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.