history of its origin and development, and in order to understand this
curious reversal of the ordinary rule in the cactus tribe we must look
at the circumstances under which the race was evolved in the howling
waste of American deserts. (All deserts have a prescriptive right to
howl, and I wouldn't for worlds deprive them of the privilege.) Some
familiar analogies will help us to see the utility of this arrangement.
Everybody knows our common English stone-crops--or if he doesn't he
ought to, for they are pretty and ubiquitous. Now stone-crops grow for
the most part in chinks of the rock or thirsty sandy soil; they are
essentially plants of very dry positions. Hence they have thick and
succulent little stems and leaves, which merge into one another by
imperceptible gradations. All parts of the plant alike are stumpy,
green, and cylindrical. If you squash them with your finger and thumb
you find that though the outer skin or epidermis is thick and firm, the
inside is sticky, moist, and jelly-like. The reason for all this is
plain; the stone-crops drink greedily by their roots whenever they get
a chance, and store up the water so obtained to keep them from
withering under the hot and pitiless sun that beats down upon them for
hours in the baked clefts of their granite matrix. It's the camel trick
over again. So leaves and stem grow thick and round and juicy within;
but outside they are enclosed in a stout layer of epidermis, which
consists of empty glassy cells, and which can be peeled off or flayed
with a knife like the skin of an animal. This outer layer prevents
evaporation, and is a marked feature of all succulent plants which grow
exposed to the sun on arid rocks or in sandy deserts.
The tendency to produce rounded stems and leaves, little
distinguishable from one another, is equally noticeable in many seaside
plants which frequent the strip of thirsty sand beyond the reach of the
tides. That belt of dry beach that stretches between high-water mark
and the zone of vegetable mould, is to all intents and purpose a
miniature desert. True, it is watered by rain from time to time; but
the drops sink in so fast that in half an hour, as we know, the entire
strip is as dry as Sahara again. Now there are many shore weeds of this
intermediate sand-belt which mimic to a surprising degree the chief
external features of the cactuses. One such weed, the common
salicornia, which grows in sandy bottoms or hollows of the beach, has a
jointed stem, branched and succulent, after the true cactus pattern,
and entirely without leaves or their equivalents in any way. Still more
cactus-like in general effect is another familiar English seaside weed,
the kali or glasswort, so called because it was formerly burnt to
extract the soda. The glasswort has leaves, it is true, but they are
thick and fleshy, continuous with the stem, and each one terminating in
a sharp, needle-like spine, which effectually protects the weed against
all browsing aggressors.
Now, wherever you get very dry and sandy conditions of soil, you get
this same type of cactus-like vegetation--_plantes grasses_, as the
French well call them. The species which exhibit it are not necessary
related to one another in any way; often they belong to most widely
distinct families; it is an adaptive resemblance alone, due to
similarity of external circumstances only. The plants have to fight
against the same difficulties, and they adopt for the most part the
same tactics to fight them with. In other words, any plant of whatever
family, which wishes to thrive in desert conditions, must almost, as a
matter of course, become thick and succulent, so as to store up water,
and must be protected by a stout epidermis to prevent its evaporation
under the fierce heat of the sunlight. They do not necessarily lose
their leaves in the process; but the jointed stem usually answers the
purpose of leaves under such conditions far better than any thin and
exposed blade could do in the arid air of a baking desert. And
therefore, as a rule, desert plants are leafless.
In India, for example, there are no cactuses. But I wouldn't advise you
to dispute the point with a peppery, fire-eating Anglo-Indian colonel.
I did so once, myself, at the risk of my life, at a _table d'hôte_ on
the Continent; and the wonder is that I'm still alive to tell the
story. I had nothing but facts on my side, while the colonel had fists,
and probably pistols. And when I say no cactuses, I mean, of course, no
indigenous species; for prickly pears and epiphyllums may naturally be
planted by the hand of man anywhere. But what people take for thickets
of cactus in the Indian jungle are really thickets of cactus-like
spurges. In the dry soil of India, many spurges grow thick and
succulent, learn to suppress their leaves, and assume the bizarre forms
and quaint jointed appearance of the true cactuses. In flower and
fruit, however, they are euphorbias to the end; it is only in the thick
and fleshy stem that they resemble their nobler and more beautiful
Western rivals. No true cactus grows truly wild anywhere on earth
except in America. The family was developed there, and, till man
transplanted it, never succeeded in gaining a foothold elsewhere.
Essentially tropical in type, it was provided with no means of
dispersing its seeds across the enormous expanse of intervening ocean
which separated its habitat from the sister continents.
But why are cactuses so almost universally prickly? From the grotesque
little melon-cactuses of our English hothouses to the huge and ungainly
monsters which form miles of hedgerows on Jamaican hillsides, the
members of this desert family are mostly distinguished by their
abundant spines and thorns, or by the irritating hairs which break off
in your skin if you happen to brush incautiously against them. Cactuses
are the hedgehogs of the vegetable world; their motto is _Nemo me
impune lacessit_. Many a time in the West Indies I have pushed my hand
for a second into a bit of tangled 'bush,' as the negroes call it, to
seize some rare flower or some beautiful insect, and been punished for
twenty-four hours afterwards by the stings of the almost invisible and
glass-like little cactus-needles. When you rub them they only break in
pieces, and every piece inflicts a fresh wound on the flesh where it
rankles. Some of the species have large, stout prickles; some have
clusters of irritating hairs at measured distances; and some rejoice in
both means of defence at once, scattered impartially over their entire
surface. In the prickly pear, the bundles of prickles are arranged
geometrically with great regularity in a perfect quincunx. But that is
a small consolation indeed to the reflective mind when you've stung
yourself badly with them.
The reason for this bellicose disposition on the part of the cactuses
is a tolerably easy one to guess. Fodder is rare in the desert. The
starving herbivores that find themselves from time to time belated on
the confines of such thirsty regions would seize with avidity upon any
succulent plant which offered them food and drink at once in their last
extremity. Fancy the joy with which a lost caravan, dying of hunger and
thirst in the byways of Sahara, would hail a great bed of melons,
cucumbers, and lettuces! Needless to say, however, under such
circumstances melon, cucumber, and lettuce would soon be exterminated:
they would be promptly eaten up at discretion without leaving a
descendant to represent them in the second generation. In the ceaseless
war between herbivore and plant, which is waged every day and all day
long the whole world over with far greater persistence than the war
between carnivore and prey, only those species of plant can survive in
such exposed situations which happen to develop spines, thorns, or
prickles as a means of defence against the mouths of hungry and
desperate assailants.
Nor is this so difficult a bit of evolution as it looks at first sight.
Almost all plants are more or less covered with hairs, and it needs but
a slight thickening at the base, a slight woody deposit at the point,
to turn them forthwith into the stout prickles of the rose or the
bramble. Most leaves are more or less pointed at the end or at the
summits of the lobes; and it needs but a slight intensification of this
pointed tendency to produce forthwith the sharp defensive foliage of
gorse, thistles, and holly. Often one can see all the intermediate
stages still surviving under one's very eyes. The thistles, themselves,
for example, vary from soft and unarmed species which haunt
out-of-the-way spots beyond the reach of browsing herbivores, to such
trebly-mailed types as that enemy of the agricultural interest, the
creeping thistle, in which the leaves continue themselves as prickly
wings down every side of the stem, so that the whole plant is amply
clad from head to foot in a defensive coat of fierce and bristling
spearheads. There is a common little English meadow weed, the
rest-harrow, which in rich and uncropped fields produces no defensive
armour of any sort; but on the much-browsed-over suburban commons and
in similar exposed spots, where only gorse and blackthorn stand a
chance for their lives against the cows and donkeys, it has developed a
protected variety in which some of the branches grow abortive, and end
abruptly in stout spines like a hawthorn's. Only those rest-harrows
have there survived in the sharp struggle for existence which happened
most to baffle their relentless pursuers.
Desert plants naturally carry this tendency to its highest point of
development. Nowhere else is the struggle for life so fierce; nowhere
else is the enemy so goaded by hunger and thirst to desperate measures.
It is a place for internecine warfare Hence, all desert plants are
quite absurdly prickly. The starving herbivores will attack and devour
under such circumstances even thorny weeds, which tear or sting their
tender tongues and palates, but which supply them at least with a
little food and moisture: so the plants are compelled in turn to take
almost extravagant precautions. Sometimes the leaves end in a stout
dagger-like point, as with the agave, or so-called American aloe;
sometimes they are reduced to mere prickles or bundles of needle-like
spikes; sometimes they are suppressed altogether, and the work of
defence is undertaken in their stead by irritating hairs intermixed
with caltrops of spines pointing outward from a common centre in every
direction. When one remembers how delicately sensitive are the tender
noses of most browsing herbivores, one can realize what an excellent
mode of defence these irritating hairs must naturally constitute. I
have seen cows in Jamaica almost maddened by their stings, and even
savage bulls will think twice in their rage before they attempt to make
their way through the serried spears of a dense cactus hedge. To put it
briefly, plants have survived under very arid or sandy conditions
precisely in proportion as they displayed this tendency towards the
production of thorns, spines, bristles, and prickles.
It is a marked characteristic of the cactus tribe to be very tenacious
of life, and when hacked to pieces, to spring afresh in full vigour
from every scrap or fragment. True vegetable hydras, when you cut down
one, ten spring in its place: every separate morsel of the thick and
succulent stem has the power of growing anew into a separate cactus.
Surprising as this peculiarity seems at first sight, it is only a
special desert modification of a faculty possessed in a less degree by
almost all plants and by many animals. If you cut off the end of a rose
branch and stick it in the ground under suitable conditions, it grows
into a rose tree. If you take cuttings of scarlet geraniums or common
verbenas, and pot them in moist soil, they bud out apace into new
plants like their parents. Certain special types can even be propagated
from fragments of the leaf; for example, there is a particularly
vivacious begonia off which you may snap a corner of one blade, and
hang it up by a string from a peg or the ceiling, when, hi, presto!
little begonia plants begin to bud out incontinently on every side from
its edges. A certain German professor went even further than that; he
chopped up a liverwort very fine into vegetable mincemeat, which he
then spread thin over a saucerful of moist sand, and lo! in a few days
the whole surface of the mess was covered with a perfect forest of
sprouting little liverworts. Roughly speaking, one may say that every
fragment of every organism has in it the power to rebuild in its
entirety another organism like the one of which it once formed a
component element.
Similarly with animals. Cut off a lizard's tail, and straightway a new
tail grows in its place with surprising promptitude. Cut off a
lobster's claw, and in a very few weeks that lobster is walking about
airily on his native rocks, with two claws as usual. True, in these
cases the tail and the claw don't bud out in turn into a new lizard or
a new lobster. But that is a penalty the higher organisms have to pay
for their extreme complexity. They have lost that plasticity, that
freedom of growth, which characterizes the simpler and more primitive
forms of life; in their case the power of producing fresh organisms
entire from a single fragment, once diffused equally over the whole
body, is now confined to certain specialized cells which, in their
developed form, we know as seeds or eggs. Yet, even among animals, at a
low stage of development, this original power of reproducing the whole
from a single part remains inherent in the organism; for you may chop
up a fresh-water hydra into a hundred little bits, and every bit will
be capable of growing afresh into a complete hydra.
Now, desert plants would naturally retain this primitive tendency in a
very high degree; for they are specially organized to resist
drought--being the survivors of generations of drought-proof
ancestors--and, like the camel, they have often to struggle on through
long periods of time without a drop of water. Exactly the same thing
happens at home to many of our pretty little European stone-crops. I
have a rockery near my house overgrown with the little white sedum of
our gardens. The birds often peck off a tiny leaf or branch; it drops
on the dry soil, and remains there for days without giving a sign of
life. But its thick epidermis effectually saves it from withering; and
as soon as rain falls, wee white rootlets sprout out from the under
side of the fragment as it lies, and it grows before long into a fresh
small sedum plant. Thus, what seem like destructive agencies
themselves, are turned in the end by mere tenacity of life into a
secondary means of propagation.
That is why the prickly pear is so common in all countries where the
climate suits it, and where it has once managed to gain a foothold. The
more you cut it down, the thicker it springs; each murdered bit becomes
the parent in due time of a numerous offspring. Man, however, with his
usual ingenuity, has managed to best the plant, on this its own ground,
and turn it into a useful fodder for his beasts of burden. The prickly
pear is planted abundantly on bare rocks in Algeria, where nothing else
would grow, and is cut down when adult, divested of its thorns by a
rough process of hacking, and used as food for camels and cattle. It
thus provides fresh moist fodder in the African summer when the grass
is dried up and all other pasture crops have failed entirely.
The flowers of the prickly pear, as of many other cactuses, grow
apparently on the edge of the leaves, which alone might give the
observant mind a hint as to the true nature of those thick and
flattened expansions. For whenever what look like leaves bear flowers
or fruit on their edge or midrib, as in the familiar instance of
butcher's broom, you may be sure at a glance they are really branches
in disguise masquerading as foliage. The blossoms in the prickly pear
are large, handsome, and yellow; at least, they would be handsome if
one could ever see them, but they are generally covered so thick in
dust that it is difficult properly to appreciate their beauty. They
have a great many petals in numerous rows, and a great many stamens in
a rosette in the centre; and, to the best of my knowledge and belief,
as lawyers put it, they are fertilized for the most part by tropical
butterflies; but on this point, having observed them but little in
their native habitats, I speak under correction.
The fruit itself, to which the plant owes its popular name, is
botanically a berry, though a very big one, and it exhibits in a highly
specialized degree the general tactics of all its family. As far as
their leaf-like stems go, the main object in life of the cactuses
is--not to get eaten. But when it comes to the fruit, this object in
life is exactly reversed; the plant desires its fruit to be devoured by
some friendly bird or adapted animal, in order that the hard little
seeds buried in the pulp within may be dispersed for germination under
suitable conditions. At the same time, true to its central idea, it
covers even the pear itself with deterrent and prickly hairs, meant to
act as a defence against useless thieves or petty depredators, who
would eat the soft pulp on the plant as it stands (much as wasps do
peaches) without benefiting the species in return by dispersing its
seedlings. This practice is fully in accordance with the general habit
of tropical or sub-tropical fruits, which lay themselves out to deserve
the kind offices of monkeys, parrots, toucans, hornbills, and other
such large and powerful fruit-feeders. Fruits which arrange themselves
for a _clientèle_, of this character have usually thick or nauseous
rinds, prickly husks, or other deterrent integuments; but they are full
within of juicy pulp, embedding stony or nutlike seeds, which pass
undigested through the gizzards of their swallowers.
For a similar reason, the actual prickly pears themselves are
attractively coloured. I need hardly point out, I suppose, at the
present time of day, that such tints in the vegetable world act like
the gaudy posters of our London advertisers. Fruits and flowers which
desire to attract the attention of beasts, birds, or insects, are
tricked out in flaunting hues of crimson, purple, blue, and yellow;
fruits and flowers which could only be injured by the notice of animals
are small and green, or dingy and inconspicuous.
PRETTY POLL.
It is an error of youth to despise parrots for their much talking.
Loquacity isn't always a sign of empty-headedness, nor is silence a
sure proof of weight and wisdom. Biologists, for their part, know
better than that. By common consent, they rank the parrot group as the
very head and crown of bird creation. Not, of course, because pretty
Poll can talk (in a state of nature, parrots only chatter somewhat
meaninglessly to one another), but because the group display on the
whole, all round, a greater amount of intelligence, of cleverness, and
of adaptability to circumstances than any other birds, including even
their cunning and secretive rivals, the ravens, the jackdaws, the
crows, and the magpies.
What are the efficient causes of this exceptionally high intelligence
in parrots? Well, Mr. Herbert Spencer, I believe, was the first to
point out the intimate connection that exists throughout the animal
world between mental development and the power of grasping an object
all round so as to know exactly its shape and its tactile properties.
The possession of an effective prehensile organ--a hand or its
equivalent--seems to be the first great requisite for the evolution of
a high order of intellect. Man and the monkeys, for example, have a
pair of hands; and in their case one can see at a glance how dependent
is their intelligence upon these grasping organs. All human arts base
themselves ultimately upon the human hand; and even the apes approach
nearest to humanity in virtue of their ever-active and busy little
fingers. The elephant, again, has his flexible trunk, which, as we have
all heard over and over again, _usque ad nauseam_, is equally well
adapted to pick up a pin or to break the great boughs of tropical
forest trees. (That pin, in particular, is now a well-worn classic.)
The squirrel, once more, celebrated for his unusual intelligence when
judged by a rodent standard, uses his pretty little paws as veritable
hands, by which he can grasp a nut or fruit all round, and so gain in
his small mind a clear conception of its true shape and properties.
Throughout the animal kingdom generally, indeed, this correspondence,
or rather this chain of causation, makes itself everywhere felt; no
high intelligence without a highly developed prehensile and grasping
organ.
Perhaps the opossum is the very best and most crucial instance that
could possibly be adduced of the intimate connection which exists
between touch and intellect. For the opossum is a marsupial; it belongs
to the same group of lowly-organized, antiquated, and pouch-bearing
animals as the kangaroo, the wombat, and the other belated Australian
mammals. Now everybody knows the marsupials as a class are nothing
short of preternaturally stupid. They are just about the very dullest
and silliest of all existing quadrupeds. And this is reasonable enough,
when one comes to think of it, for they represent a very antique and
early type, the first rough sketch of the mammalian idea, if I may so
describe them, with wits unsharpened as yet by contact with the world
in the fierce competition of the struggle for life as it displays
itself on the crowded stage of the great continents. They stand, in
short, to the lions and tigers, the elephants and horses, the monkeys
and squirrels, of Europe and America, as the Australian blackfellow
stands to the Englishman or the Yankee. They are the last relic of the
original secondary quadrupeds, stranded for ages in a remote southern
island, and still keeping up among Australian forests the antique type
of life that went out of fashion in Europe, Asia, and America before
the chalk was laid down or the London Clay deposited on the bed of our
northern oceans. Hence they have still very narrow brains, and are so
extremely stupid that a kangaroo, it is said--though I don't vouch for
it myself--when struck a smart blow, will turn and bite the stick that
hurts him instead of expending his anger on the hand that holds it.