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.'
VIII.
_CUCKOO-PINT._
In the bank which supports the hedge, beside this little hanger on
the flank of Black Down, the glossy arrow-headed leaves of the common
arum form at this moment beautiful masses of vivid green foliage.
'Cuckoo-pint' is the pretty poetical old English name for the plant;
but village children know it better by the equally quaint and fanciful
title of 'lords and ladies.' The arum is not now in flower: it
blossomed much earlier in the season, and its queer clustered fruits
are just at present swelling out into rather shapeless little
light-green bulbs, preparatory to assuming the bright coral-red hue
which makes them so conspicuous among the hedgerows during the autumn
months. A cut-and-dry technical botanist would therefore have little to
say to it in its present stage, because he cares only for the flowers
and seeds which help him in his dreary classifications, and give him so
splendid an opportunity for displaying the treasures of his Latinised
terminology. But to me the plant itself is the central point of
interest, not the names (mostly in bad Greek) by which this or that
local orchid-hunter has endeavoured to earn immortality.
This arum, for example, grows first from a small hard seed with a
single lobe or seed-leaf. In the seed there is a little store of starch
and albumen laid up by the mother-plant, on which the young arum feeds,
just as truly as the growing chick feeds on the white which surrounds
its native yolk, or as you and I feed on the similar starches and
albumens laid by for the use of the young plant in the grain of wheat,
or for the young fowl in the egg. Full-grown plants live by taking in
food-stuffs from the air under the influence of sunlight: but a young
seedling can no more feed itself than a human baby can; and so food is
stored up for it beforehand by the parent stock. As the kernel swells
with heat and moisture, its starches and albumens get oxidised and
produce the motions and rearrangements of particles that result in the
growth of a new plant. First a little head rises towards the sunlight
and a little root pushes downward towards the moist soil beneath. The
business of the root is to collect water for the circulating
medium--the sap or blood of the plant--as well as a few mineral matters
required for its stem and cells; but the business of the head is to
spread out into leaves, which are the real mouths and stomachs of the
compound organism. For we must never forget that all plants mainly
grow, not, as most people suppose, from the earth, but from the air.
They are for the most part mere masses of carbon-compounds, and the
carbon in them comes from the carbonic acid diffused through the
atmosphere around, and is separated by the sunlight acting in the
leaves. There it mixes with small quantities of hydrogen and nitrogen
brought by the roots from soil and water; and the starches or other
bodies thus formed are then conveyed by the sap to the places where
they will be required in the economy of the plant system. That is the
all-important fact in vegetable physiology, just as the digestion and
assimilation of food and the circulation of the blood are in our own
bodies.
The arum, like the grain of wheat, has only a single seed-leaf; whereas
the pea, as we all know, has two. This is the most fundamental
difference among flowering plants, as it points back to an early and
deep-seated mode of growth, about which they must have split off from
one another millions of years ago. All the one-lobed plants grow with
stems like grasses or bamboos, formed by single leaves enclosing
another; all the double-lobed plants grow with stems like an oak,
formed of concentric layers from within outward. As soon as the arum,
with its sprouting head, has raised its first leaves far enough above
the ground to reach the sunlight, it begins to form fresh starches and
new leaves for itself, and ceases to be dependent upon the store laid
up in its buried lobe. Most seeds accordingly contain just enough
material to support the young seedling till it is in a position to
shift for itself; and this, of course, varies greatly with the habits
and manners of the particular species. Some plants, too, such as the
potato, find their seeds insufficient to keep up the race by
themselves, and so lay by abundant starches in underground branches or
tubers, for the use of new shoots; and these rich starch receptacles we
ourselves generally utilise as food-stuffs, to the manifest detriment
of the young potato-plants, for whose benefit they were originally
intended. Well, the arum has no such valuable reserve as that; it is
early cast upon its own resources, and so it shifts for itself with
resolution. Its big, glossy leaves grow apace, and soon fill out, not
only with green chlorophyll, but also with a sharp and pungent essence
which makes them burn the mouth like cayenne pepper. This acrid juice
has been acquired by the plant as a defence against its enemies. Some
early ancestor of the arums must have been liable to constant attacks
from rabbits, goats, or other herbivorous animals, and it has adopted
this means of repelling their advances. In other words, those arums
which were most palatable to the rabbits got eaten up and destroyed,
while those which were nastiest survived, and handed down their
pungency to future generations. Just in the same way nettles have
acquired their sting and thistles their prickles, which efficiently
protect them against all herbivores except the patient, hungry donkey,
who gratefully accepts them as a sort of _sauce piquante_ to the
succulent stems.
And now the arum begins its great preparations for the act of
flowering. Everybody knows the general shape of the arum blossom--if
not in our own purple cuckoo-pint, at least in the big white 'Æthiopian
lilies' which form such frequent ornaments of cottage windows. Clearly,
this is a flower which the plant cannot produce without laying up a
good stock of material beforehand. So it sets to work accumulating
starch in its root. This starch it manufactures in its leaves, and then
buries deep underground in a tuber, by means of the sap, so as to
secure it from the attacks of rodents, who too frequently appropriate
to themselves the food intended by plants for other purposes. If you
examine the tuber before the arum has blossomed, you will find it large
and solid; but if you dig it up in the autumn after the seeds have
ripened, you will see that it is flaccid and drained; all its starches
and other contents have gone to make up the flower, the fruit, and the
stalk which bore them. But the tuber has a further protection against
enemies besides its deep underground position. It contains an acrid
juice like that of the leaves, which sufficiently guards it against
four-footed depredators. Man, however, that most persistent of
persecutors, has found out a way to separate the juice from the starch;
and in St. Helena the big white arum is cultivated as a food-plant, and
yields the meal in common use among the inhabitants.
When the arum has laid by enough starch to make a flower it begins to
send up a tall stalk, on the top of which grows the curious hooded
blossom known to be one of the earliest forms still surviving upon
earth. But now its object is to attract, not to repel, the animal
world; for it is an insect-fertilised flower, and it requires the aid
of small flies to carry the pollen from blossom to blossom. For this
purpose it has a purple sheath around its head of flowers and a tall
spike on which they are arranged in two clusters, the male blossoms
above and the female below. This spike is bright yellow in the
cultivated species. The fertilisation is one of the most interesting
episodes in all nature, but it would take too long to describe here in
full. The flies go from one arum to another, attracted by the colour,
in search of pollen; and the pistils, or female flowers, ripen first.
Then the pollen falls from the stamens or male flowers on the bodies of
the flies, and dusts them all over with yellow powder. The insects,
when once they have entered, are imprisoned until the pollen is ready
to drop, by means of several little hairs, pointing downwards, and
preventing their exit on the principle of an eel-trap or lobster-pot.
But as soon as the pollen is discharged the hairs wither away, and then
the flies are free to visit a second arum. Here they carry the
fertilising dust with which they are covered to the ripe pistils, and
so enable them to set their seed; but, instead of getting away again as
soon as they have eaten their fill, they are once more imprisoned by
the lobster-pot hairs, and dusted with a second dose of pollen, which
they carry away in turn to a third blossom.
As soon as the pistils have been impregnated, the fruits begin to set.
Here they are, on their tall spike, whose enclosing sheath has now
withered away, while the top is at this moment slowly dwindling, so
that only the cluster of berries at its base will finally remain. The
berries will swell and grow soft, till in autumn they become a
beautiful scarlet cluster of living coral. Then once more their object
will be to attract the animal world, this time in the shape of
field-mice, squirrels, and small birds; but with a more treacherous
intent. For though the berries are beautiful and palatable enough they
are deadly poison. The robins or small rodents which eat them,
attracted by their bright colours and pleasant taste, not only aid in
dispersing them, but also die after swallowing them, and become huge
manure heaps for the growth of the young plant. So the whole cycle of
arum existence begins afresh, and there is hardly a plant in the field
around me which has not a history as strange as this one.
IX.
_BERRIES AND BERRIES._
This little chine, opening toward the sea through the blue lias cliffs,
has been worn to its present pretty gorge-like depth by the slow action
of its tiny stream--a mere thread of water in fine weather, that
trickles down its centre in a series of mossy cascades to the shingly
beach below. Its sides are overgrown by brambles and other prickly
brushwood, which form in places a matted and impenetrable mass: for it
is the habit of all plants protected by the defensive armour of spines
or thorns to cluster together in serried ranks, through which cattle or
other intrusive animals cannot break. Amongst them, near the down
above, I have just lighted upon a rare plant for Southern Britain--a
wild raspberry-bush in full fruit. Raspberries are common enough in
Scotland among heaps of stones on the windiest hillsides; but the south
of England is too warm and sickly for their robust tastes, and they can
only be found here in a few bleak spots like the stony edges of this
weather-beaten down above the chine. The fruit itself is quite as good
as the garden variety, for cultivation has added little to the native
virtues of the raspberry. Good old Izaak Walton is not ashamed to quote
a certain quaint saying of one Dr. Boteler concerning strawberries, and
so I suppose I need not be afraid to quote it after him. 'Doubtless,'
said the Doctor, 'God _could_ have made a better berry, but doubtless
also God never did.' Nevertheless, if you try the raspberry, picked
fresh, with plenty of good country cream, you must allow that it runs
its sister fruit a neck-and-neck race.
To compare the structure of a raspberry with that of a strawberry is a
very instructive botanical study. It shows how similar causes may
produce the same gross result in singularly different ways. Both are
roses by family, and both have flowers essentially similar to that of
the common dog-rose. But even in plants where the flowers are alike,
the fruits often differ conspicuously, because fresh principles come
into play for the dispersion and safe germination of the seed. This
makes the study of fruits the most complicated part in the unravelling
of plant life. After the strawberry has blossomed, the pulpy receptacle
on which it bore its green fruitlets begins to swell and redden, till
at length it grows into an edible berry, dotted with little yellow
nuts, containing each a single seed. But in the raspberry it is the
separate fruitlets themselves which grow soft and bright-coloured,
while the receptacle remains white and tasteless, forming the 'hull'
which we pull off from the berry when we are going to eat it. Thus the
part of the raspberry which we throw away answers to the part of the
strawberry which we eat. Only, in the raspberry the separate fruitlets
are all crowded close together into a single united mass, while in the
strawberry they are scattered about loosely, and embedded in the soft
flesh of the receptacle. The blackberry is another close relative; but
in its fruit the little pulpy fruitlets cling to the receptacle, so
that we pick and eat them both together; whereas in the raspberry the
receptacle pulls out easily, and leaves a thimble-shaped hollow in the
middle of the berry. Each of these little peculiarities has a special
meaning of its own in the history of the different plants.
Yet the main object attained by all is in the end precisely similar.
Strawberries, raspberries, and blackberries all belong to the class of
attractive fruits. They survive in virtue of the attention paid to them
by birds and small animals. Just as the wild strawberry which I picked
in the hedgerow the other day procures the dispersion of its hard and
indigestible fruitlets by getting them eaten together with the pulpy
receptacle, so does the raspberry procure the dispersion of its soft
and sugary fruitlets by getting them eaten all by themselves. While the
strawberry fruitlets retain throughout their dry outer coating, in
those of the raspberry the external covering becomes fleshy and red,
but the inner seed has, notwithstanding, a still harder shell than the
tiny nuts of the strawberry. Now, this is the secret of nine fruits out
of ten. They are really nuts, which clothe themselves in an outer tunic
of sweet and beautifully coloured pulp. The pulp, as it were, the plant
gives in, as an inducement to the friendly bird to swallow its seed;
but the seed itself it protects by a hard stone or shell, and often by
poisonous or bitter juices within. We see this arrangement very
conspicuously in a plum, or still better in a mango; though it is
really just as evident in the raspberry, where the smaller size renders
it less conspicuous to human sight.
It is a curious fact about the rose family that they have a very marked
tendency to produce such fleshy fruits, instead of the mere dry
seed-vessels of ordinary plants, which are named fruits only by
botanical courtesy. For example, we owe to this single family the
peach, plum, apricot, cherry, damson, pear, apple, medlar, and quince,
all of them cultivated in gardens or orchards for their fruits. The
minor group known by the poetical name of Dryads, alone supplies us
with the strawberry, raspberry, blackberry, and dewberry. Even the
wilder kinds, refused as food by man, produce berries well known to our
winter birds--the haw, rose-hip, sloe, bird-cherry, and rowan. On the
other hand, the whole tribe numbers but a single thoroughgoing nut--the
almond; and even this nut, always somewhat soft-shelled and inclined to
pulpiness, has produced by a 'sport' the wholly fruit-like nectarine.
The odd thing about the rose tribe, however, is this: that the pulpy
tendency shows itself in very different parts among the various
species. In the plum it is the outer covering of the true fruit which
grows soft and coloured: in the apple it is a swollen mass of the
fruit-stalk surrounding the ovules: in the rose-hip it is the hollowed
receptacle: and in the strawberry it is the same receptacle, bulging
out in the opposite direction. Such a general tendency to display
colour and collect sugary juices in so many diverse parts may be
compared to the general bulbous tendency of the tiger-lily or the
onion, and to the general succulent tendency of the cactus or the
house-leek. In each case, the plant benefits by it in one form or
another; and whichever form happens to get the start in any particular
instance is increased and developed by natural selection, just as
favourable varieties of fruits or flowers are increased and developed
in cultivated species by our own gardeners.
Sweet juices and bright colours, however, could be of no use to a plant
till there were eyes to see and tongues to taste them. A pulpy fruit is
in itself a mere waste of productive energy to its mother, unless the
pulpiness aids in the dispersion and promotes the welfare of the young
seedlings. Accordingly, we might naturally expect that there would be
no fruit-bearers on the earth until the time when fruit-eaters, actual
or potential, arrived upon the scene: or, to put it more correctly,
both must inevitably have developed simultaneously and in mutual
dependence upon one another. So we find no traces of succulent fruits
even in so late a formation as that of these lias or cretaceous cliffs.
The birds of that day were fierce-toothed carnivores, devouring the
lizards and saurians of the rank low-lying sea-marshes: the mammals
were mostly primæval kangaroos or low ancestral wombats, gentle
herbivores, or savage marsupial wolves, like the Tasmanian devil of our
own times. It is only in the very modern tertiary period, whose soft
muddy deposits have not yet had time to harden under superincumbent
pressure into solid stone, that we find the earliest traces of the rose
family, the greatest fruit-bearing tribe of our present world. And side
by side with them we find their clever arboreal allies, the ancestral
monkeys and squirrels, the primitive robins, and the yet shadowy
forefathers of our modern fruit-eating parrots. Just as bees and
butterflies necessarily trace back their geological history only to the
time of the first honey-bearing flowers, and just as the honey-bearing
flowers in turn trace back their pedigree only to the date of the
rudest and most unspecialised honey-sucking insects, so are fruits and
fruit-eaters linked together in origin by the inevitable bond of a
mutual dependence. No bee, no honey; and no honey, no bee: so, too, no
fruit, no fruit-bird; and no fruit-bird, no fruit.
X.
_DISTANT RELATIONS._
Behind the old mill, whose overshot wheel, backed by a wall thickly
covered with the young creeping fronds of hart's-tongue ferns, forms
such a picturesque foreground for the view of our little valley, the
mill-stream expands into a small shallow pond, overhung at its edges by
thick-set hazel-bushes and clambering honeysuckle. Of course it is only
dammed back by a mud wall, with sluices for the miller's water-power;
but it has a certain rustic simplicity of its own, which makes it
beautiful to our eyes for all that, in spite of its utilitarian origin.
At the bottom of this shallow pond you may now see a miracle daily
taking place, which but for its commonness we should regard as an
almost incredible marvel. You may there behold evolution actually
illustrating the transformation of life under your very eyes: you may
watch a low type of gill-breathing gristly-boned fish developing into
the highest form of lung-breathing terrestrial amphibian. Nay,
more--you may almost discover the earliest known ancestor of the whole
vertebrate kind, the first cousin of that once famous ascidian larva,
passing through all the upward stages of existence which finally lead
it to assume the shape of a relatively perfect four-legged animal. For
the pond is swarming with fat black tadpoles, which are just at this
moment losing their tails and developing their legs, on the way to
becoming fully formed frogs.