preserving for us in all its native simplicity the primitive aspect of
the vertebrate type. Beasts, birds, reptiles, and fishes have all
descended from an animal whose shape closely resembled that of these
wriggling little black creatures which dart up and down like imps
through the clear water, and raise a cloud of mud above their heads
each time that they bury themselves comfortably in the soft mud of the
bottom. But while the birds and beasts, on the one hand, have gone on
bettering themselves out of all knowledge, and while the ascidian, on
the other hand, in his adult form has dropped back into an obscure and
sedentary life--sans eyes, sans teeth, sans taste, sans everything--the
tadpole alone, at least during its early days, remains true to the
ancestral traditions of the vertebrate family. When first it emerges
from its egg it represents the very most rudimentary animal with a
backbone known to our scientific teachers. It has a big hammer-looking
head, and a set of branching outside gills, and a short distinct body,
and a long semi-transparent tail. Its backbone is a mere gristly
channel, in which lies its spinal cord. As it grows, it resembles in
every particular the ascidian larva, with which, indeed, Kowalewsky and
Professor Ray Lankester have demonstrated its essential identity. But
since a great many people seem wrongly to imagine that Professor
Lankester's opinion on this matter is in some way at variance with Mr.
Darwin's and Dr. Haeckel's, it may be well to consider what the
degeneracy of the ascidian really means. The fact is, both larval
forms--that of the frog and that of the ascidian--completely agree in
the position of their brains, their gill-slits, their very rudimentary
backbones, and their spinal cords. Moreover, we ourselves and the
tadpole agree with the ascidian in a further most important point,
which no invertebrate animal shares with us; and that is that our eyes
grow out of our brains, instead of being part of our skin, as in
insects and cuttle-fish. This would seem _à priori_ a most inconvenient
place for an eye--inside the brain; but then, as Professor Lankester
cleverly suggests, our common original ancestor, the very earliest
vertebrate of all, must have been a transparent creature, and therefore
comparatively indifferent as to the part of his body in which his eye
happened to be placed. In after ages, however, as vertebrates generally
got to have thicker skulls and tougher skins, the eye-bearing part of
the brain had to grow outward, and so reach the light on the surface of
the body: a thing which actually happens to all birds, beasts, and
reptiles in the course of their embryonic development. So that in this
respect the ascidian larva is nearer to the original type than the
tadpole or any other existing animal.
The ascidian, however, in mature life, has grown degraded and fallen
from his high estate, owing to his bad habit of rooting himself to a
rock and there settling down into a mere sedentary swallower of passing
morsels--a blind, handless, footless, and degenerate thing. In his
later shape he is but a sack fixed to a stone, and with all his limbs
and higher sense-organs so completely atrophied that only his earlier
history allows us to recognise him as a vertebrate by descent at all.
He is in fact a representative of retrogressive development. The
tadpole, on the contrary, goes on swimming about freely, and keeping
the use of its eyes, till at last a pair of hind legs and then a pair
of fore legs begin to bud out from its side, and its tail fades away,
and its gills disappear, and air-breathing lungs take their place, and
it boldly hops on shore a fully evolved tailless amphibian.
There is, however, one interesting question about these two larvæ which
I should much like to solve. The ascidian has only _one_ eye inside its
useless brain, while the tadpole and all other vertebrates have _two_
from the very first. Now which of us most nearly represents the old
mud-loving vertebrate ancestor in this respect? Have two original
organs coalesced in the young ascidian, or has one organ split up into
a couple with the rest of the class? I think the latter is the true
supposition, and for this reason: In our heads, and those of all
vertebrates, there is a curious cross-connection between the eyes and
the brain, so that the right optic nerve goes to the left side of the
brain and the left optic nerve goes to the right side. In higher
animals, this 'decussation,' as anatomists call it, affects all the
sense-organs except those of smell; but in fishes it only affects the
eyes. Now, as the young ascidian has retained the ancestral position of
his almost useless eye so steadily, it is reasonable to suppose that he
has retained its other peculiarities as well. May we not conclude,
therefore, that the primitive vertebrate had only one brain-eye; but
that afterwards, as this brain-eye grew outward to the surface, it
split up into two, because of the elongated and flattened form of the
head in swimming animals, while its two halves still kept up a memory
of their former union in the cross-connection with the opposite halves
of the brain? If this be so, then we might suppose that the other
organs followed suit, so as to prevent confusion in the brain between
the two sides of the body; while the nose, which stands in the centre
of the face, was under no liability to such error, and therefore still
keeps up its primitive direct arrangement.
It is worth noting, too, that these tadpoles, like all other very low
vertebrates, are mud-haunters; and the most primitive among adult
vertebrates are still cartilaginous mud-fish. Not much is known
geologically about the predecessors of frogs; the tailless amphibians
are late arrivals upon earth, and it may seem curious, therefore, that
they should recall in so many ways the earliest ancestral type. The
reason doubtless is because they are so much given to larval
development. Some ancestors of theirs--primæval newts or
salamanders--must have gone on for countless centuries improving
themselves in their adult shape from age to age, yet bringing all their
young into the world from the egg, as mere mud-fish still, in much the
same state as their unimproved forefathers had done millions of æons
before. Similarly, caterpillars are still all but exact patterns of the
primæval insect, while butterflies are totally different and far higher
creatures. Thus, in spite of adult degeneracy in the ascidian and adult
progress in the frog, both tadpoles preserve for us very nearly the
original form of their earliest backboned ancestor. Each individual
recapitulates in its own person the whole history of evolution in its
race. This is a very lucky thing for biology; since without these
recapitulatory phases we could never have traced the true lines of
descent in many cases. It would be a real misfortune for science if
every frog had been born a typical amphibian, as some tree-toads
actually are, and if every insect had emerged a fully formed adult, as
some aphides very nearly do. Larvæ and embryos show us the original
types of each race; adults show us the total amount of change produced
by progressive or retrogressive development.
XI.
_AMONG THE HEATHER._
This is the worst year for butterflies that I can remember.
Entomologists all over England are in despair at the total failure of
the insect crop, and have taken to botanising, angling, and other bad
habits, in default of means for pursuing their natural avocation as
beetle-stickers. Last year's heavy rains killed all the mothers as they
emerged from the chrysalis; and so only a few stray eggs have survived
till this summer, when the butterflies they produce will all be needed
to keep up next season's supply. Nevertheless, I have climbed the
highest down in this part of the country to-day, and come out for an
airing among the heather, in the vague hope that I may be lucky enough
to catch a glimpse of one or two old lepidopterous favourites. I am not
a butterfly-hunter myself. I have not the heart to drive pins through
the pretty creatures' downy bodies, or to stifle them with reeking
chemicals; though I recognise the necessity for a hardened class who
will perform that useful office on behalf of science and society, just
as I recognise the necessity for slaughtermen and knackers. But I
prefer personally to lie on the ground at my ease and learn as much
about the insect nature as I can discover from simple inspection of the
living subject as it flits airily from bunch to bunch of
bright-coloured flowers.
I suppose even that apocryphal person, the general reader, would be
insulted at being told at this hour of the day that all bright-coloured
flowers are fertilised by the visits of insects, whose attentions they
are specially designed to solicit. Everybody has heard over and over
again that roses, orchids, and columbines have acquired their honey to
allure the friendly bee, their gaudy petals to advertise the honey, and
their divers shapes to ensure the proper fertilisation by the correct
type of insect. But everybody does not know how specifically certain
blossoms have laid themselves out for a particular species of fly,
beetle, or tiny moth. Here on the higher downs, for instance, most
flowers are exceptionally large and brilliant; while all Alpine
climbers must have noticed that the most gorgeous masses of bloom in
Switzerland occur just below the snow-line. The reason is, that such
blossoms must be fertilised by butterflies alone. Bees, their great
rivals in honey-sucking, frequent only the lower meadows and slopes,
where flowers are many and small: they seldom venture far from the hive
or the nest among the high peaks and chilly nooks where we find those
great patches of blue gentian or purple anemone, which hang like
monstrous breadths of tapestry upon the mountain sides. This heather
here, now fully opening in the warmer sun of the southern counties--it
is still but in the bud among the Scotch hills, I doubt not--specially
lays itself out for the bumblebee, and its masses form about his
highest pasture-grounds; but the butterflies--insect vagrants that they
are--have no fixed home, and they therefore stray far above the level
at which bee-blossoms altogether cease to grow. Now, the butterfly
differs greatly from the bee in his mode of honey-hunting; he does not
bustle about in a business-like manner from one buttercup or
dead-nettle to its nearest fellow; but he flits joyously, like a
sauntering straggler that he is, from a great patch of colour here to
another great patch at a distance, whose gleam happens to strike his
roving eye by its size and brilliancy. Hence, as that indefatigable
observer, Dr. Hermann Müller, has noticed, all Alpine or hill-top
flowers have very large and conspicuous blossoms, generally grouped
together in big clusters so as to catch a passing glance of the
butterfly's eye. As soon as the insect spies such a cluster, the colour
seems to act as a stimulant to his broad wings, just as the
candle-light does to those of his cousin the moth. Off he sails at
once, as if by automatic action, towards the distant patch, and there
both robs the plant of its honey and at the same time carries to it on
his legs and head fertilising pollen from the last of its congeners
which he favoured with a call. For of course both bees and butterflies
stick on the whole to a single species at a time; or else the flowers
would only get uselessly hybridised instead of being impregnated with
pollen from other plants of their own kind. For this purpose it is that
most plants lay themselves out to secure the attention of only two or
three varieties among their insect allies, while they make their
nectaries either too deep or too shallow for the convenience of all
other kinds. Nature, though eager for cross-fertilisation, abhors
'miscegenation' with all the bitterness of an American politician.
Insects, however, differ much from one another in their æsthetic
tastes, and flowers are adapted accordingly to the varying fancies of
the different kinds. Here, for example, is a spray of common white
galium, which attracts and is fertilised by small flies, who generally
frequent white blossoms. But here, again, not far off, I find a
luxuriant mass of the yellow species, known by the quaint name of
'lady's bedstraw'--a legacy from the old legend which represents it as
having formed Our Lady's bed in the manger at Bethlehem. Now why has
this kind of galium yellow flowers, while its near kinsman yonder has
them snowy white? The reason is that lady's bedstraw is fertilised by
small beetles; and beetles are known to be one among the most
colour-loving races of insects. You may often find one of their number,
the lovely bronze and golden-mailed rose-chafer, buried deeply in the
very centre of a red garden rose, and reeling about when touched as if
drunk with pollen and honey. Almost all the flowers which beetles
frequent are consequently brightly decked in scarlet or yellow. On the
other hand, the whole family of the umbellates, those tall plants with
level bunches of tiny blossoms, like the fool's parsley, have all but
universally white petals; and Müller, the most statistical of
naturalists, took the trouble to count the number of insects which paid
them a visit. He found that only 14 per cent. were bees, while the
remainder consisted mainly of miscellaneous small flies and other
arthropodous riff-raff; whereas in the brilliant class of composites,
including the asters, sunflowers, daisies, dandelions, and thistles,
nearly 75 per cent. of the visitors were steady, industrious bees.
Certain dingy blossoms which lay themselves out to attract wasps are
obviously adapted, as Müller quaintly remarks, 'to a less æsthetically
cultivated circle of visitors.' But the most brilliant among all
insect-fertilised flowers are those which specially affect the society
of butterflies; and they are only surpassed in this respect throughout
all nature by the still larger and more magnificent tropical species
which owe their fertilisation to humming-birds and brush-tongued
lories.
Is it not a curious, yet a comprehensible circumstance, that the tastes
which thus show themselves in the development, by natural selection, of
lovely flowers, should also show themselves in the marked preference
for beautiful mates? Poised on yonder sprig of harebell stands a little
purple-winged butterfly, one of the most exquisite among our British
kinds. That little butterfly owes its own rich and delicately shaded
tints to the long selective action of a million generations among its
ancestors. So we find throughout that the most beautifully coloured
birds and insects are always those which have had most to do with the
production of bright-coloured fruits and flowers. The butterflies and
rose-beetles are the most gorgeous among insects: the humming-birds and
parrots are the most gorgeous among birds. Nay more, exactly like
effects have been produced in two hemispheres on different tribes by
the same causes. The plain brown swifts of the North have developed
among tropical West Indian and South American orchids the metallic
gorgets and crimson crests of the humming-bird: while a totally unlike
group of Asiatic birds have developed among the rich flora of India and
the Malay Archipelago the exactly similar plumage of the exquisite
sun-birds. Just as bees depend upon flowers, and flowers upon bees, so
the colour-sense of animals has created the bright petals of blossoms;
and the bright petals have reacted upon the tastes of the animals
themselves, and through their tastes upon their own appearance.
XII.
_SPECKLED TROUT._
It is a piece of the common vanity of anglers to suppose that they know
something about speckled trout. A fox might almost as well pretend that
he was intimately acquainted with the domestic habits of poultry, or an
Iroquois describe the customs of the Algonquins from observations made
upon the specimens who had come under his scalping-knife. I will allow
that anglers are well versed in the necessity for fishing up-stream
rather than in the opposite direction; and I grant that they have
attained an empirical knowledge of the æsthetic preferences of trout in
the matter of blue duns and red palmers; but that as a body they are
familiar with the speckled trout at home I deny. If you wish to learn
all about the race in its own life you must abjure rod and line, and
creep quietly to the side of the pools in an unfished brooklet, like
this on whose bank I am now seated; and then, if you have taken care
not to let your shadow fall upon the water, you may sit and watch the
live fish themselves for an hour together, as they bask lazily in the
sunlight, or rise now and then at cloudy moments with a sudden dart at
a May-fly who is trying in vain to lay her eggs unmolested on the
surface of the stream. The trout in my little beck are fortunately too
small even for poachers to care for tickling them: so I am able
entirely to preserve them as objects for philosophical contemplation,
without any danger of their being scared away from their accustomed
haunts by intrusive anglers.
Trout always have a recognised home of their own, inhabited by a pretty
fixed number of individuals. But if you catch the two sole denizens of
a particular scour, you will find another pair installed in their place
to-morrow. Young fry seem always ready to fill up the vacancies caused
by the involuntary retirement of their elders. Their size depends
almost entirely upon the quantity of food they can get; for an adult
fish may weigh anything at any time of his life, and there is no limit
to the dimensions they may theoretically attain. Mr. Herbert Spencer,
who is an angler as well as a philosopher, well observes that where the
trout are many they are generally small; and where they are large they
are generally few. In the mill-stream down the valley they measure only
six inches, though you may fill a basket easily enough on a cloudy day;
but in the canal reservoir, where there are only half-a-dozen fish
altogether, a magnificent eight-pounder has been taken more than once.
In this way we can understand the origin of the great lake trout, which
weigh sometimes forty pounds. They are common trout which have taken to
living in broader waters, where large food is far more abundant, but
where shoals of small fish would starve. The peculiarities thus
impressed upon them have been handed down to their descendants, till at
length they have become sufficiently marked to justify us in regarding
them as a separate species. But it is difficult to say what makes a
species in animals so very variable as fish. There are, in fact, no
less than twelve kinds of trout wholly peculiar to the British Islands,
and some of these are found in very restricted areas. Thus, the Loch
Stennis trout inhabits only the tarns of Orkney; the Galway sea trout
lives nowhere but along the west coast of Ireland; the gillaroo never
strays out of the Irish loughs; the Killin charr is confined to a
single sheet of water in Mayo; and other species belong exclusively to
the Llanberis lakes, to Lough Melvin, or to a few mountain pools of
Wales and Scotland. So great is the variety that may be produced by
small changes of food and habitat. Even the salmon himself is only a
river trout who has acquired the habit of going down to the sea, where
he gets immensely increased quantities of food (for all the trout kind
are almost omnivorous), and grows big in proportion. But he still
retains many marks of his early existence as a river fish. In the first
place, every salmon is hatched from the egg in fresh water, and grows
up a mere trout. The young parr, as the salmon is called in this stage
of its growth, is actually (as far as physiology goes) a mature fish,
and is capable of producing milt, or male spawn, which long caused it
to be looked upon as a separate species. It really represents, however,
the early form of the salmon, before he took to his annual excursion to
the sea. The ancestral fish, only a hundredth fraction in weight of his
huge descendant, must have somehow acquired the habit of going
seaward--possibly from a drying up of his native stream in seasons of
drought. In the sea, he found himself suddenly supplied with an
unwonted store of food, and grew, like all his kind under similar
circumstances, to an extraordinary size. Thus he attains, as it were,
to a second and final maturity. But salmon cannot lay their eggs in the
sea; or at least, if they did, the young parr would starve for want of
their proper food, or else be choked by the salt water, to which the
old fish have acclimatised themselves. Accordingly, with the return of
the spawning season there comes back an instinctive desire to seek once
more the native fresh water. So the salmon return up stream to spawn,
and the young are hatched in the kind of surroundings which best suit
their tender gills. This instinctive longing for the old home may
probably have arisen during an intermediate stage, when the developing
species still haunted only the brackish water near the river mouths;
and as those fish alone which returned to the head waters could
preserve their race, it would soon grow hardened into a habit engrained
in the nervous system, like the migration of birds or the clustering of
swarming bees around their queen. In like manner the Jamaican
land-crabs, which themselves live on the mountain-tops, come down every
year to lay their eggs in the Caribbean; because, like all other crabs,
they pass their first larval stage as swimming tadpoles, and afterwards
take instinctively to the mountains, as the salmon takes to the sea.
Such a habit could only have arisen by one generation after another
venturing further and further inland, while always returning at the
proper season to the native element for the deposition of the eggs.
These trout here, however, differ from the salmon in one important
particular beside their relative size, and that is that they are
beautifully speckled in their mature form, instead of being merely
silvery like the larger species. The origin of the pretty speckles is
probably to be found in the constant selection by the fish of the most
beautiful among their number as mates. Just as singing birds are in
their fullest and clearest song at the nesting period, and just as many
brilliant species only possess their gorgeous plumage while they are
going through their courtship, and lose the decoration after the young
brood is hatched, so the trout are most brightly coloured at spawning
time, and become lank and dingy after the eggs have been safely
deposited. The parent fish ascend to the head-waters of their native
river during the autumn season to spawn, and then, their glory dimmed,
they return down-stream to the deep pools, where they pass the winter
sulkily, as if ashamed to show themselves in their dull and dusky
suits. But when spring comes round once more, and flies again become
abundant, the trout begin to move up-stream afresh, and soon fatten out
to their customary size and brilliant colours. It might seem at first
sight that creatures so humble as these little fish could hardly have
sufficiently developed aesthetic tastes to prefer one mate above
another on the score of beauty. But we must remember that every species
is very sensitive to small points of detail in its own kind, and that
the choice would only be exerted between mates generally very like one
another, so that extremely minute differences must necessarily turn the
scale in favour of one particular suitor rather than his rivals.
Anglers know that trout are attracted by bright colours, that they can
distinguish the different flies upon which they feed, and that
artificial flies must accordingly be made at least into a rough
semblance of the original insects. Some scientific fishermen even
insist that it is no use offering them a brown drake at the time of
year or the hour of day when they are naturally expecting a red
spinner. Of course their sight is by no means so perfect as our own,
but it probably includes a fair idea of form, and an acute perception
of colour, while there is every reason to believe that all the trout
family have a decided love of metallic glitter, such as that of silver
or of the salmon's scales. Mr. Darwin has shown that the little
stickleback goes through an elaborate courtship, and I have myself
watched trout which seemed to me as obviously love-making as any pair
of turtle-doves I ever saw. In their early life salmon fry and young
trout are almost quite indistinguishable, being both marked with blue
patches (known as 'finger-marks') on their sides, which are remnants of
the ancestral colouring once common to the whole race. But as they grow
up, their later-acquired tastes begin to produce a divergence, due
originally to this selective preference of certain beautiful mates; and
the adult salmon clothes himself from head to tail in sheeny silver,
while the full-grown trout decks his sides with the beautiful speckles
which have earned him his popular name. Countless generations of slight
differences, selected from time to time by the strongest and handsomest
fish, have sufficed at length to bring about these conspicuous
variations from the primitive type, which the young of both races still
preserve.
XIII.
_DODDER AND BROOMRAPE._