Kitabı oku: «The Evolutionist at Large», sayfa 3
V.
SLUGS AND SNAILS
Hoeing among the flower-beds on my lawn this morning – for I am a bit of a gardener in my way – I have had the ill-luck to maim a poor yellow slug, who had hidden himself among the encroaching grass on the edge of my little parterre of sky-blue lobelias. This unavoidable wounding and hacking of worms and insects, despite all one's care, is no small drawback to the pleasures of gardening in propriâ personâ. Vivisection for genuine scientific purposes in responsible hands, one can understand and tolerate, even though lacking the heart for it oneself; but the useless and causeless vivisection which cannot be prevented in every ordinary piece of farm-work seems a gratuitous blot upon the face of beneficent nature. My only consolation lies in the half-formed belief that feeling among these lower creatures is indefinite, and that pain appears to affect them far less acutely than it affects warm-blooded animals. Their nerves are so rudely distributed in loose knots all over the body, instead of being closely bound together into a single central system as with ourselves, that they can scarcely possess a consciousness of pain at all analogous to our own. A wasp whose head has been severed from its body and stuck upon a pin, will still greedily suck up honey with its throatless mouth; while an Italian mantis, similarly treated, will calmly continue to hunt and dart at midges with its decapitated trunk and limbs, quite forgetful of the fact that it has got no mandibles left to eat them with. These peculiarities lead one to hope that insects may feel pain less than we fear. Yet I dare scarcely utter the hope, lest it should lead any thoughtless hearer to act upon the very questionable belief, as they say even the amiable enthusiasts of Port Royal acted upon the doctrine that animals were mere unconscious automata, by pushing their theory to the too practical length of active cruelty. Let us at least give the slugs and beetles the benefit of the doubt. People often say that science makes men unfeeling: for my own part, I fancy it makes them only the more humane, since they are the better able dimly to figure to themselves the pleasures and pains of humbler beings as they really are. The man of science perhaps realises more vividly than all other men the inner life and vague rights even of crawling worms and ugly earwigs.
I will take up this poor slug whose mishap has set me preaching, and put him out of his misery at once, if misery it be. My hoe has cut through the soft flesh of the mantle and hit against the little embedded shell. Very few people know that a slug has a shell, but it has, though quite hidden from view; at least, in this yellow kind – for there are other sorts which have got rid of it altogether. I am not sure that I have wounded the poor thing very seriously; for the shell protects the heart and vital organs, and the hoe has glanced off on striking it, so that the mantle alone is injured, and that by no means irrecoverably. Snail flesh heals fast, and on the whole I shall be justified, I think, in letting him go. But it is a very curious thing that this slug should have a shell at all! Of course it is by descent a snail, and, indeed, there are very few differences between the two races except in the presence or absence of a house. You may trace a curiously complete set of gradations between the perfect snail and the perfect slug in this respect; for all the intermediate forms still survive with only an almost imperceptible gap between each species and the next. Some kinds, like the common brown garden snail, have comparatively small bodies and big shells, so that they can retire comfortably within them when attacked; and if they only had a lid or door to their houses they could shut themselves up hermetically, as periwinkles and similar mollusks actually do. Other kinds, like the pretty golden amber-snails which frequent marshy places, have a body much too big for its house, so that they cannot possibly retire within their shells completely. Then come a number of intermediate species, each with progressively smaller and thinner shells, till at length we reach the testacella, which has only a sort of limpet-shaped shield on his tail, so that he is generally recognised as being the first of the slugs rather than the last of the snails. You will not find a testacella unless you particularly look for him, for he seldom comes above ground, being a most bloodthirsty subterraneous carnivore who follows the burrows of earthworms as savagely as a ferret tracks those of rabbits; but in all the southern and western counties you may light upon stray specimens if you search carefully in damp places under fallen leaves. Even in testacella, however, the small shell is still external. In this yellow slug here, on the contrary, it does not show itself at all, but is buried under the closely wrinkled skin of the glossy mantle. It has become a mere saucer, with no more symmetry or regularity than an oyster-shell. Among the various kinds of slugs, you may watch this relic or rudiment gradually dwindling further and further towards annihilation; till finally, in the great fat black slugs which appear so plentifully on the roads after summer showers, it is represented only by a few rough calcareous grains, scattered up and down through the mantle; and sometimes even these are wanting. The organs which used to secrete the shell in their remote ancestors have either ceased to work altogether or are reduced to performing a useless office by mere organic routine.
The reason why some mollusks have thus lost their shells is clear enough. Shells are of two kinds, calcareous and horny. Both of them require more or less lime or other mineral matters, though in varying proportions. Now, the snails which thrive best on the bare chalk downs behind my little combe belong to that pretty banded black-and-white sort which everybody must have noticed feeding in abundance on all chalk soils. Indeed, Sussex farmers will tell you that South Down mutton owes its excellence to these fat little mollusks, not to the scanty herbage of their thin pasture-lands. The pretty banded shells in question are almost wholly composed of lime, which the snails can, of course, obtain in any required quantity from the chalk. In most limestone districts you will similarly find that snails with calcareous shells predominate. But if you go into a granite or sandstone tract you will see that horny shells have it all their own way. Now, some snails with such houses took to living in very damp and marshy places, which they were naturally apt to do – as indeed the land-snails in a body are merely pond-snails which have taken to crawling up the leaves of marsh-plants, and have thus gradually acclimatised themselves to a terrestrial existence. We can trace a perfectly regular series from the most aquatic to the most land-loving species, just as I have tried to trace a regular series from the shell-bearing snails to the shell-less slugs. Well, when the earliest common ancestor of both these last-named races first took to living above water, he possessed a horny shell (like that of the amber-snail), which his progenitors used to manufacture from the mineral matters dissolved in their native streams. Some of the younger branches descended from this primæval land-snail took to living on very dry land, and when they reached chalky districts manufactured their shells, on an easy and improved principle, almost entirely out of lime. But others took to living in moist and boggy places, where mineral matter was rare, and where the soil consisted for the most part of decaying vegetable mould. Here they could get little or no lime, and so their shells grew smaller and smaller, in proportion as their habits became more decidedly terrestrial. But to the last, as long as any shell at all remained, it generally covered their hearts and other important organs; because it would there act as a special protection, even after it had ceased to be of any use for the defence of the animal's body as a whole. Exactly in the same way men specially protected their heads and breasts with helmets and cuirasses, before armour was used for the whole body, because these were the places where a wound would be most dangerous; and they continued to cover these vulnerable spots in the same manner even when the use of armour had been generally abandoned. My poor mutilated slug, who is just now crawling off contentedly enough towards the hedge, would have been cut in two outright by my hoe had it not been for that solid calcareous plate of his, which saved his life as surely as any coat of mail.
How does it come, though, that slugs and snails now live together in the self-same districts? Why, because they each live in their own way. Slugs belong by origin to very damp and marshy spots; but in the fierce competition of modern life they spread themselves over comparatively dry places, provided there is long grass to hide in, or stones under which to creep, or juicy herbs like lettuce, among whose leaves are nice moist nooks wherein to lurk during the heat of the day. Moreover, some kinds of slugs are quite as well protected from birds (such as ducks) by their nauseous taste as snails are by their shells. Thus it happens that at present both races may be discovered in many hedges and thickets side by side. But the real home of each is quite different. The truest and most snail-like snails are found in greatest abundance upon high chalk-downs, heathy limestone hills, and other comparatively dry places; while the truest and most slug-like slugs are found in greatest abundance among low water-logged meadows, or under the damp fallen leaves of moist copses. The intermediate kinds inhabit the intermediate places. Yet to the last even the most thorough-going snails retain a final trace of their original water-haunting life, in their universal habit of seeking out the coolest and moistest spots of their respective habitats. The soft-fleshed mollusks are all by nature aquatic animals, and nothing can induce them wholly to forget the old tradition of their marine or fresh-water existence.
VI.
A STUDY OF BONES
On the top of this bleak chalk down, where I am wandering on a dull afternoon, I light upon the blanched skeleton of a crow, which I need not fear to handle, as its bones have been first picked clean by carrion birds, and then finally purified by hungry ants, time, and stormy weather. I pick a piece of it up in my hands, and find that I have got hold of its clumped tail-bone. A strange fragment truly, with a strange history, which I may well spell out as I sit to rest a minute upon the neighbouring stile. For this dry tail-bone consists, as I can see at a glance, of several separate vertebræ, all firmly welded together into a single piece. They must once upon a time have been real disconnected jointed vertebræ, like those of the dog's or lizard's tail; and the way in which they have become fixed fast into a solid mass sheds a world of light upon the true nature and origin of birds, as well as upon many analogous cases elsewhere.
When I say that these bones were once separate, I am indulging in no mere hypothetical Darwinian speculation. I refer, not to the race, but to the particular crow in person. These very pieces themselves, in their embryonic condition, were as distinct as the individual bones of the bird's neck or of our own spines. If you were to examine the chick in the egg you would find them quite divided. But as the young crow grows more and more into the typical bird-pattern, this lizard-like peculiarity fades away, and the separate pieces unite by 'anastomosis' into a single 'coccygean bone,' as the osteologists call it. In all our modern birds, as in this crow, the vertebræ composing the tail-bone are few in number, and are soldered together immovably in the adult form. It was not always so, however, with ancestral birds. The earliest known member of the class – the famous fossil bird of the Solenhofen lithographic stone – retained throughout its whole life a long flexible tail, composed of twenty unwelded vertebræ, each of which bore a single pair of quill-feathers, the predecessors of our modern pigeon's train. There are many other marked reptilian peculiarities in this primitive oolitic bird; and it apparently possessed true teeth in its jaws, as its later cretaceous kinsmen discovered by Professor Marsh undoubtedly did. When we compare side by side those real flying dragons, the Pterodactyls, together with the very birdlike Deinosaurians, on the one hand, and these early toothed and lizard-tailed birds on the other, we can have no reasonable doubt in deciding that our own sparrows and swallows are the remote feathered descendants of an original reptilian or half-reptilian ancestor.
Why modern birds have lost their long flexible tails it is not difficult to see. The tail descends to all higher vertebrates as an heirloom from the fishes, the amphibia, and their other aquatic predecessors. With these it is a necessary organ of locomotion in swimming, and it remains almost equally useful to the lithe and gliding lizard on land. Indeed, the snake is but a lizard who has substituted this wriggling motion for the use of legs altogether; and we can trace a gradual succession from the four-legged true lizards, through snake-like forms with two legs and wholly rudimentary legs, to the absolutely limbless serpents themselves. But to flying birds, on the contrary, a long bony tail is only an inconvenience. All that they need is a little muscular knob for the support of the tail-feathers, which they employ as a rudder in guiding their flight upward or downward, to right or left. The elongated waving tail of the Solenhofen bird, with its single pair of quills, must have been a comparatively ineffectual and clumsy piece of mechanism for steering an aërial creature through its novel domain. Accordingly, the bones soon grew fewer in number and shorter in length, while the feathers simultaneously arranged themselves side by side upon the terminal hump. As early as the time when our chalk was deposited, the bird's tail had become what it is at the present day – a single united bone, consisting of a few scarcely distinguishable crowded rings. This is the form it assumes in the toothed fossil birds of Western America. But, as if to preserve the memory of their reptilian origin, birds in their embryo stage still go on producing separate caudal vertebræ, only to unite them together at a later point of their development into the typical coccygean bone.
Much the same sort of process has taken place in the higher apes, and, as Mr. Darwin would assure us, in man himself. There the long prehensile tail of the monkeys has grown gradually shorter, and, being at last coiled up under the haunches, has finally degenerated into an insignificant and wholly embedded terminal joint. But, indeed, we can find traces of a similar adaptation to circumstances everywhere. Take, for instance, the common English amphibians. The newt passes all its life in the water, and therefore always retains its serviceable tail as a swimming organ. The frog in its tadpole state is also aquatic, and it swims wholly by means of its broad and flat rudder-like appendage. But as its legs bud out and it begins to fit itself for a terrestrial existence, the tail undergoes a rapid atrophy, and finally fades away altogether. To a hopping frog on land, such a long train would be a useless drag, while in the water its webbed feet and muscular legs make a satisfactory substitute for the lost organ. Last of all, the tree-frog, leading a specially terrestrial life, has no tadpole at all, but emerges from the egg in the full frog-like shape. As he never lives in the water, he never feels the need of a tail.
The edible crab and lobster show us an exactly parallel case amongst crustaceans. Everybody has noticed that a crab's body is practically identical with a lobster's, only that in the crab the body-segments are broad and compact, while the tail, so conspicuous in its kinsman, is here relatively small and tucked away unobtrusively behind the legs. This difference in construction depends entirely upon the habits and manners of the two races. The lobster lives among rocks and ledges; he uses his small legs but little for locomotion, but he springs surprisingly fast and far through the water by a single effort of his powerful muscular tail. As to his big fore-claws, those, we all know, are organs of prehension and weapons of offence, not pieces of locomotive mechanism. Hence the edible and muscular part of a lobster is chiefly to be found in the claws and tail, the latter having naturally the firmest and strongest flesh. The crab, on the other hand, lives on the sandy bottom, and walks about on its lesser legs, instead of swimming or darting through the water by blows of its tail, like the lobster or the still more active prawn and shrimp. Hence the crab's tail has dwindled away to a mere useless historical relic, while the most important muscles in its body are those seated in the network of shell just above its locomotive legs. In this case, again, it is clear that the appendage has disappeared because the owner had no further use for it. Indeed, if one looks through all nature, one will find the philosophy of tails eminently simple and utilitarian. Those animals that need them evolve them; those animals that do not need them never develop them; and those animals that have once had them, but no longer use them for practical purposes, retain a mere shrivelled rudiment as a lingering reminiscence of their original habits.
VII.
BLUE MUD
After last night's rain, the cliffs that bound the bay have come out in all their most brilliant colours; so this morning I am turning my steps seaward, and wandering along the great ridge of pebbles which here breaks the force of the Channel waves as they beat against the long line of the Dorset downs. Our cliffs just at this point are composed of blue lias beneath, with a capping of yellow sandstone on their summits, above which in a few places the layer of chalk that once topped the whole country-side has still resisted the slow wear and tear of unnumbered centuries. These three elements give a variety to the bold and broken bluffs which is rare along the monotonous southern escarpment of the English coast. After rain, especially, the changes of colour on their sides are often quite startling in their vividness and intensity. To-day, for example, the yellow sandstone is tinged in parts with a deep russet red, contrasting admirably with the bright green of the fields above and the sombre steel-blue of the lias belt below. Besides, we have had so many landslips along this bit of shore, that the various layers of rock have in more than one place got mixed up with one another into inextricable confusion. The little town nestling in the hollow behind me has long been famous as the head-quarters of early geologists; and not a small proportion of the people earn their livelihood to the present day by 'goin' a fossiling.' Every child about the place recognises ammonites as 'snake-stones;' while even the rarer vertebrae of extinct saurians have acquired a local designation as 'verterberries.' So, whether in search of science or the picturesque, I often clamber down in this direction for my daily stroll, particularly when, as is the case to-day, the rain has had time to trickle through the yellow rock, and the sun then shines full against its face, to light it up with a rich flood of golden splendour.
The base of the cliffs consists entirely of a very soft and plastic blue lias mud. This mud contains large numbers of fossils, chiefly chambered shells, but mixed with not a few relics of the great swimming and flying lizards that swarmed among the shallow flats or low islands of the lias sea. When the blue mud was slowly accumulating in the hollows of the ancient bottom, these huge saurians formed practically the highest race of animals then existing upon earth. There were, it is true, a few primæval kangaroo-mice and wombats among the rank brushwood of the mainland; and there may even have been a species or two of reptilian birds, with murderous-looking teeth and long lizard-like tails – descendants of those problematical creatures which printed their footmarks on the American trias, and ancestors of the later toothed bird whose tail-feathers have been naturally lithographed for us on the Solenhofen slate. But in spite of such rare precursors of higher modern types, the saurian was in fact the real lord of earth in the lias ocean.
For him did his high sun flame, and his river billowing ran,
And he felt himself in his pride to be nature's crowning race.
We have adopted an easy and slovenly way of dividing all rocks into primary, secondary, and tertiary, which veils from us the real chronological relations of evolving life in the different periods. The lias is ranked by geologists among the earliest secondary formations: but if we were to distribute all the sedimentary rocks into ten great epochs, each representing about equal duration in time, the lias would really fall in the tenth and latest of all. So very misleading to the ordinary mind is our accepted geological nomenclature. Nay, even commonplace geologists themselves often overlook the real implications of many facts and figures which they have learned to quote glibly enough in a certain off-hand way. Let me just briefly reconstruct the chief features of this scarcely recognised world's chronology as I sit on this piece of fallen chalk at the foot of the mouldering cliff, where the stream from the meadow above brought down the newest landslip during the hard frosts of last December. First of all, there is the vast lapse of time represented by the Laurentian rocks of Canada. These Laurentian rocks, the oldest in the world, are at least 30,000 feet in thickness, and it must be allowed that it takes a reasonable number of years to accumulate such a mass of solid limestone or clay as that at the bottom of even the widest primæval ocean. In these rocks there are no fossils, except a single very doubtful member of the very lowest animal type. But there are indirect traces of life in the shape of limestone probably derived from shells, and of black lead probably derived from plants. All these early deposits have been terribly twisted and contorted by subsequent convulsions of the earth, and most of them have been melted down by volcanic action; so that we can tell very little about their original state. Thus the history of life opens for us, like most other histories, with a period of uncertainty: its origin is lost in the distant vistas of time. Still, we know that there was such an early period; and from the thickness of the rocks which represent it we may conjecture that it spread over three out of the ten great æons into which I have roughly divided geological time. Next comes the period known as the Cambrian, and to it we may similarly assign about two and a half æons on like grounds. The Cambrian epoch begins with a fair sprinkling of the lower animals and plants, presumably developed during the preceding age; but it shows no remains of fish or any other vertebrates. To the Silurian, Devonian, and Carboniferous periods we may roughly allow an æon and a fraction each: while to the whole group of secondary and tertiary strata, comprising almost all the best-known English formations – red marl, lias, oolite, greensand, chalk, eocene, miocene, pliocene, and drift – we can only give a single æon to be divided between them. Such facts will sufficiently suggest how comparatively modern are all these rocks when viewed by the light of an absolute chronology. Now, the first fishes do not occur till the Silurian – that is to say, in or about the seventh æon after the beginning of geological time. The first mammals are found in the trias, at the beginning of the tenth æon. And the first known bird only makes its appearance in the oolite, about half-way through that latest period. This will show that there was plenty of time for their development in the earlier ages. True, we must reckon the interval between ourselves and the date of this blue mud at many millions of years; but then we must reckon the interval between the lias and the earliest Cambrian strata at some six times as much, and between the lias and the lowest Laurentian beds at nearly ten times as much. Just the same sort of lessening perspective exists in geology as in ordinary history. Most people look upon the age before the Norman Conquest as a mere brief episode of the English annals; yet six whole centuries elapsed between the landing of the real or mythical Hengst at Ebbsfleet and the landing of William the Conqueror at Hastings; while under eight centuries elapsed between the time of William the Conqueror and the accession of Queen Victoria. But, just as most English histories give far more space to the three centuries since Elizabeth than to the eleven centuries which preceded them, so most books on geology give far more space to the single æon (embracing the secondary and tertiary periods) which comes nearest our own time, than to the nine æons which spread from the Laurentian to the Carboniferous epoch. In the earliest period, records either geological or historical are wholly wanting; in the later periods they become both more numerous and more varied in proportion as they approach nearer and nearer to our own time.
So too, in the days when Mr. Darwin first took away the breath of scientific Europe by his startling theories, it used confidently to be said that geology had shown us no intermediate form between species and species. Even at the time when this assertion was originally made it was quite untenable. All early geological forms, of whatever race, belong to what we foolishly call 'generalised' types: that is to say, they present a mixture of features now found separately in several different animals. In other words, they represent early ancestors of all the modern forms, with peculiarities intermediate between those of their more highly differentiated descendants; and hence we ought to call them 'unspecialised' rather than 'generalised' types. For example, the earliest ancestral horse is partly a horse and partly a tapir: we may regard him as a tertium quid, a middle term, from which the horse has varied in one direction and the tapir in another, each of them exaggerating certain special peculiarities of the common ancestor and losing others, in accordance with the circumstances in which they have been placed. Science is now perpetually discovering intermediate forms, many of which compose an unbroken series between the unspecialised ancestral type and the familiar modern creatures. Thus, in this very case of the horse, Professor Marsh has unearthed a long line of fossil animals which lead in direct descent from the extremely unhorse-like eocene type to the developed Arab of our own times. Similarly with birds, Professor Huxley has shown that there is hardly any gap between the very bird-like lizards of the lias and the very lizard-like birds of the oolite. Such links, discovered afresh every day, are perpetual denials to the old parrot-like cry of 'No geological evidence for evolution.'
