often contain bladderworts, or Utriculariæ (Fig. 23)--floating
water-plants, without roots, and with horizontally spread,
long-drawn-out, tendril-like shoots, in part thickly covered with
whorls of delicate, needle-shaped leaves, in part bearing sparse
leaves of quite peculiar structure. These are stalked, hollow
bladders (Fig. 23 _A_, _FA_), with quite a narrow entrance at the
apex, which is closed, as far as larger animals are concerned, by
projecting bristle-like hairs (_B_). Small animals, such as water-fleas
(_Daphnia_), species of _Cyclops_, and Ostracods, can swim in between
the bristles, and they then come in contact with a valve which opens
easily inwards (_B_, _kl_) and allows them to penetrate into the
interior of the trap. Once inside they are captives, for the valve
does not open outwards; therefore they soon die and decompose, and are
then taken up by special absorptive cells (_B_, _C_, _sz_) and utilized
as nourishment for the plants. In this way the Utriculariæ catch
numerous little crustaceans and insect larvæ, which slip into their
traps, presumably for concealment.
[Illustration: FIG. 24. Pitcher of _Nepenthes villosa_, after Kerner.
_St_, stalk of the leaf. _Spr_, its apex. _Fk_, the pitcher. _R_, the
margin beset with incurved spines.]
Another example is found in the marsh plants of the genus _Nepenthes_,
some species of which live as climbers on the outskirts of tropical
forests, climbing up the trees and letting their long, thin tendrils
hang downwards, often over ponds and stagnant pools, where swarms of
small flying insects abound. These plants have developed exceedingly
remarkable contrivances for catching insects and using them as food
(Fig. 24). The long stalks (_St_) of their leaves (_Spr_) are first
bent downwards, then they suddenly turn sharply upwards, and the
upturned portion is modified into a pitcher-like structure, in the
bottom of which a fluid gathers, acid in taste, containing pepsin,
and therefore a digestive fluid. Nitrogenous substances, such as
flesh, dissolve in this fluid, and insects which fall into the pitcher
from the rim are killed and dissolved. There are many species of
_Nepenthes_, but not all of them possess the trap-structure in equal
perfection, so that we are able, to some extent, to follow the course
of its evolution, from a broad leaf-stalk, somewhat bent over at the
edges, to the marvellous closed pitcher shown by _Nepenthes villosa_
(Fig. 24) of Borneo. In this species the pitchers attain a length of
fifty centimetres, and are beautifully coloured, resembling in that
respect, as well as in their form, the tobacco-pipe-like flowers of
the tropical Aristolochiæ. When we come to discuss the origin of
flowers, we shall see that the bright, conspicuous colour possesses
a very considerable value in attracting insects; and in the case of
the pitcher-plant, too, the gorgeous colour probably allures insects
to settle on the rim of the pitcher, and they are tempted to dally
the longer since it secretes honey. But the thick, swollen rim of the
pitcher is as smooth as if it were made of polished wax, and resembles
the petals of those magnificent large orchids, the Stanhopeæ; the
inner surface of the pitcher below the margin is also smooth, so that
insects which creep about seeking honey are apt to slip and fall to the
bottom. Even if many of them are not at once killed by the digestive
fluid, but are able to climb up the smooth wall again, they cannot
escape, for beneath the swollen rim, which projects inwards, there is a
circle of strong bristles or teeth, with the points directed downwards,
which, like thorns, prevent the captive's escape. Thus the pitchers
of _Nepenthes_ secure and digest a large number of insects, and we
can easily understand that the plant acquires a considerable amount
of valuable nourishment in this way, for ready-made protoplasm is a
convenient food to which the plant has to do but little in order to
convert it into its own particular kind of living matter.
The toothwort (_Lathræa squamaria_) must also be briefly noticed
here, because it does not catch insects through the medium either of
air or of water, but through the earth. As is well known, this plant
is parasitic on the roots of various foliage-trees. It is of a pale
yellowish colour, and has no green assimilating parts. For such a plant
it must be of particular value to be able to catch animals and to
use them as food. To this end the short, pale leaves, which surround
the creeping, underground stem in the form of closely appressed
scales, have been modified into snares for minute animals. The leaves
have their upper parts recurved downwards, and the edges have grown
together, so that only a small opening is left at the base, and this
leads into a system of tunnels. Aphides, rotifers, bear-animalcules,
but especially springtails (Podurids), creep into these hollow leaves,
are held fast by a sticky secretion, and are dissolved and absorbed.
Another example, also indigenous, is that graceful marsh plant, the
butterwort (_Pinguicula vulgaris_), whose broad, tongue-shaped leaves,
arranged in the form of a rosette, have been modified into an insect
trap by the turning up of their edges, while the middle is deepened
into a longitudinal groove (Fig. 25). The whole upper surface of the
leaf is covered with an enormous number of little mushroom-shaped
glands (_B_, _C_, _Dr_), which secrete a viscid slime. Insects which
settle on the leaf stick fast, and as the glands continue to pour out
more and more slime, while at the same time the edges of the leaf,
stimulated by the struggling of the insect, curl over still farther,
the victims are drowned in the slime, and ultimately absorbed; for
this secretion is so powerful that even fragments of cartilage are
dissolved by it in forty-eight hours. Midges and mayflies in particular
fall victims to this plant, which is common in marshy places both in
mountain and plain.
[Illustration: FIG. 25. Butterwort (_Pinguicula vulgaris_). _A_, the
entire plant, showing the incurved margins of the leaves and some
insects caught by the secretion. _B_, cross-section through a leaf,
enlarged 50 times. _r_, the margin. _Dr_, _Dr_^l, two kinds of glands.
_C_, a portion of the leaf-surface, magnified 180 times.]
We must also mention the sundew (_Drosera rotundifolia_), which takes
its name from the seeming dewdrops that sparkle in the sun on the
leaves, or rather on the rounded extremities of long and rather thick
cilia-like hairs which cover the whole upper surface of the leaf. In
reality the apparent dewdrops consist of a sticky, clear, viscid slime,
which is secreted by the glandular ends of the pin-shaped hairs or
'tentacles.' Insects which settle on the leaf are caught by the slime,
and in this case also an acid, pepsin-containing fluid is secreted,
which gradually effects the digestion of the soluble parts of the
insect. It is especially noteworthy that it is not only those tentacles
which are in contact with the insect that take part in its digestion
and absorption, for all the others gradually alter their position from
the moment when any nitrogenous body, be it a fragment of flesh or an
insect, touches any of them. All begin to curve slowly towards the
stimulating object (Fig. 27), so that, after one to three hours, all
the tentacles have their heads towards it, and collectively pour out
their digestive juice upon it.
[Illustration: FIG. 26. The Sundew (_Drosera rotundifolia_), after
Kerner.]
[Illustration: FIG. 27. A leaf of the Sundew, with half of the
tentacles curved in upon a captured insect; enlarged 4 times.]
The sundew grows in marshes, as, for instance, those of the Black
Forest, and also on the moss-covered ridges there, and it is easy to
observe that a leaf often shows not merely a single gnat, midge, or
little dragon-fly, but several, sometimes as many as a dozen. In this
case, again, the value of the arrangement from the point of view of
nourishment can be no inconsiderable one.
In the case of the sundew we are obviously face to face with an
exceedingly complex adaptation, for not only is there a secretion of
the peculiar digestive juices, which occur only in carnivorous plants,
but the secreting tentacles are actively motile. That the tentacles
more remote from the captive may be excited to curve towards it, it is
necessary that the stimulus exerted by it on the heads of the tentacles
connected with it be conveyed to the base, and thence to the tips of
the other tentacles, for they curve throughout their whole length.
The utility of the contrivance is obvious, but that an arrangement so
divergent from the ordinary dispositions of plants could be brought
about points to the length of time that the processes of natural
selection must have gone on, preserving every new little variation, and
adding it to the rest.
[Illustration: FIG. 28. Leaf of Venus Fly-trap (_Dionæa muscipula_),
after Kerner. _A_, leaf-blade (_Spr_) open. _St_, leaf-stalk. _Stch_,
sensitive hairs. _B_, vertical section through the closed leaf-blade.]
[Illustration: FIG. 29. _Aldrovandia vesiculosa_, a branch with the
traps _FA_.]
Two plants remain to be noticed in conclusion, both possessing movable,
closing traps for catching animals. The so-called Venus fly-trap
(_Dionæa muscipula_) is a marsh plant of North America, the leaves of
which, like those of _Pinguicula_ and _Drosera_, are arranged in a
rosette on the ground. The individual leaf has a spatula-like stalk
and a blade in two halves (Fig. 28, _A_), each edged with long and
strong spinous processes, directed obliquely inwards. The halves of
the blade, when the necessary stimulus is applied to the surface, can
close together in a very short time, from 10 to 30 seconds. The two
rows of marginal spines then cross, as the interlocking fingers of
the hands do, and thus form a cage out of which the imprisoned insect
cannot escape. The appropriate stimulus to set the mechanism in motion
is a light touch, while a more violent shock, or strong pressure,
or a current of air, does not cause the trap to close. But if a fly
comes to creep about on the leaf, and in doing so touches one of six
short jointed hairs rising erect from a minute cushion of cells, then
the leaf closes, quickly indeed, but at the same time so gently and
imperceptibly that the fly is unaware of danger and does not try to
escape. Then numerous purple mucous glands begin to surround the victim
with pepsin-containing, acid, digestive juice which gradually dissolves
it.
One of the water-plants of Southern Europe, _Aldrovandia vesiculosa_,
which is also to be found in swamps on the northern ridge of the
Alps, possesses, in addition to the capturing and digesting apparatus
proper, an active motile apparatus, which is set in motion through
sensitive hairs. When I found the plant for the first time in a swamp
at Lindau, on the Lake of Constance, I took it at first sight for an
_Utricularia_, for the two plants resemble each other in external
appearance (cf. Figs. 22 and 29), but the modification of the leaves
into traps is quite different. On both halves of the leaf-blade there
are numerous bristles (Fig. 30, _A_), and the lightest touch on these
by a little water animal acts as a releasing stimulus to the motile
elements of the leaf (_Stch_). As in the Venus fly-trap, the two halves
of the leaf close together somewhat quickly, but quite quietly, and the
animal is caught. Fig. 30 shows a section of one of these traps in its
closed state. The captive animals cannot escape, because the margins of
the leaf shut quite tightly on one another, and are beset with little
teeth. Numerous little glands (_Dr_) secrete a digestive juice, and
after some days, or even weeks, the insoluble remains of the minute
animals may be found inside the trap.
[Illustration: FIG. 30. _Aldrovandia_: its trap apparatus. _A_, open.
_St_, stalk of the leaf. _Spr_, blade of the leaf. _Stch_, sensitive
bristles. _Dr_, glands. _B_, closed, a cross-section.]
Many more cases of animal-catching plants might be adduced, but
it is far from my intention to try to describe all the existing
contrivances; those already mentioned may suffice to give an idea of
the diversity and of the detailed effectiveness of these adaptations.
They amplify--so it seems to me--our conception of the scope of natural
selection, by showing us that adaptations may arise which are quite
foreign to the original mode of life of the organism in question,
and stand, indeed, in apparent contradiction to its fundamental
physiological processes. It is hardly necessary to enter into a
special argument to show that they can only have been brought about
in the course of natural selection, since every other interpretation
of their occurrence fails. Neither climatic nor any other external
direct influence could have effected these modifications of the parts
of plants, which are all so different, yet all so well suited to
their purpose; they are different even in plants growing quite close
together, like the sundew and the butterwort. The Lamarckian principle
of use and disuse hardly enters into the question at all, since plants
do not possess a will, and we can hardly speak of 'chance' where we
have to do with such complex and diversely combined transformations.
A process of selection actually operative in each of these cases can
easily be thought out, and I shall leave it to my readers themselves
to do this, and shall only indicate that we have to do with increasing
elaboration in two different directions: first, improvements in the
ability to utilize animal substances which happened to stick to the
leaves, and second, an increase in the probability of animals sticking
to the leaves, and so becoming available. Thus there arose, on the one
hand, dissolving and digestive juices, and arrangements for absorption;
and, on the other hand, viscid slime, and traps of various kinds to
secure the animals, as well as honey and bright colours to attract them.
But it is not merely transformations in the form of the stems and
leaves which have come about; there are also important physiological
changes. The sensitiveness to stimulus of various parts of the leaf is
greatly increased, to a certain extent in the butterwort, the edges of
whose leaves turn inwards in response to stimulus, still more in the
sundew, in which the stimulus is conveyed from the tentacles touched to
all the others, but most wonderfully of all in the Venus fly-trap and
_Aldrovandia_, whose sensitive hairs so transmit the stimulus that the
whole leaf is affected by it, and is set in motion, in a manner quite
comparable to the effects of a nerve-stimulus in animals.
Thus the case of carnivorous or insectivorous plants shows us that,
in the course of natural selection, quite new organs can be produced
in a plant by a thoroughgoing transformation of old ones, as, for
instance, the pitchers of _Nepenthes_, and that, furthermore, even
the physiological capacities of the plant may be changed in the most
far-reaching manner, increasing and varying until they come to resemble
the functions of the animal body.
LECTURE VIII
THE INSTINCTS OF ANIMALS
The robber-wasp--Statement of the problem--Material basis of
instincts--Instincts are not 'inherited habits'--Instinct of
self-preservation--Fugitive instinct: death-feigning--Masking
of crabs--Nutritive instinct--Monophagous caterpillars--Diverse
modes of acquiring food: May-flies, sea-cucumbers, fishes that
snare--'Aberration' of instinct--Change of instinct during
metamorphosis: Eristalis, Sitaris--Imperfection of adaptation points
to origin through natural selection--Instinct and will--Instincts and
protective coloration--Leisurely flight of Heliconiidæ--Rapid flight
of Papilionidæ--Instincts which act only once in a lifetime--Pupation
of butterflies--Pupation of the Longicorns--Pupation of the
silk-moth--The emperor moth--The cocoons of Atlas--Oviposition of
butterflies.
WE have hitherto considered animals with especial regard to the
variation and re-adaptation of morphological characters, e.g.
modifications of form and colour; and we have now to ask whether their
behaviour also is to be referred as to its origin, in whole or in
part, to the principle of selection. All around us we can see that
animals know how to use their parts or organs in a purposeful manner:
the duckling swims at once upon the water; the chicken which has just
been hatched from the egg pecks at the seeds lying on the ground; the
butterfly but newly emerged from the pupa, as soon as its wings have
dried and hardened, knows how to use them in flight; and the predatory
wasp requires no instruction to recognize her victim, a particular
caterpillar, a grasshopper, or some other definite insect; she knows
how to attack it, to paralyse it by stings, and then hesitates not
a moment as to what she has to do next; she drags it to her nest,
deposits it in one of the cells already prepared for her future brood,
lays a single egg upon it, and roofs the cell carefully over. It is
only because all these complex acts are so precisely performed, as
precisely as if the wasp knew why she performed them, that the species
is able to maintain its existence, for only thus can the rearing of the
next generation be secured. Out of the egg there slips a little larva,
which at once makes for the paralysed victim, feeds upon it, and grows
thereby, then, within the shelter of the closed cell, passes through
the pupa stage and is transformed into a perfect wasp. Many species
of these predatory wasps do not lay the egg directly beside or upon
their prey, but lest its movements should endanger their offspring,
they hang the egg above it by a silken thread. It is thus in security,
and the young larva, too, when it appears, can withdraw to its safely
swinging resting-place as soon as danger threatens from the convulsive
struggles of the unfortunate victim at whose body it is gnawing.
Every animal has a great many such 'instincts,' which lead it, indeed
force it, to act appropriately towards an end, without having any
consciousness of that end. For how should the butterfly know what
flying is, or that it possessed the power of flight at all, or who
could have shown the predatory wasp, when she wakened from the pupa
sleep to quite a new kind of life, all that she had to do in order to
procure food for herself and to secure shelter and nourishment for
the brood which was still enclosed within her ovary? Since species
have developed from other species, these regulators of the body, the
instincts, cannot have been the same in earlier times; they must have
evolved out of the instincts of ancestors, and the questions we have to
ask are: By what factors? In what way? Has the principle of selection
been operative here too, or can we refer instincts to the inherited
effects of use and disuse?
Before I enter upon this question it is necessary to consider for a
little the physiological basis of instinct. We can distinguish three
kinds of actions: purely reflex, purely instinctive, and purely
conscious actions. In the case of the first, we see most clearly that
they depend on an existing mechanism, for they follow of necessity on
a particular stimulus, and cannot always be suppressed. Bright light
striking our eye makes the pupil narrower by a contraction of the iris,
and in the same way our eyelids close if a finger be thrust suddenly
towards them. We know, too, the principle of these reflex mechanisms;
they depend on nerve connexions. Sensory nerves are so connected in the
nerve-centres with motor nerves, that a stimulus affecting the former
at the periphery of the body, as at the eye, is carried to certain
nerve-cells of the brain, and from these it excites to activity certain
motor centres, so that definite movements are set up. It is rarely
only one muscle that is thus excited to activity, there are usually
several, and here we have the transition to instinctive action, which
consists in a longer or shorter series of actions, that is, of motor
combinations. These, too, are originally, at least, set a-going by a
sense impression, an external stimulus which affects a sensory nerve
exactly in the same way as in the reflex mechanism, and this stimulus
is carried to a particular group of sensory nerve-cells in the central
nervous organ, and from these transmitted by very fine inter-connexions
to motor centres. There are extraordinarily complex instinctive
actions, and in these the completion of one action is obviously the
stimulus to the second, the completion of the second to the third, and
so on, until the entire chain of inter-dependent movements which make
up the whole performance has been completed.
Instincts have thus a material basis in the cells and fibres of
the nervous system, and through variations in the connexions and
irritability of these nervous parts they too can be modified, like any
of the other characters of the body, such as form and colour.
Conscious actions depend directly on the will, and they have a close
connexion with instinctive actions in as far as these also can be
controlled by the will, that is, can be set a-going or inhibited, and
also, on the other hand, in as far as purely voluntary actions may
become instinctive through frequent repetition. The first case is
illustrated, for instance, when the suckling of a child at the mother's
breast is continued into the second year of life, as not infrequently
happens in the southern countries of Europe. Such a child knows exactly
why it wants the breast, and its action is a conscious one, while the
newborn child seeks about with the mouth instinctively, and when it has
found what it sought performs the somewhat complex sucking movements
automatically. The second case is illustrated, when, for instance, we
have made a habit of winding up a watch on going to bed, and do it when
we happen to change our clothes through the day, although it is then
purposeless and would have been omitted if the action had required
a conscious effort of will. One can often observe on oneself in how
short a time a conscious action may become instinctive. I once sent my
keyless watch to a watchmaker for repairs, and received from him for
the time an ordinary watch, which had to be wound with a key, which
key I kept for safety in my purse. At the end of eight days I got
back my own watch, and on undressing the first night I found myself
'instinctively' taking my purse from my pocket in order to get the key,
which, as I very well knew, I no longer needed. And that a long series
of complex movements, originally performed only consciously, may be
gone through instinctively, is shown by the fact that pieces of music
which have been learnt by heart can often be played without mistake
from beginning to end while the player is thinking of quite other
things. The complex instinctive actions of animals are performed in
quite a similar manner.
There is thus no sharp boundary line between reflex and instinctive
actions, nor between instinctive and conscious actions, but one passes
over into the other, and the thought suggests itself, that in the
phyletic development also transitions from one kind of action to the
other must have taken place. As long as one believes the Lamarckian
principle to be really operative one can suppose that actions, which
were originally dependent on the will, when they were often repeated,
became instinctive, or, in other words, that instincts, many of them at
least, are inherited habits.
I shall endeavour to show later on that this assumption, plausible as
it seems at first sight, cannot be correct; but in the meantime I must
confine myself to saying that there are a great number of instincts
which must be referred to the process of selection, and that the rest
can be similarly interpreted in their essentials at least.
The instinct of self-preservation is universally distributed, and it
is exhibited in many animals by flight from their enemies. The hare
flees from the fox and from men; the bird flies away at the approach of
the cat; the butterfly flies from even the shadow of the net spread to
catch it. These might be regarded as purely conscious actions, and in
the case of the hare and the bird experience and will have undoubtedly
some part in them, but even in these the basis of the action is an
organic impulse; this, and not reflection, causes the animal to flee
at sight of an enemy. In the butterfly, indeed, this must be purely
instinctive, since it is done with the same precision immediately on
leaving the pupa state, before the animal has had any experience. But
even in the case of the hare and the bird, taking to flight would in
most cases come too late if reflection were necessary first; if it is
to be effective it must take place as instantaneously as the shutting
of the lids when danger threatens the eye.