the incurved tip of every young shoot. As soon as a young leaf ofAzollaunfolds from the bud it receives itsAnabænacells from this deposit, and no one has yet found either twigs or leaves which were free from the algæ. But no one has succeeded in discovering any benefit derived by theAzollafrom this partnership.

This looks like a contradiction of the theory of selection, but there remains the possibility that there is some benefit rendered to theAzollaby the alga, though we cannot see it as yet. There is also the possibility that the cavity is an organ which was of use to the plant at an earlier time, perhaps as an insect-trap, but has now lost its significance, and is utilized by the alga as a dwelling-place. This, however, is contradicted by the remarkable distribution of the four known species ofAzolla. Two of these are widely distributed in America; the third lives in Australia, Asia, and Africa; the fourth in the region of the Nile: all four have cavities in their leaves, and in all these forms the cavity is inhabited by the same species ofAnabæna. This indicates that the leaf-cavity and the partnership with the alga must have originated in remote antiquity; the symbiosis must date from a time before the four modern species ofAzollahad split off from a single parent-species. But no rudimentary organ, that is to say, no organ not of use to the plant itself,would have been preserved through such a vast period of time, as we shall see later, for useless organs disappear in the course of ages. As the cavity has not yet disappeared, we may assume with some probability that it is useful to the plant, whether by means of theAnabæna, or in some other unknown way. To draw an argument against the reality of the processes of selection from our lack of knowledge of what this advantage may be would be as unreasonable as if, notwithstanding our experience that stones sink in the water, we were to assume of a particular stone which we did not see sink, because it was hidden from our sight by bushes, that perhaps it had not sunk, but was capable of floating.

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