prevents the access of spores of fungi and bacteria to the living tissues of the stem.

When a leaf falls and its stalk breaks away from the twig that bears it, one would expect to find an open wound on the twig, through which harmful bacteria and fungi might enter the plant. But it is not found. When the leaf is ready to fall two layers or plates of cork cells are formed, at first in contact with each other but soon to part company. The outermost plate of cork falls with the falling leaf, the inmost remains on the twig as an impermeable 'dressing' sealing up what without it would have been an open wound.

It is perfectly easy to divide with a sharp scalpel some of the larger of the unicellular animals (Protozoa), and as long as each half contains part of the nucleus, each half regenerates all the lost parts, and after a short time no one could tell that they represent one animal cleft in twain. Everybody knows that bath-sponges are propagated artificially by cutting one of them up into many portions and planting each portion out in a favourable spot, where it soon grows to the size of the parent sponge. Should an earth-worm be cut in two by the gardener's spade, the hinder portion will regenerate a head and the front portion will regenerate a tail, and by this unmeditated action of the gardener we find two earth-worms where hitherto but one had existed. Sometimes, however, things go wrong and the hinder portion develops a tail instead of a head, and then we have a worm with two tails, one 'fore' and one 'aft.' The worms with many bristles, which are so common in the sea, have even greater regenerative powers than the earth-worm. If but a process of Syllis ramosa be knocked off, an entire new individual will arise at the point of injury. This method of regeneration after division disappears, however, in the higher animals, and even amongst the Invertebrates there are certain groups where it does not exist.

The body of the round worms and of that curious torpedo-shaped, floating, transparent Sagittas which at some times of the year abounds in such quantities that the sea is almost stiff with them, shows no trace of such a power of reconstruction; nor do the Brachiopods or Gephyrea, but a starfish can lose one

or even two arms

and regenerate one or both, and the same is true of other groups of Echinodermata such as the sea-lilies. Many of the sea-worms are as capable of reconstructing their bodies after cleavage as the earth-worm or more so, but this is not true of leeches. Crustacea, such as the lobster, crayfish, or crab, and hundreds of other genera, will in time regenerate a limb which has been broken off, and there is a mechanism in their body for closing the severed blood vessels, otherwise they would bleed to death. If a joint of the limb of a crayfish be injured, the animal generally casts off the whole of that joint, sundering it at the next articulation above the wound. At the articulation which has now been broken a clot is at once formed which soon becomes covered with a cuticle or hardened coat. Under this cuticle a small bud arises which in time forms a miniature of the lost portion of the limb. Next time the crayfish casts its skin the miniature limb will emerge and straighten out, and though at first small, it has all the organisation appropriate to the lost part. At each successive moult it reappears larger, but it takes a long time to attain its full growth. That is why you frequently find Crustacea with limbs of unequal size on each side of the body. This habit of throwing off a limb when seized by an enemy is a great source of safety to a crayfish or other Crustacean, for although the enemy goes off with the limb the crayfish goes off with its life. Insects and spiders also can regenerate lost limbs, and cuttle-fish their arms.

When we come to the power of recovery from injuries of this kind amongst the Vertebrata we find that fishes show little power of regeneration. Although capable of surviving severe injuries they do not reproduce substantial parts of their bodies which may be injured. A pike will live after it has lost its tail. A sea-perch will survive the loss of a portion of the same organ; and a carp will live with but half a snout. Other fishes are, however, much more sensitive. The Lung-fish (Dipnoi) will regenerate the end of the tail, but it does not reform the representative that exists there of the backbone. In bony fishes the ends of the fin rays and various filaments such as barbles are, if nipped off, regrown. Sometimes these filaments become frayed out and mimic the waving

seaweeds amongst which they hide. Not only accident but mere wear and tear help to destroy these structures; and as they are essential for the life of the fish they are reproduced when necessary. It looks almost as if those organs which are most essential for the well-being of the animal in question are most easily regenerated. Those parts which are most liable to be lost in the ordinary rough-and-tumble of life, and those parts which are essential to the life of the organism, are more readily regenerated than are, for instance, the internal organs which are to a less extent subject to non-fatal injury. Amongst the amphibia the weak and little used limbs of Siren and Proteus are not replaced, though their gills are, and so are the strong and active limbs of the Triton. The newt readily regenerates a lost eye. A recent writer in the 'Times' has pointed out that there is a close connexion between the possibility of transplantation and the power of regeneration. Newts, for example, can regenerate both limbs and tail, and in their case transplantation of limbs or tail is easy to accomplish. Reptiles can only regenerate their tails, and in their case transplantation of limbs has not been successful. The eyeball of the larval common newt has a high power of regeneration, and when, as frequently happens, it is lost in fighting, it is replaced, although, so far as is known, sight is not restored. The optical vesicle of the larva of a newt has been removed and transplanted further back in the tissues of the animal; a new lens is then produced from the area of skin opposite the vesicle, although under normal circumstances no lens would have been formed there. But the eye is sightless."

Amongst reptiles the power is less, but lizards which lose their tail are capable of regenerating it, and as the tail is a likely part of the body to be seized by the enemy, they have a special provision in the vertebræ of that appendix for breaking off just above the region that has been seized, say, by a raptorial bird. This apparatus consists of a thin unossified disk of cartilage which cuts vertically across the vertebra, and it is in the plane of this disk that the fracture takes place. The new tail, which may be double should the fracture be incomplete, has but an imperfect skeleton, and the scales on it often differ from those of the lost part.

Any one, and I suppose this means every one, who has seen the moulting of a bird finds it perfectly obvious that birds can at any rate replace their feathers. Feathers arise from certain skin papillæ which become active at the moulting season. The new feather pushes the old one out of its place, and in the Cassowaries and Emus the new feather grows up into the hollow of its predecessor, so that the feathers of these birds for a time wear their old coat over their new one. The regenerative power of the papilla and its surrounding follicle seems to be almost unlimited under healthy conditions unless mechanically injured. The accidental loss of a feather sets the papilla in action, and, regardless of the moulting season, a new feather replaces the lost one. This, of course, is a matter of considerable importance to a bird whose powers of flight may depend upon the presence or absence of a certain feather. The Japanese know a lot of things that we are ignorant of, and they are able in some mysterious way to prevent the cock-birds of their poultry from moulting. Consequently the quill feathers of these cocks continue to grow instead of being shed from time to time, and may reach the astounding length of ten or twelve feet. Certain birds such as the stork, can also regenerate a lost beak.

Mammals have little power of regenerating. Hairs may be replaced, but not noses or ears, arms or legs, fingers or toes. St Patrick is said to have swum across a river-I think it was the Shannon-' carrying his head in his teeth,' but that of course was a miracle, and there is no record that he regenerated his lost head.

The whole problem of regeneration reveals how great is the tendency of live animal tissues to react on isolation by an attempt to form an entire animal. It is as though any one tissue were only stable in the presence of all the other tissues of the body: if one tissue is absent or incomplete equilibrium can only be restored by its regeneration. This shows that the fundamental Physiological Unit is neither tissue nor cell, but the whole animal.

The chief enemies of plants are animals that feed upon them. Many plants are grown by mankind merely to be consumed-a very painful proceeding to the people who are averse to the destruction of any life-and certain

tropical ants have regular vegetable gardens, consisting of certain moulds or fungi which they cultivate and harvest for the consumption of themselves and their offspring. As many of these fungi are injurious to plants, the ants unconsciously help to spread disease. An innumerable number of insects feed on plants, burrowing into their surface, and at times the whole vegetation may be swept away by the incursion of locusts and their young, which eat it up. The burrowing larvæ of insects can destroy whole forests. This is specially true in countries like the United States, Russia, and Canada, where the area of forest used to be almost limitless.

Many plant diseases, like so many animal diseases, are caused by fungi, and although in the animal world it is usually, but by no means always, bacteria that do the damage, plants are attacked by much more highly developed moulds. They fill up the cavities between the cells of the leaves, which are so important for transpiration, and in many ways do incalculable harm. The temperature of a sick orange or lemon attacked by a common fungus such as Penicillium digitatum rises a degree and a half to two degrees.

Comparatively few protozoa (unicellular animals) are found living in plants, but a species of Trypanosoma, an animal allied to that which causes sleeping sickness in Africa, and syphilis all over the world, has been found lately to live in the pungent latex of certain herbaceous Euphorbias, which, when badly infected, become blanched and eventually wither. A plant can be artificially inoculated with this unicellular animal, but the natural infection is brought about by a certain genus of bug named Stenocephalus which lives on and amongst the spurges. Allied species of Trypanosoma occur in the latex of other plants. Further, there is an infection which causes one of the most serious diseases in sugarbeet in America, known as curly-leaf. The leaves crumple and are put out of action. This disease is invariably associated with a small insect known as Eutettix, or the beet-leaf-hopper. They also are true bugs. All of them are minute, but what they lack in size they make up in numbers. It has been shown that an acre of pasture will support a million of these pests, and that they