More Science From an Easy Chair Part 11

CHAPTER XXI

CARRIERS OF DISEASE

It has now been discovered that a great number of human diseases are caused by microscopic parasites, which are spoken of in a general way by the name invented by the great Pasteur, viz. "microbes."

Wool-sorter's disease, Eastern relapsing fever, lock-jaw, glanders, leprosy, phthisis, diphtheria, cholera, Oriental plague, typhoid fever, Malta fever, septic poisoning and gangrene have been shown to be caused each by a peculiar species of the excessively minute parasitic vegetables known as bacteria (or Schizophyta). Others, for example, malaria and sleeping sickness, have been shown to be caused by almost equally minute microbes, which are of an animal nature, and similar to the free-living animalcules which we call Protozoa, or "simplest animals," whilst a third lot of diseases--rabies, smallpox, yellow fever, scarlet fever, and typhus--are held to be caused by similar minute parasites, although these have not yet actually been seen and cultivated, but are surely inferred (from the nature and spread of these diseases) to exist.

The difference of the microbes called bacteria from the disease-causing microbes cla.s.sed as "Protozoa" consists in their simpler structure and mode of growth. They are essentially filaments which continually multiply by fission--a process often carried so far that the little organisms present themselves as short rods, or as curved (comma-shaped), or even spherical particles (micrococci)--and only in favourable conditions arrest their self-division so as to grow for a time into the thread-like or filament shape. Often these filaments are not straight, but spirally twisted, and are called "spirilla." Some of them are blood parasites, but the larger number attack the tissues, and others occur in the digestive ca.n.a.l.

The parasitic disease-producing protozoa, on the other hand, are of softer substance, often have the habit of twisting themselves in a corkscrew-like manner, and usually are provided with an undulating membrane or frill, as well as with one or with two whip-like swimming processes (the latter are present also and are often numerous in the actively swimming phases of bacteria), and have a more complicated life-history. They divide, as a rule, longitudinally and not transversely, and pa.s.s from one "host" to a second, where they a.s.sume distinct forms--males and females, which conjugate and break up (each conjugated or fused pair) into a ma.s.s of very numerous, excessively minute, young. The disease-producing protozoa of this kind are frequently parasitic in the blood of man and animals, and were only recently recognised, after the disease-producing bacteria of many kinds had been thoroughly studied. These animal microbes are often spoken of as "blood-flagellates" or haemo-flagellata, and the larger kinds are called "Trypanosomes," or "screw-form parasites," or whilst a series of more minute ones are called "Piroplasma," or "pear-shaped parasites." Many, but not all, are found during a certain period of their life, actually inside the corpuscles of the blood. The fact that many of these blood-flagellates (if not all) have, besides their life in the blood of one species of animal, a second period of existence in the juices or the gut of another animal, has made it very difficult to trace their migrations, since in the second phase of their history their appearance differs considerably from that which they presented in the first. And often they exist in one kind of animal without doing any harm, and are only poisonous when introduced by insects into the blood of other kinds of animals!

There is, further, another set of disease-causing protozoan parasites which are similar to the amoeba or proteus-animalcule, and a third, which belong to the group of "ciliated infusoria." They are not so minute as the preceding set, and are not usually referred to as "microbes." They inhabit the intestine of man and animals, and cause, in some instances, dysentery. These two later kinds of protozoan parasites I will at the moment leave out of consideration, as well as the "coccidia," which multiply in the tissue-cells of animals--for instance, rabbits and mice--and cause an unhealthy growth and excessive multiplication of the cells of the tissues, which in some respects resembles that seen in the terrible disease known as cancer.

Indeed, it is held by many investigators that some such parasite--though not yet discovered--is the cause of cancer.

A very important question is: How do these poison-producing parasites (for it is by the poison which they manufacture that they upset the healthy life of their hosts) make their way into the human body? The surface of the body of animals, like man, is protected by a delicate, h.o.r.n.y covering--the epidermis--through which none of these parasites can make their way. They can only get through it, and so into the soft, juicy tissues and the fine blood-vessels which it covers, when it is cracked, broken, pierced, or cut. But they also have a way to open them through the softer moist surfaces of the inner pa.s.sages, such as the digestive ca.n.a.l and the lungs. They enter (some kinds only and not a few) with food and drink into the digestive ca.n.a.l, and with the air into the air-pa.s.sages and the lungs; and once in these chambers, which have only soft lining-surfaces, they are able to penetrate into the substance of the body. Many of those which enter the digestive ca.n.a.l do not require to penetrate further, but multiply excessively in the contents of the bowel, and there produce poisons, which are absorbed and produce deadly results--such are the bacteria which produce Indian cholera and ordinary diarrhoea--whilst the kind causing typhoid fever not only multiplies in the gut, but penetrates its surface.

The protective surface of man's body is broken, and the way laid open for the entrance of microbes in various ways. A slight scratch, abrasion, or even "chapping" is enough. Thus, a mere breaking of the skin of the knuckles by a fall on to dirty ground lets in the deadly bacterium of lock-jaw (teta.n.u.s), which is lurking in the soil. Leprosy is communicated from a leper in the same way. The almost ubiquitous bacteria of blood-poisoning (septicaemia) may enter by the smallest fissure of the skin, still more readily by large cuts or wounds. The bites and stabs of small and large animals--wolves, dogs, flies, gnats, fleas and bugs, also open the way, and often the deadly microbe has a.s.sociated itself with the biting animal and is carried by it, ready to effect an entrance. Thus rabies (hydrophobia) is introduced by the bites of wolves and dogs, and a whole series of diseases, such as plague, malaria, sleeping-sickness, gaol-fever (typhus), yellow fever, relapsing fever, and others, are introduced into the human body by blood-sucking insects. Hence the immense importance of treating every slightest wound and scratch with chemicals (called "antiseptics"), which at once destroy the invading microbe--and of keeping a wounded surface covered and protected from their approach.

In ways at one time unsuspected, such openings may be made by which poisonous microbes enter the body. Thus the little hard-skinned parasitic thread-worms which are often brought in by uncooked food into man's intestine, though by themselves comparatively harmless, scratch the soft lining of the bowel and enable poison-making microbes to enter the deeper tissues, and cause dangerous abscesses and appendicitis.

The carriers of disease germs thus become a very important subject of study. There are carriers which make no selection, but are, so to speak, "casual" in their proceedings, and there are others which have the most special and elaborate relations to some one kind of disease-causing microbe for which alone they are responsible, and to the life of which they are necessary. Let us look first at the more casual group. Man himself is a great carrier and distributor of his own diseases. Unless and until he has learned to be careful and guard against thoughtless proceedings, he is always spreading the microbes of his diseases and pa.s.sing them on to his fellow men. He pollutes the waters, rivers, lakes, and pools from which others drink. He manures his crops, and then eats some of them uncooked. His hands are polluted by disease-causing microbes, and he handles (to an alarming and unnecessary extent) the food, such as bread and fruit, which is swallowed by his fellows, without cleansing it by heat. It has lately been shown that apparently healthy men and women often harbour within them the microbes of typhoid fever or of cholera (and probably other diseases), without themselves suffering in health, and that unsuspected they thus become distributing centres of these diseases.

The names "typhoid carrier" and "cholera carrier" have actually been introduced to describe the condition of such persons. Then, again, by his breath, and by coughing and spitting, a man acts as a carrier to others of disease-microbes already lodged in him, as well as by actual contact in the case of those infections which are called "contagious."

The numerous animals which surround and are a.s.sociated with man act very largely as casual carriers and distributors of disease microbes.

Thus dogs and even the cleanly cat are frequently carriers of disease.

But more especially those creatures which visit man's food stores and food ready for consumption (such as bread, fruits, cold meat, etc.) are active carriers. Rats and mice run over such stores and pollute them. But the most widely active in this way is the common house-fly.

Whilst white men have developed an almost automatic resistance and objection to the visits of flies to their lips, eyelids, and any wound or scratch of the skin--a resistance which is not shown by many savage races--they yet allow house-flies to swarm in their dwellings, to run about and sample their food, with an indifference which is, when the truth is known, truly horrible in its fatuity and foolhardiness. For the fact is that the feet and proboscis of the common house-fly are covered with microbes of all sorts, picked up by his explorations upon every kind of filth. At every step which he takes he plants a few dozen microbes, which include those of infantile diarrhoea, typhoid, and other prevalent diseases. This is easily shown by allowing him to walk over a smooth plate of sterilised nutritive gelatine and preserving it afterwards free from the access of microbes from the air. In twenty-four hours every footstep of the fly on the gelatine is marked by an abundant and varied crop of microbes, which have multiplied from the individuals let drop by the little pedestrian.

There is no doubt whatever that the house-fly is a main source of the dissemination of the microbe of infantile diarrhoea, and the cause annually of hundreds of thousands of deaths of children in the great cities of Europe and America. Also in camps and infected districts he is largely responsible for the introduction of the microbe of typhoid fever into the human food to which he has free access after his previous visits to open latrines. The house-fly is himself a product of dirt and neglect. The eggs are laid in old manure heaps and kitchen middens, and the maggots, which eventually are transformed into flies, nourish themselves in those acc.u.mulations. When this refuse is rapidly and regularly removed by the care of the sanitary officials of a town, the flies diminish in number, as they have diminished in London within the last thirty years. We no longer are overrun by flies in London in the summer months. The man selling sheets of sticky paper is no longer heard in our streets calling "Catch 'em alive, oh!" But in country places, where a neglected stable-yard is near the dining room of the inn, house-flies are as great a nuisance and danger as ever. There is no difficulty, if the simplest rules of cleanliness are observed, in abolishing them altogether from human a.s.sociation, but combined and simultaneous action against them is an essential condition of success.

CHAPTER XXII

IMMUNITY AND CURATIVE INOCULATIONS

During the last twenty years the whole att.i.tude of the study and investigation of disease-causing microbes has advanced from the preliminary step of merely identifying certain microbes as the causes of certain diseases to a further step, viz. that of attempting to defend the animal and the human body against their attacks in the manner already so finely started by Pasteur. For many years disease after disease was examined and found to be caused by special bacteria or other microbes. Even non-infectious diseases or diseases only communicable under very special conditions were found to be due to microbes, so that it is probable that all disease that is not due to congenital malformation or to mechanical injury, or to poison fabricated in the weapons of larger animals and plants, or by man himself, is due to microbes. "Life," says Lord Justice Moulton, "is one ceaseless war against these enemies, and the periods of our too-transient successes are known as health." One of the last diseases traced to microbes is that sad condition known as "infantile paralysis," by which so many of the brightest and best members of the community have been crippled, from childhood onwards, through life.

Of late we have been making rapid strides in arriving at a knowledge as to how Nature herself protects higher creatures from the excesses and exuberance of destructive microbes, and we are now able to see that it is in adopting her methods that our best hope of increasing that protection lies. Nature is satisfied if the efficacy of her defence is sufficient to save enough individuals to carry on the race.

Man desires in the case of his own fellows to out-do Nature and to save all.

A century and a half ago, before the true character of infective disease was understood, it was observed that an individual who was attacked by the smallpox and recovered became incapable of receiving the infection again. He was "protected" or "immune." The practice of "inoculation" was introduced from the East by Lady Montague. The infectious matter was introduced from a smallpox patient into the person to be protected by rubbing it into a scarified part of the skin. A much less severe attack of smallpox was thus produced than that which usually followed the natural infection, which (though we do not know precisely its mode of entrance) is more widely spread through the blood. At the same time the condition of "immunity" after the attack was brought about with equal efficacy. When Jenner introduced inoculation with "cowpox" for the purpose of establishing "immunity"

in the vaccinated person, inoculation with smallpox itself was a very usual practice. It was open to the objection that sometimes an unexpectedly violent attack of the disease was produced, resulting in death, and that the active infection was kept alive and ever present in the community. The notion with regard to the mode in which "immunity" was produced by either the Montacutian or Jennerian inoculation was, even after the general knowledge of microbes as the living contagion of disease had been arrived at, that the mild attack due to inoculation "used up" something in the blood--in fact, exhausted the soil, so that the infective matter or microbe could no longer flourish in the blood. And this view was accepted as the explanation of the "immunity" to the anthrax disease conferred on cattle and sheep by Pasteur's inoculations of weakened, but still actively growing, cultures of the anthrax bacillus. Another theory was that they produced something in the blood by their own life-processes which checked their further growth, just as yeast will not grow in wort in which it has produced 8 per cent. of alcohol, and as a fire may be choked by its own smoke or ashes.

We now know that both these explanations of "immunity" are incorrect.

Nature provides at least three varieties of defence within the blood of higher animals against disease-producing microbes which have broken through the outer line of fortification, the skin. These three methods are effective in different cases (one in this disease, the other in that), and, on the whole, are sufficient to preserve the races of animals (including man) from complete destruction. These are (1) the production in the blood of an antidote to the toxin or poison elaborated by the invading microbe--an ant.i.toxin, which chemically neutralises the toxin; (2) the production in the blood of the attacked animal of a "germicidal" poison which repels and kills the attacking microbes themselves (not merely neutralising their poisonous products); (3) the extermination of the intrusive, disease-producing microbes by a kind of police, which scour the blood channels and tissues and "eat up"--actually engulf and digest--the hostile intruders. These latter agents, actual particles of the living animal in which they exist, are the "eater-cells," or "phagocytes"--minute, viscid, actively moving cells, resembling the animalcules called "amoeba." They are only the one two-thousandth of an inch in diameter, and are known as the white or colourless corpuscles of the blood. They are far less numerous than the red blood-corpuscles, which are the agents for carrying oxygen, but there are eight thousand million of them in a large spoonful of blood. They are the really important agents in protecting us from microbes, since they not only engulf and digest and so destroy those intruders, but it is probable (not certain) that they also are the manufacturers of the ant.i.toxins and of the germicidal poisons.

If these three defensive processes given us by Nature are in working order, that is to say, if we are "healthy," they should secure to us a sufficient "immunity"--at at any rate, "recovery"--from any attack of disease-producing microbes. But they are not in "unselected," widely ranging mankind always equal (in their unaided natural state) to their task.

The attempts to produce immunity by vaccination with weakened or localised disease germs is really an attempt to train and develop to a high point the activities of the phagocytes or eater-cells of the blood.

The introduction of ant.i.toxins by injection of them into the blood (as in the treatment of diphtheria, lock-jaw, and snake-bite) is an attempt to bring to the rescue of a patient who would sooner or later produce his own ant.i.toxins (but perhaps too late or in insufficient quant.i.ty) the similar ant.i.toxin obtained from the blood of another animal which has been artificially made to produce in its blood an excessive quant.i.ty of that substance.

Mithridates, King of Pontus, was, according to ancient legend, in consequence of his studies and experiments, soaked with all kinds of poisons to which he had become habituated by gradually increasing doses, and he had at last reached a condition in which no poison could harm him, so that when he was captured by the Romans and wished to kill himself (which was the correct thing in those days for a fallen king to do), he wept because he was unable to get any poisons which would act upon him. He was "immune" to all poisons. This real or supposed immunity resulting from the introduction into the living body at intervals of a series of doses of a poison gradually increasing strength has been called "Mithridatism," and animals and men so treated have been said to be "mithradatized." The toleration of poisonous drugs--such as tobacco and alcohol, and even of mineral poisons, such as a.r.s.enic--was, until lately, regarded as merely a special exhibition of that habituation of "adaptation by use" which living things often show in regard to some of the conditions of their life. Unusual cold, unusual heat, unusual moisture, salinity or the reverse, unusual deprivation of food, unusual muscular effort may be tolerated by animals without injury provided that they have been "gradually accustomed" to the unusual thing, or, in other words, that the unusual has been gradually made the usual; so that there is a saying that eels after a time even get used to being skinned. There was no attempt to explain the details of this process of habituation; it was a.s.sumed to be a part of the general "educability" of living matter.

The study of the education of living matter, in regard to various conditions which can act upon it, has yet to be further carried out, but the way in which the poisons made by disease germs and the like, and the disease germs themselves, are dealt with in the blood and tissues has, on account of its urgent importance, from a medical point of view, been already profoundly studied by experimental and microscopic methods of late years. The old notion as to "mithridatism"

was that an animal or a man would have to be separately prepared and "immunised" by habituation for every distinct kind of poison. We now know that this is not the usual way in which Nature confers immunity to poisons. Most astonishing, and at first sight magical or mysterious, powers exist in the living protoplasmic cells in and around the blood of man and higher animals, which enable their possessors to resist and combat the poison-producing microbes, and also the poison itself, of all kinds, by which the race is liable to be attacked.

Few of us realise what a wonderful and exceptional fluid the blood of a higher animal is. The Australian natives attach so little importance to it that they actually cut themselves and use their blood as a sort of paste for sticking decorative feathers on to a pole! The Papuans are more advanced, since they regard the flow of blood from a cut or graze as an evil portent. And some respect to the greatness and wonder of blood is shown by those persons among civilised peoples (more frequently men than women) who faint when they see blood, or even at the mention of its name! This stream of red fluid within us (of which an average man has about fifteen pints in his vessels) courses at a tremendous rate from the heart through all the endless branches and networks of arteries, capillaries and veins, and back to the heart. It feeds, cleanses, warms and takes "vital air" (the old name for oxygen gas) dissolved in it to every particle of our bodies, fresh and fresh at every pulse-beat as it rushes on. It not only absorbs crude digested food through the walls of the gut, but conveys it to where it is worked up and distributes the worked-up product. It removes the quickly used-up substances from every part, and the choke-damp or carbonic acid which would stop the whole machine, and kill us, were it not got rid of through the lungs as the blood hurries through the walls of these air-sacs, whilst other used-up materials are carried by it to the kidneys and pa.s.sed out of the body through them. Every part of the body is brought into common life with every other part by this impetuous blood-stream--which is here, there, and everywhere, right round, and back again, in twenty-five seconds! It is obviously a very serious thing if a poison-producing microbe gets into this blood-stream and multiplies within it, or if poison-producing microbes lodge somewhere beneath the skin in a wound, and keep on discharging virulent poison into the blood! The mischief is spread all over the body at once.

It is not surprising, then, that the long course of natural selection and survival of the fittest has resulted in the fixing in the blood and the living cells immediately connected with it of extraordinary protective powers. The floating scavenger cells (eater-cells or phagocytes, first recognised as such and so named by Metchnikoff) are already found in the blood of quite simple animals in worms, sh.e.l.l-fish and insects. I have watched them with the microscope at work in transparent minute living water-fleas eating up, and digesting microbes which had got into the water-flea's blood. In higher animals what we call "inflammation" is a condition--the result of a new and advantageous mechanism--which consists in a local r.e.t.a.r.ding of the blood-current, effected by the action of the nerves on the muscular walls of the blood-vessels, and the consequent escape of the eater-cells into the injured or infected tissue, there to eat up and destroy the injurious microbes or other particles. Special and remarkable properties--chemical activities of an extraordinary character--have been gradually developed in the floating phagocytes and in similar non-floating fixed cells over which the blood flows.

These special chemical activities are of several distinct kinds. The first is the power to convert the poison of a microbe into a destroyer of that poison--toxin into ant.i.toxin. The atoms of these poisons are elaborately composed combinations of the organic elements. By a "shake" or a "twist" (so to speak) administered by the living cells of the blood the combination is altered, and the toxin becomes an ant.i.toxin, destroying by chemically combining with it the very toxin from which it was formed. This is a far more efficacious method than the supposed mithridatic "habituation" or "toleration" of a poison, with small doses of which you have to be gradually prepared. The healthy blood converts any one of a large series of microbe poisons into ant.i.toxins. It is true that apparent "opposites" are often closely allied in Nature. Evil smells and tastes are closely allied to sweet perfumes and flavours, and what is healthy and agreeable to some men acts as virulent poison to others (_e.g._ sh.e.l.l-fish, egg, quinine, opium). The smallest change in the substance administered or the smallest difference in the living substance of an individual (what is called "idiosyncrasy") makes all the difference between "poison"

and "meati."

If the phagocytes and similar cells in the blood of a man or animal exposed to the poison produced by localised microbes (such as those of teta.n.u.s, diphtheria and septic growths) cannot produce enough ant.i.toxin so as to quickly destroy the poison, we can, and do, nowadays, save his life, by injecting into his blood the required ant.i.toxin, obtained from another animal which we have caused (by injection of the toxin) to produce the ant.i.toxin in excess. That is one sort of "immunity" or "resistance" which we can confer, and is largely in use at the present day--the "ant.i.toxin" treatment.

The second poison-repelling chemical activity of the blood, produced by the living cells in and about it, consists in the blood becoming directly poisonous to injurious microbes. It becomes "bactericidal,"

produces a bactericidal poison (called an alexin) which is usually present in normal blood, but is greatly increased when large numbers of certain poisonous microbes (_e.g._ those of typhoid fever) get into the blood. Again, by other chemical substances produced in it, the blood may, without actually killing the invading bacteria, only paralyse them, and cause them to "agglutinate" (that is, to adhere to one another as an inactive "clot" or "lump"). As the "agglutinating"

poison is peculiar (or nearly so) for each kind of microbe, we can tell whether a patient has typhoid by drawing a drop of his blood into a tube, and adding some fresh living typhoid bacilli to it. If the patient had typhoid he will have begun to form the "typhoid-agglutinating" or "typhoid-paralysing" poison in his blood, and the experiment will result in the "agglutination" (sticking together in a lump) of the typhoid bacilli. And so we prove, in a doubtful case, that the patient has typhoid.

The third chemical activity of the blood in dealing with poisonous microbes is also one which is conferred upon it by its living cells when excited by the presence of those microbes. It is the production of a "relish" (for so it must be called) which attaches itself to the microbes and renders them attractive to the eater-cells (the phagocytes), so that those swarming amoeba-like floating particles at once proceed to engulf the microbes with avidity. In the absence of the relish (the Greek word for it used by Sir Almroth Wright, its discoverer, is "opsonin"), the eater-cells are sluggish--too sluggish--in their work. They resemble a child who will not eat dry toast, or, at best, only slowly, but will devour rapidly many pieces when the toast is b.u.t.tered. It is of the utmost importance to us that our white corpuscles, or eater-cells, should not be sluggish but greedy.

There are some microbes which will produce deadly poison if grown in the clear fluid (serum) of the blood of an animal (as, for instance, the cholera-microbe when grown in the serum of the frog's blood), yet when inoculated living into the blood of that animal never cause the slightest illness! Why? Because they are at once eaten by the vigilant phagocytes of the blood before they can produce any appreciable amount of poison. That is easily demonstrated by experiment. Our main means of defence against microbial disease, says Metchnikoff--though cleanliness and precaution against access of microbes are all very well in their way--is the activity of our phagocytes. Now it appears that just as in the other cases I have been considering, so in the production of "relish," the power to produce it resides in the blood (and perhaps the cells of its vessels), but is not set at work until the enemy is in the blood. Suppose there is an infection, an invasion of the blood and tissues by one or other disease-causing microbe.

Gradually if the body is healthy the "relish" is produced and becomes attached to the invading microbes. The phagocytes swallow them greedily and make an end of the invasion.

It is proved that this aroused avidity of the phagocytes is due to no change in the phagocytes themselves; since if they are transferred to the serum of a normal man they show no such predilection for the special invading microbe. The "opsonin," or "relish," is something exuded into or produced in the blood fluid when the attacking microbe arrives. It attaches itself to them: that is the essential fact. In many of us the phagocytes are not at a given moment so "avid" of this or that disease-microbe as they should be in order to protect us from its multiplication and poison production. But it is found that by injecting boiled and cooled (therefore dead) microbes of a particular kind into the blood of a man, you can start the production of the "relish" appropriate to that kind. The dead microbes answer this purpose; they excite the production of the opsonin appropriate to them and yet are not themselves dangerous, since they are dead. When subsequently (or possibly concurrently in small quant.i.ty) living microbes of the same disease enter the blood, the opsonin is ready for them. They are, to put it picturesquely, like oysters at the oyster-bar, peppered and vinegared "in no time," and then swallowed by the phagocytes by the dozen. This seems almost too comic a view of the deadly struggle of man and higher animals for health and freedom from the swarming pests which everywhere invade him. Yet it is correct, and involves a simple and fundamental truth. Our properties and appet.i.tes are but the sum of those of the protoplasmic organisms--the cells--of which we are built up. Our need for a relish with oysters is the same thing as the need of the phagocyte for a relish with its microbes, not something "poetically" compared to it. The story of "the oysters and the carpenter" might be replaced by that of "the microbes and the phagocyte." The saying, "Fine words b.u.t.ter no parsnips," finds a parallel in the remark that "The drinking of drugs does not opsonise microbes."

Half-way between us and the amoeba-like unicellular organisms we find the earth-worm preparing his piece of lettuce (as Darwin showed) with a juice exuded from his mouth, a "relish" reminding one of the Kava drink of the South Sea Islanders. To "opsonise" or render attractive by the application of chemical "relish" is a proceeding which we find in operation in the feeding of the minute colourless corpuscles which engorge the still more minute bacteria--and also in the preparation of their food by various lower animals, and finally in the elaborate flavouring and cooking of his food by civilised man!

CHAPTER XXIII

THE STRANGE STORY OF ANIMAL LIFE IN NEW ZEALAND

New Zealand consists of two islands, together more than 1,000 miles long and of about 200,000 square miles area. It is 1,000 miles distant from New Caledonia, the nearest island of any considerable size, and is 1,500 miles from the great Continental island of Australia. There is no other island in the world so large and at the same time so remote from other considerable tracts of land. Australia is closely connected by island groups at a distance of only 100 miles to Asia.

The isolation of New Zealand is unique. The seas around it are of vast depth and of proportionately great age. During the chalk period--before the great deposits and changes of the earth's face which we a.s.sign to the Tertiary period--New Zealand consisted of a number of small scattered islands, which gradually, as the floor of the sea rose in that part of the world, became a continent stretching northward and joining New Guinea. In that very ancient time the land was covered with ferns and large trees. Birds (as we now know them) had only lately come into existence in the northern hemisphere, and when New Zealand for a time joined that area the birds, as well as a few lizards and one kind of frog, migrated south and colonised the new land. It is probable that the very peculiar lizard-like reptile of New Zealand--the "tuatara" or Sphenodon--entered its area at a still earlier stage of surface change. That creature (only 20 in. long) is the only living representative of very remarkable extinct reptiles which lived in the area which now is England, and, in fact, in all parts of the world, during the Tria.s.sic period, further behind the chalk in date than the chalk is behind our own day. For ages, this "type" with its peculiar beak-like jaws, has survived only in New Zealand. Living specimens have been brought to this country, and are to be seen at the Zoological Gardens in Regent's Park. Having received, as it were, a small cargo of birds and reptiles, but no hairy, warm-blooded quadruped, no mammal, New Zealand became at the end of the chalk-period detached from the northern continent, and isolated, and has remained so ever since. Migratory birds from the north visited it, and at a late date two kinds of bat reached it and established themselves.

Thus we are prepared for the very curious state of things in this large tract of land. Looking at New Zealand as it was a thousand years ago, we find there were no mammals living on it excepting a couple of bats and the seals (so-called sea lions, sea elephants, and others) which frequent its coasts. There were 180 species of birds, and many of these quite peculiar to the island. Many of the birds showed in the absence of any predatory enemies--there being no carnivorous quadrupeds to hunt them or their young--a tendency to lose the power of flight, and some had done so altogether. The gigantic, wingless Moas--allied to the ostrich and the ca.s.sawary--had grown up there, and were the masters of the situation. There were many species of these--one of great height--one fourth taller than the biggest known ostrich; others with short legs of monstrous thickness and strength.

Allied to these are the four species of Kiwi or apteryx, still existing there. They are very strange wingless birds, about the size of a large Dorking fowl. The Kiwis are still in existence, but the Moas and some of the other flightless birds have died out since the arrival of the Maori man, who killed and ate them.