SCIENCE FROM AN EASY CHAIR.

Press, Volume L, Issue 14916, 14 March 1914, Page 9

SCIENCE FROM AN EASY CHAIR.

EARTHWORMS OF THE WORLD

AND THEIit WORK

(Special rights secured by "Tho Press.")

(By Sir Ray Laxkester, X.C.8., F.R.S.)

The notion which used to bo largely prevalent that every animal and plant has somo special "use" in the scheme of Nature, and has been definitely created by Providence in order to perform its part in tho daily work of the world, has been very much modified of late years. Though it is obvious and iudisputable that man is the most elaborate and gifted of tho living creatures on tho face of the earth, and differs from them all in his self-con-sciousness anT mental power, yet we do not now judge of every animal and plant as something (specially created and appointed for mans service. Tho •long process of life on tho globe, and the gradual development of more and moro complex forms, with increased powers and immense diversity of activities, has, it must, bo admitted —so far as things nave at present gone^—culminated in the production of man. He, with all his faults and disabilities, with his injjmite faculty, his admirable form and movement, ins god-like apprehension, is, in Shakespeare's words, "the paragon of animals, the beauty of the world." He is the flower ot all the ages.

The many thousands of living things which served, as it were, as, the phases through which man developed, wero not obliterated as he passed on, but left countless offspring, his remote cousins, whose descendants survive in endless variety at the present day. They are the by-products of tho process which brought him forth, and their activities and capacities, however well they serve him, have not become fitted to him, but on the contrary he has become fitted ,to them. And it is by no means to all his fellow-beings that man has become adapted so as to mvi in t"iem use and service. iSome, on the contrary, ho lias to evade, to destroy, or to control, and though he may, when surveying them in safety, admire their brantv and the evidence of their power, yet ho knows that thoy do not exist for his "benefit," and that he and they must keep apart.

The earthworms would serve well for the text of those who desire to interpret al) living things as having their appointed use for the benefit or man. Unuiie some other worms — parasitic in his flesh or as maggots devouring his stores of food and his woodwork —earthworms aro di oxtraondinary value to man, since they make and distribute the soil which ho has learnt to use for his crops and gardens. Curiously enough, it was Charles Darwin, the destroyer of tho old belief in special creations and tho existence of a designed and special utility or* purpose for the benefit of n.an in every living thing, who first drew attention to the vast operations of earthworms by which man profits.. According to Darwin's view of the gradual evolution of tho kinds of living things by the natural selection of favourable varieties in the strugglo for existenco, no animal or plant can have acquired in a state of nature (that is, in tho absence of man's interference) any character, form, habit, or activity, because* such was beneficial to some other animal or even to human bo:'ngs, but only throush the value of such character, form, habit, or activity to its own species, enabling that species to survive, to mate, and to produco offspring which inherit that character, form, habit, or activity. In the course of generations in which tho natural e'.imination of those failing to present the favoured characteristics inexorably proceeds, those characteristic* are intensified in tho surviving race and further developed. That any form of activity thus established should, prove to be useful to man, is the result of man's subsequent arrival on the scene and his power of examining, trying, and availing himself of an endless diversity of circumstances in the world around him, a power possessed in some degree by -*other organisms, but by none, to any degree' comparable with that shown uy man.

We must, whilst admitting the soundness of this point of view, nevertheless recognise that if we take a large -and comprehensive view of the living things, botli animals and plants, in any extensive area sepamtea by seas or mountain ranges neighbouring regions, there is in that area such an intimate association of all tho living things with one another, such a complete inter-working, intor-dependence, and consequent, adaptation of each plant and each animal to tho activities and results produced by tho others tbat rach is, in a certain sense, "necessary" to the others. Even the predatory and competitive kinds exert a check upon the excessive multiplication or the deterioration of those species of which they destroj' the feebler members. In this ser.ro every kind of living thing mpy correctly bo, said to have, if not its "use.'' at any rate its effective place in maintaining tho balance of nature in the region under examination. In this way Darwin restores to us the conception of a .harmoniously working woiT* of living things, and whilst 'abolishing the old notion that everything in the world was "created" nrA "designed for the use and delight of man, reforms what used to he called "teleology"—the teleology of the older naturalists —and unfolds to our view a world the units of which have become adjusted ( to their associated units—whether hostile. or friendly—continuously and_ progressively by natural selection of favonrab'oVariations and the survival of the fittest in tTTe struggle for existence.

Earthworms of somo hundreds of species, grouped in many diverse, "genera" of like species—yet ail in external appearance wonderfully resembling our common earthworm —aro found in all parts of the globe where the land bears vegetation, and its sands and clays are mixed with the fallen leaves, twigs, and fruits of plants. The earthworms feed on such refuse, and on the fragments o~ small dead animals which are mixed with it. But it is probable that thoy get their food not on'v from such fragments—which they soften with a digestive fluid poured from the mouth before nibblinr, gnawing and sucking them into thfir muscular -gullets. They Uurrow like many water-worms (the lug-worms of our sea shores, for instance) into thc fine mudlike or sandy deposits overlying the harrier rocks of the earth's surface, and to facilitate their boring or burrowinsc they swallow tbe "soil" or "earth" into whi"h they penetrate. Probably in origin this procedure was purely concerned with the raoid making of a bore or tubular retreat»-a safe habitation. B"t owing to the abundance of vegetable and animal particles in the f *Va r th." it is nutritious, and it i* generally held (though it has not been demonstrated by chemical examination) that the swallowing of earth in boring, and the pissage of it through 17ie worm's intestine, is accompanied by a "digestion" of the nutrient organic particles in the earth, and. consequently, that the earth-swal-lowing has become a part of'the "feediucr" of the worm. The reasons for doubting this are that worms, as shown by the coiled "casiings" which they piss from the vent when the hinder end of the body is just below the surface, sw.'Ulow chalk and sand in large quan-

tity when living in ground consisting of either of these materials, and their castings may consist of nearly pure chalk or of pure sand. They do not show any exclusive preference for vegetable mould, though they will burrow in it rather than in hard gritty chalk washings. Another reason is ,that—as Mr Darwin has more especially shown — worms feed on fresh vegetable and animal matters when given the opportunity. They are especially attracted by some Istrong-flavoured vegetables, such as onions, celery, and carrot leaves, and by raw fat, which they prefer to raw meat. Tiiey also eat the dead bodies of other worms. Nevertheless, it is quite possible that they get somo nourishment from the soil wLTch they gorge.

The burrow of a large English earthworm may extend three, four, and even seven feet below the surface, and is a little wider than a worm's body. As it goes in, head foremost" ono may well enquire how it manages to turn round and come out again (as it does) head foremost. Enlargements of the boring aro found, ono or more, afc different depths which aro spacious enough to allow tho worm to turn round. Then it can crawl up again and out of its burrow. New burrows are continually made, and the old ones deserted in moist weather. It is clear that, in order to discharge its castings on to tho surface, when the worm has burrowed by swallowing earth, at a depth of three feet or more, it must also be ablo to crawl backwards up tho tubular boring and bring tho hind-end of its body near tho surface. Tho interior surface of the worm's burrow is kept sn.-ooth and firm by the slimy secretion of the body, and as the worm breathes tho oxygen of the air through the fine blood-vessels of its delicate sßin, it is important that it should not fit tightly to its burrow, but that »ii- should have access to it. Not only does the worm rospire tho air in the loose soil, but it keeps tho mouth of its burrow open, loosely plugged with a leaf drawn in or by a small pebble. These allow the air to pass, and also prevent flooding by rain-water —an accident which might result in the drowning of the worm. Leaves aro dragged by worms into tho burrows for some distance. They aro not only used to close the entrance, but to line the first few inches of it.

The common earthworm is not only liable to bo killed by long submersion in non-aerated water, but its delicate skin is mortally injured by saline solutions and by even very weak acid (no stronger than a drop of acetic acid to half a pint of water;. Any acid, even diluted vinegar, is a terrible wormkiller. Son water is (ienyiy to our common earthworm and all others, so fa>- as known, except one or two special kinds which actually flourish on the seashore amongst seaweed (Pontodnlus o 1 tho Mediterranean coast and Pontosoolex in Jamaica). The former of these two "genera" includes a. phosphorescent species which has been found both in the north of Franco and in Ireland. Many exotic earthworms are reported as being phosphorescent, but the exact nature of ■ the phosphorescence of earthworms has not been studied. It may quite wed bo duo to the production by the woim itself of a phosphorescent slinic (as is the case with many other animals), or it may bo due to the infesting of the worm's surface by phosphorescent bacteria (as in the case of phosphorescent bones and dead fish and diseased sandhoppers), or possibly in some cases a phosphorescent centipede, which exudes a luminous fluid from the base of its legs (ono such centipede I have otten found in English gardens in late summer), may have been attacking the earthworm with a view to eatmgjt, and have smeared it with luminous s..me in tho process!

The fact that sea water is deadly to all the regular soil-inhabiting earthworms makes their wide distribution over tho surface of the eartn or special interest. They cannot have floated in the cracks of sea-driven tree-trunks nor have been carried when young, or in tho egg capsule by the feet of wading hirds across wide seas to distant lands. They would have been killed by sea water. Accordingly, wo find that large continental, tracts of; the earth's surface are marked by being inhabited by numerous special kinds of earthworms which aro not found in other parts of tho world. We can distinguish five such great regions—first, the whole of the northern hemisphere, ]>oth of the Old World and the New ./orld.: second, thp region of tropical Africa; third, that of tropical South and Central America; fourth. India, with the East Indies and Australia; and fifth, the remains of what seems to have been an Antarctic continent, represented by Now Zealand, Kerguelen Island, and some other small islands now inhabited by a peculiar group of earthworms called "Acanthodrilids." These earthworms appear to have passed by a land connexion from thc Antarctic" Tar.* Into South Africa and into South America.

Though enrthworms, obviously enough, cannot exist in earth which is perpetually frozen, yet they occur in cold climates, for instance,'in Iceland and Northern Scandinavia. In very dry regions they aro absent; exposure for a single night in a dry room kiHs an earthworm. But they-manage to escape the deadly action of prolonged dryness and frost when not actually permanent, by burrowing deeply into the earth. Thus, in a long cold or dry season, or Tn an unfavourable locality, they may retreat far into tho earth in order to reach warmer and''moist soil, and in consequence t'»ey escapo notice. This, no doubt, explains the_ astonishing statement made to mc by a correspondent that there are, according to his observation, no earthworms in Canada, or, rather, in such parts of it as he knew, and at those times when he observed.

A remarkable fact in the distribution of earthworms is the unintentional spiending by civilised man of the earthworms of one region into another. Ho does this by carrying plants in pots or with roots embedded in earth from ono port of the world to another. The greenhouses in Kew Gardens harbour many such foreign kinds. Thus, in Australia and New Zealand the common earthworm ,in gardens and wellsettlod regions are those of England, which are thoso of the Northern Hemisphere, whilst to find the native kinds you have to go ont into the Vilds. How far the European earthworms prevail over the native species whejt thus introduced, or how they prevail, if they do, we do not know. Animals and plants 'from a large continental area as a rulo prevail over and crowd out tho natives of a small island area. European earthworms are found now m every part of the globe whore the European man has settled and made his garden. But exotic earthworms have, hero and there, their revenge. About Nice and other parts of the Riviera the East Indian earthworms of the genus Pericha?ta, the peculiarities of which I have described in earlier articles, are very abundant in certain tracts of land. They havo been brouglC there with Oriental plants (oranges, lemons, palms, and other trees), which for ages have been continually introduced and cultivated in this favoured garden of Europe, and they seem to be able to hold their own there.

Tlie effect of earthworms on the soil depends on their -enormous numbers and on their incessantly burrowing in i; as they do so (for they have not the strength to push it aside as the mole does) and evacuating it on the surface in the form of little twisted "castings." often only an inch or less high in this country, but reaching a milch greater eize in the case of the griat tropical earthworms cf 2ft to 4ft Ion?, namely a height of 6in and weigning when dried more than a quarter of a pound. The earthworms thus not only affect the soil by canalisijig it—making burrows in it which admit air and eventuallj' water —but

they continually throw up the deeplying soft earth on to the surface, separating it from stones and coarse fragments. Thoy bring it up often from a depth of 2ft or 3ft. Thus tho deeplying soil, containing much. organic debris aud abundant bacteria is brought ud and exposed to atmospheric oxygen. In these conditions tho oxygen-needing or "aerobic" bacteria, whk-n oxidise nitrogenous organic debris and nitrify it i —that is, convert it into nitrates fitted j to be tho food of green plants —have their chance. The soil is incessantly rendered fit to nourish plants by this activity of earthworms in bringing it to the surface. Man tho agriculturist has supplemented the efforts of his friends and allies the earthworms by inventing and labouring with tho plough. But most of mankind, excepting the inhabitants ot Europe west of Russia, do not plough deep, if at all, and leavo tho job to the earthworms. Ido not know what tho modern scientific agriculturists may havo been doing lately in the matter. But it would certainly bo of great interest to know what is the influence ot the earthworm's digestive canal on tho bacteria and other micro-organisms ot tho soil. So far as Tarn aware, a complete knowledge of the micro-organ-isms in the earthworm's digestive canal does not exist, nor is it known what micro-organisms are present in the earth which tho worm swallows and what are present in the earth which he evacuates; nor, again, what chemical changes, if any, have been effected in the soil in its passage through the worm. Tlio attemnt has been made to estimate tho number of worms in the soil in this part of the world. It, of course, varies according to circumstances, but a first-rate naturalist, Henscn. came to the conclusion that there were about 54.000 worms, weighing m the mass 3561b in an acre of good garaeu ground, and probably about half that number in an acre ot corn held VVe have some of us heard lately wifn jegret of the enormous slaughter of earthworms by a new worm-killing PP"a- retion applied to golf greens and vel\et lawns.'whicrh seem to confirm Hensen* figures Of course, the little "towers o? "casting" of the industrious worms spoil the "green" and the "lawn." and s!> they have to go. But they do n* suffer so much destruction from man as from the mole, who lives on them, and the blackbirds and thrushes, who every moraine in certain seasons of the >ear on all tho lawns throughout the countrj draw out of their holes an astonishing number of earthworms who jncautiouslj are lying with their heads very ' close to tho openings of these retreats, in order .(according to Mr Darwin) to enjoy the warmth of tho sun s rays.

The direct mechanical effect of the burrowing and up-throwing ot soil by earthworms is so vast, owing to their numbers, that, as Darwin showed, it « ,moortant in moulding the face of the land-surface. In many parts of Ensland more than ten tons of dry earth annually oasaes through their bodies and-is brought to the surface, on each aero of laud" so that in a few years tho whole of the superficial vegetable mould passes through their bodies. By the collapsing of their old burrows as well as the throwmg-uu of their castin"*,'' the soil is continually kept in movement, and the coarser narticles axe worn, ground, and dissolved. iho "castings" flow during rainy weather down anY moderate slope, and are washed far, down. When dry the castrings" roll down a sloping surface, and they are driven as dust by prevalent winds in one direction. Thus, the superficial mould, tends to be carried into the small streams in the shallow valleys, and is prevented from accumulating to a great thickness. This favours the disintegration of the underlying rock, which would be protected by a deep deposit of mould. The amount of mould thus annually moved downwards into the beds of valleys, and so ultimately by streams to the ocean, is, it is estimated, one-fifth of an inch in»thickness. In a million years this would lead 'to a lowering oM6,oooft. and a million years is not a long period to the geologist. But the "castings" of earthworms not only result in the removal and spreading' out of the superficial mould; they also protect and preserve every object dropped on tho surface of the laud. Roman pavements and other ancient objects, and even massive walls are undermined by them, and subside and become covered in and protected by tho 'worms' up-throwing of soil. But it is by preparing the soil for plants, the roots of which often make use of their burrows, and by burying seeds which owe their germination to having been thus covered in by castings that tho earthworm's incessant operations are most clearly interwoven with the needs and life T processes of other living things. It may be doubted whether there are any other animals which have played Sv> definite a mechanical part in the* history of the world, as have these lowly organised creatures. The work of tho coral-polyps in constructing innumerable reefs and islands in the great oceans, is, perhaps, more conspicuous. But when we come to tlie enormous chemical operations of the übiquitou* bacteria and to tho incessant liberation of oxygen and the manufacture of the food of animals and of unnumberad valuable products by green plants, oven earthworms and coral-polyps cease to appear of exceptional importance. Such vast series of living things are necessary to one another and inter-do-pondent.