up to the snail's horns, where they pulsate with great rapidity, sometimes twice a second. The swollen and agitated horns are all the more conspicuous because they have become banded with red and green pigment. They have been, so to speak, painted by the parasite. If a Blackcap or some similar perching songster is attracted by the snail on a leaf and pecks off the pulsating horn, the branch of the parasite, which is now full of larvæ of another generation, closes up automatically at its base, so that there is no loss of the multitudinous microscopic progeny. Now the investigators tell us that if the Blackcap swallows the horn then and there, nothing happens. The fluke-parasites are digested. But if the bird gives the tit-bit to its nestling, which has a weaker digestion, then infection occurs and the extraordinary life-cycle begins again.

The new biology has prepared us then for the linking together of lives apparently far out of touch with one another. The malarial parasites pass from man to man by aid of the mosquito; the Trypanosome microbe, that causes Sleeping Sickness, is disseminated by the tse-tse fly; bubonic plague is transferred from rat to man by the intermediation of the rat-flea. Some of the finest pearls appear to be formed in the pearl-oyster as the sepulchres of the minute parasitic larvæ of flukes and tapeworms.

If the idea of the web of life is Darwinian, as it distinctively is, then it cannot be called new. But the point is that it has now been recognised as a leading idea, as expressing a fundamental fact in the economy of Nature, with illustrations more abundant and farreaching than was previously supposed. Let us take a few modern instances. One of the most striking biological facts in many parts of Britain is the success of the heather. It grows exuberantly on mountain and moorland where few other flowering plants can make a living. There is soil, but it is unready; there is water, but it is apt to be physiologically unavailable. How does the heather flourish so well? The answer is that it is involved in a very intimate partnership with a fungus, which penetrates through and through the heather, from root to stem, into every leaf, even into the flower and its seed. What an individual could not

do, a firm achieves. The heather is a dual organism; it is like a flowering lichen! If it stood alone it would be a remarkable curiosity, but it is only an instance of a kind of partnership that is now known to be common, between the highest plants and the lowest. The list of flowering plants with fungi (mycorhiza) living in profitable partnership with their roots is already a long one. The root-tubercles of Leguminosa, due to nests of symbiotic Bacteria, are familiar, and of considerable importance in agriculture. Familiar, we say, but it does not seem at all clear as yet how it is that the partnership enables the quiet-living plant to capture the nitrogen of the air—a feat which man accomplishes by harnessing waterfalls to electric machines and sending terrific lightning discharges through the air.

Considerable progress has been made with the physiology of animal luminescence. Indeed the chemistry of the transformation of energy has outrun our knowledge of what the light means in the life of the creature. The gleams, so badly called 'phosphorescent,' may be lanterns, or lures, or danger-signals, or love-lights, or recognition-marks, or what-not. Speculation is rife because ecological observations and experiments have been few. As to the physiology, however, it seems to have been well-established in the case of the fire-fly, the crustacean Cypridina, and the rock-boring bivalve, the piddock or Pholas, that a ferment-like substance, luciferase, acts on an oxidisable substance, luciferin, changing chemical energy into radiant energy. But our present point, in connexion with the linkages of organisms, is illustrated by the evidence that in some cases, such as luminous cuttlefishes, the light is produced by nests of Bacteria. They are like those that we see in the dark on the glistening surface of the haddock hung up to dry; but they are living in regularised partnership with the animal. In some cases, therefore, the luminescent animal shines with a borrowed light.

Many beetles eat wood, but that is Spartan diet. It is not surprising, therefore, that some of the beetles which habitually bore in fresh wood have learned to grow a mould that yields what is called 'ambrosia.' The fungus lives on the wood and its sap, and spreads over the walls of the tunnels that the beetles make.

The fungus collects, concentrates, and prepares the food, making ambrosial bodies for the beetles and their grubs. In some cases the beetles do not swallow the dust of the wood through which they bore; they feed on ambrosia. The fungus does not seem to form spores or propagative elements, so it is probable that the beetles infect a new tree with surplus vegetative ambrosia cells which have passed out undigested from the food-canal.

Sometimes there is a triple alliance. Thus certain gall-midges that attack flowers of mulleins, scrophularias, and capers, provoke strange galls, inside which an ambrosia fungus flourishes. Gall-midge, mullein, fungus -a triple alliance. The hollow petioles of the Tachygalia tree in British Guiana are the homes of certain little beetles that have established an alimentary partnership with minute mealy bugs. These share shelter with their hosts and yield food to them in response to quaintly urgent massage. Beetle, tree, mealy bug-a triple alliance.

In his Edge of the Jungle' Mr Charles Beebe has recently given a vivid picture of the habits of the leafcutting ants, and verified an interesting linkage. When he took a pick-axe and broke into the underground city of the ants, into which they carry the semicircles of leaf dexterously cut off from the branches, he disclosed a remarkable sight. For he saw hordes of worker-ants chewing at the leaves that had been brought in and making them into a green paste, which is the culturemedium for a particular kind of fungus not known elsewhere. This forms the sole food of the ants as long as they are beneath the ground. But there are wheels within wheels, for when a queen-ant leaves the community and rises high in the air on her nuptial flight, she takes with her a minute pill of the fungus, carrying it in a depression beneath her mouth. Eventually, she comes to earth again, settles down to maternity, and has a family of workers. When they are strong enough and numerous enough they bring in segments of leaves and begin to make the green paste; then the queen takes the fungus-pill from beneath her mouth, where it has been safely kept all this time, and starts a new culture. The story is almost too good to be true, but it is well documented. Animate Nature becomes subtler under

our eyes.

We must not linger over this seductive section of our subject, but a final illustration may be permitted. Every one knows the little beetles called 'death-watches' that make tapping noises in the wainscot. The male thumps his head against the wood, signalling to his desired mate, speaking therefore of love not of death. The larval death-watches bore in wood and other dry materials, including books-poor food! Now, it has been shown that at the beginning of the digestive part of the foodcanal of the larval death-watch there are two minute pockets which are crammed with yeast-plants. These work on the unpromising wood-pulp, and there is a little brewery inside the larval death-watch. Careful examination showed Prof. Buchner that there were no yeast-plants in the eggs, yet they were always present in the young grubs. The solution of this puzzle is almost incredible. Associated with the egg-laying apparatus in the female there are two minute reservoirs opening to the exterior, and these are full of yeast-plants. When an egg is laid, some yeast-plants are expelled along with it, and they adhere to the rough surface of the egg-shell. When the beetle-grub is ready to hatch out, it nibbles at the egg-shell, and thus its food-canal becomes infected with yeast-plants. A little leaven goes a long way with the death-watch. The details are interesting, but even more important is the general fact that a partnership of yeasts and insects has been demonstrated in scores of cases. It is no curiosity.'

The correlation of organisms is very important practically, for there cannot be an evolving control of life which does not give the intricacy of the web full recognition. But the fact is also of great importance theoretically, for the complex system of inter-relations forms an external registration of evolutionary gains, and a sieve by which new variations-sometimes subtle nuances, one might think-are effectively sifted.

The observations on the habits of insects made by Réaumur in the first half of the 18th century have never been surpassed, not even by Fabre, whom Darwin called 'that inimitable observer.' Yet it is fair to say that a great change for the better has come over the study of animal behaviour. It has become more exact and

experimental; its interpretations are more critical. We owe the welcome change to the pioneer work of Lord Avebury (on ants, bees, and wasps) and Romanes, but especially to Prof. Lloyd Morgan, who laid the firm foundations of an experimental comparative psychology, and insisted on the principle that no act shall be ascribed to a higher mental faculty if it can be adequately described in terms of a lower one. From another salient a big advance is due to Prof. Jacques Loeb, who has pressed to its limit-or beyond it-the physiological, as opposed to the psychological, mode of interpretation. We are aware, in some measure at least, of the value of the resolute analytical work done by the extreme 4 apsychic school of behaviourists,' who regard the psychical aspect as purely epiphenomenal-like bubbles : on the stream-and not counting, but we think they are trying to have the play of 'Hamlet' without the part of the Prince of Denmark. Perhaps the main fact is that anecdotalism is being left behind, and precise science is emerging.

There is a growing discernment of an ascending curve on which may be arranged a long series of different kinds of behaviour. A Slipper Animalcule has a simple answer to almost every question: it reverses the action of its cilia, backs away from the disturbing stimulus, moves slightly on its own axis, and then advances again. If it does not clear the stimulus, it repeats the performance. Another Infusorian, the trumpet-shaped Stentor, has a variety of reactions, and in difficult circumstances it tries one after another, and may in this way solve its difficulty. There is much of the 'trial and error' method among simple animals.

Sponges have no nerve-cells at all, yet there are some that can narrow an osculum in the face of an intruding worm. That is to say, the muscle-cells forming the sphincter are themselves sensitive to stimulus; the effectors are also receptors. In sea-anemones we find a pre-established linkage between nerve-cells and musclecells, and a very simple kind of reflex action is now possible. The tentacles immediately contract on the worm which has fallen into their midst. This structural enregistering of a type of answer-back which is effective nine times out of ten, is a time-saving, energy-saving