come to be, we may leave habit as _primum movens_ out of the question
altogether, for there are no external conditions which could have
induced the butterfly to take to slower flight than its ancestors
exhibited. That it is now advantageous for it--since it acts as a
signal of its nauseousness--to be as clearly seen and recognized as
possible can exercise no direct influence on its manner of flight,
since it knows nothing about it. Even if we assume that individual
variations cropped up which had an instinct for slower flight, there
would still, without selection, be no reason why this variation in
particular should multiply, still less why the originally slight
slowing of the flight should become more marked in the course of
generations. On the contrary, the butterflies fly a great deal, just
as all other diurnal butterflies do; they exert their power of flight
as long as the sun shines, and if the exercise of one generation
influences the next, they ought to become gradually more capable of
rapid flight. In this case exactly the opposite takes place to what is
ascribed to the Lamarckian principle; more constant use must here have
brought about a diminution of the activity of the relevant parts. It is
quite otherwise when we look at it from the point of view of selection.
The variants which cropped up by chance with slower flight survived
because they were most easily recognized and avoided; they are the most
frequent survivors; they leave descendants which inherit the slower
flight-instinct, and this goes on increasing in them as long as the
increase carries any advantage with it. As soon as this ceases to be
the case the variation comes to a standstill, for it is adapted to the
average of the conditions at a given time.
We may picture to ourselves the thousand kinds of regulations of animal
movements through instinct as having come about in a similar way; in
the majority of cases we _must_ picture it thus. For it is only in the
case of those with high intelligence that we can ask whether the animal
did not by deliberation help in establishing the purposive variation
in its movements. Among insects in any case this could only be taken
into account to a very limited extent, although I do not dispute that
the more intelligent among them may learn, and may make experiments,
and can modify their actions accordingly. But in fleeing from an enemy
experience has nothing to do with it, for the first time it is caught
it pays the penalty with its life. Without care, and with no idea of
the dangers surrounding them on all sides, the butterflies float about,
guided only by their instinct, which, however, is so exactly adapted
to the conditions of their life that a sufficient number of them to
preserve the species always happily escapes all the many dangers. I
may remind you of Hahnel's case of the butterfly, already mentioned,
which escaped the agile lizard by flying rapidly up from the sweet
bait, but settled again upon it without fear immediately afterwards, to
fly from the lizard as before, and did so several times in succession.
We usually judge such actions far too much from the human standpoint;
the butterfly does not wish to escape the death which threatens it; it
knows nothing about death; it is not with it as it was with Dr. Hahnel
himself, who when he was once in danger from a jaguar in a thicket
was so affected by the thought of the death he had happily escaped
that he never cared to pass the place again, but made a long circuit
to his home. The butterfly does not act according to reflection and
imagination; it flies up with lightning-like rapidity when the lizard
rushes at it, because this rapid movement, which it _sees_, acts as
the stimulus which liberates the flight-instinct, and this works so
promptly that in most cases the insect is rescued from danger. Its
disposition, however, is not otherwise affected by its narrow escape,
and it obeys anew the food-instinct which impels it to settle again
on the bait, until the flight-instinct is again set a-going by the
visual impression of the re-advance of the lizard. It is the plaything
of its instincts, a machine which works exactly as it must. That it
is only sense-impressions and not conceptions which here liberate the
actions can be well seen in the case of shy species of butterfly like
our purple emperor (_Apatura iris_), which flies up like lightning from
the moist wood-paths on which it loves to settle as soon as any rapidly
moving visual image, even if it be only a shadow, strikes its eyes. For
this reason the collector tries to approach it so as not to throw his
shadow before him, for then the insect lets the advancing enemy get
quite close, and only flies up when the net is quickly thrust towards
it. In all probability the eye of this insect is particularly well
adapted for perceiving movements, and certainly the flight-instinct
reacts very promptly to such visual impressions, and we can understand
that it must have been so regulated if, as we assume, the regulation
came about through processes of selection, for the enemies of the
butterflies, such as birds, dragon-flies, and lizards shoot quickly
out on their prey, and therefore those butterflies must always have
survived whose instinct impelled them to take to flight most quickly.
In this, then, as in a thousand other cases, the instinct of flight,
or indeed any other mode of movement, cannot be interpreted as an
'inherited habit,' because there is no evidence of the possession of
that degree of intelligence which could have induced the variation in
the previous habit, that is, in manner of movement. The same is true
of animals of low intelligence in regard to all the other instincts,
which otherwise might seem to be explicable in terms of the Lamarckian
principle.
In addition, there is a whole large group of instincts in regard to
which the idea of the Lamarckian principle cannot be entertained, as
I showed years ago, and it consists of all those instincts which are
only exercised once in the course of a lifetime. These cannot possibly
depend on practice in an individual lifetime, and transmission of
the results of this exercise to the following generation; they can
therefore only be interpreted in terms of selection, unless we are to
give up all attempts at a scientific interpretation, and simply accept
them as 'marvels.'
To this class belong all the diverse instincts by which insects
protect themselves against attack during the pupa stage. Even the way
in which the caterpillars of many diurnal butterflies hang themselves
up in pupation is not by any means a very simple instinctive action.
The caterpillar first spins, in a suitable place, a small round disk
of silk threads, to which it then attaches the posterior end of its
body, so securely that it cannot be easily torn away. More complicated
still is the securing of the pupa when it does not hang freely, but
is to remain pressed against a wall or a tree, as is the case in the
Papilionidæ and the Pieridæ. In this case the caterpillar must, in
addition to the usual cradle, spin a thread of silk, in an ingenious
way, diagonally across the thorax, so that it may cross about the
middle of the wing rudiments, and not be too loose, lest the pupa fall
out, yet not too tight, lest the thread cut too deeply into the wing
rudiments and hinder their development. When one remembers that it is
the caterpillar that does all this, before it has taken the form of the
pupa, and that it must all be adapted to the pupa's form, we are amazed
at the extraordinary exactness with which instinct prescribes all the
individual movements which make the whole of the complex performance
effective. And yet, as each caterpillar only accomplishes this
performance once in its life, it could at no time in the development
of the species have become a habit in the case of any individual
caterpillar, and it cannot therefore be an 'inherited habit.'
But however diverse are the methods of securing the safety of the pupæ
in the different families of butterflies, they must all be referred
back to a single root, if the butterfly pedigree can be traced back to
a single ancestral group. The caterpillar of the Sphingidæ does not
creep up walls and trees when it is ready to enter on the pupa stage,
as so many of the caterpillars of the diurnal butterflies do, but
instead its instinct compels it to run about on the ground until it has
found a spot which seems to it suited for boring into the earth, or,
to speak less metaphorically, until it comes to a place which, from
its nature, acts as a liberating stimulus to the instinct to burrow.
Then it penetrates more or less deeply, according to the species, and
makes a small chamber, which it lines with silken threads to prevent
it collapsing; this done, it moults, and enters on the pupa stage.
The exactness with which the individual movements are prescribed
by instinct is seen in the way in which the size of the chamber is
regulated so as to be exactly as large as is necessary to give the
pupa room enough without leaving any superfluous free space. This is
not so simple as it seems, and is not directly conditioned by the
size of the animal, for the caterpillar is longer and altogether of
greater volume than the pupa. The same thing is seen in the stag-beetle
(_Lucanus cervus_), the largest of our indigenous beetles, which gets
its name from the powerful antler-like jaws which distinguish the male.
It also undergoes its pupal metamorphosis in the earth, and makes a
large hard ball of clay, hollow inside, and as smooth as if polished,
and its cavity is exactly the size of the future pupa, or to speak more
precisely, of the fully-formed beetle. For, as Rösel von Rosenhof in
his day 'observed with amazement,' the balls in which the males lie
have a much longer cavity than those built by the females, and for
this reason, that when the fully-formed beetle emerges from the pupa
it must, if it is a male, have room to stretch out its horns, which
have till then lain upon the breast. 'For the beetles do not leave
their dwelling-place until all their parts are sufficiently strong and
properly hardened, and till the season has arrived in which they are
wont to fly about.' The male larva thus makes a much longer pupa-house
than the female larva, in anticipation, so to speak, of the enormous
size of the jaws which will grow out later!
Here the instinct has two modes of expression, according as the bodily
parts are male or female. Here we have to do with an action which is
performed once in a lifetime, and thus the possibility of any other
explanation of the origin of this instinct than through natural
selection is excluded.
Not less significant is the case of the silk-cocoons. The cocoons spun
by the silkworm are egg-shaped, and consist of a single thread many
thousand yards in length, which is wound round the spinning caterpillar
so that not a space is left uncovered. The web is firm, tough, and very
difficult to tear; therefore we must grant that the pupa resting within
will enjoy a very considerable degree of security against injury. But
the moth must be able to get out, and that this may be possible the
caterpillar is impelled by instinct to make its spinning movements such
that the cocoon is eventually looser at the anterior end, so that the
insect, when it is ready to emerge, can tear it asunder with its feet
and make a way out for itself. For this very reason, because the silk
must be torn and spoilt by the emerging insect, silk-breeders kill the
pupating insect before it begins to make its way out.
But there are species whose cocoons are provided from the very start
with an outlet, for the caterpillar spins the silk round itself in
such a way that a round opening is left. But this opening would be not
only a convenient door for the butterfly to emerge by, but an equally
convenient entrance for all its enemies. It is, therefore, closed up.
In the case of the 'emperor moth' (_Saturnia carpini_) this is effected
by means of a circle of stiff bristles of silk on the inside (Fig. 33),
the points of which bend outwards like those of a weir-basket (_r_);
from the inside the emerging moth can easily push aside the bristles,
while the threatening enemy from without is scared off by the stiff
points of the bristles.
[Illustration: FIG. 33. Cocoon of the Emperor Moth (_Saturnia
carpini_), after Rösel. _A_, enclosed pupa. _B_, emerging moth. _r_,
hedge of bristles. _fl_, wings.]
Such a cocoon is comparable to a work of art in which every part
harmonizes with the rest, and all together are adapted as well as
possible to their purpose. And yet it is all accomplished without the
caterpillar having the remotest conception of what it is aiming at when
it winds the endless silken thread about itself in the artistic and
precisely prescribed coils. Nor has it any time for trying experiments
or for learning; it must make all the complex bendings and turnings
of the head which spins the thread, and of the anterior part of the
body which guides the thread, quite exactly and correctly the first
time if a good cocoon is to be produced. Here every possibility of
interpreting this instinct as 'an inherited habit' is excluded, for
each caterpillar becomes a pupa only once; and it is just as impossible
to suppose that it can be directed by intelligence, since it can
neither know that it is about to become a pupa, nor that, in the pupa
stage, it will be in danger from enemies which will attempt to force
their way into the cocoon, nor that the hedge of bristles will protect
it from such enemies. Our only clue to an interpretation is in the slow
process by which minute useful variations in the primitive instinct
of spinning are accumulated through selection; and it is wonderful to
see how exactly these spinning powers are adapted to the particular
life-conditions of individual species.
Thus there are several of the Saturnides whose enormous caterpillars
live on large-leaved trees, and these make use of the large leaves to
form a shelter for the pupa stage, spinning them together so that the
cocoon is for the most part surrounded by leaf. But as the leaf might
easily fall off with the weight of the pupa, they make the leaf-stalk
fast to the twig from which it grows by binding the two firmly together
with a broad, strong, closely-apposed silken band. Seitz relates of
the largest of all these spinners, the Chinese _Attacus atlas_, that
this silk sheath 'is continued to the nearest strong branch, so that
it is impossible with the hand to detach the leaves that conceal an
Atlas-pupa from the tree.' To be sure, this pupa weighs about eleven
grammes!
Since instincts vary, as well as the visible parts of an animal, a
fulcrum is afforded by means of which selection can bring about all
these very special adaptations to given conditions, since it always
preserves for breeding the best suited variations of an already
existing instinct. Any other interpretation is once more excluded.
The same may be said of insects and their egg-laying. This, too,
is in many cases only performed once in a lifetime, and the insect
dies before it has seen the fruit of its labour. Yet egg-laying is
performed in the most effective manner, and with the most perfect
security of result. It seems as if the insect knew, so to speak,
exactly where, in what numbers, and how it should lay its eggs. Many
Mayflies (Ephemeridæ) let their eggs fall all at once into the water
in which the larvæ live; many Lepidoptera, such as _Macroglossa
stellatarum_, lay their eggs singly, and on definite plants--the
humming-bird hawk-moth, just referred to, on _Galium mollugo_; others,
like _Melitæa cinxia_, lay their eggs in heaps on the leaves of the
way-bread (_Plantago media_), or, like _Aglia tau_, on the bark of a
large beech-tree. Nothing in these different modes of egg-laying is
due to chance or caprice; all is determined and regulated by instinct,
and all, as far as we can see, is as well adapted to its purpose as
possible. When, for instance, _Macroglossa stellatarum_ lays her eggs
singly, or in twos or threes, on the green leaves of the food-plant, it
thereby obviates the danger of scarcity of food for the comparatively
large caterpillars, since not many of them could subsist together on
a single plant of Galium, while _Aglia tau_ can place several hundred
eggs on the same beech-tree trunk without having to fear that its
caterpillars will not find abundant nourishment. The precision with
which the egg-laying instinct works is even greater in other species
in which there are more special requirements, e.g. when the eggs have
to be laid on the under side of the leaves, as in _Vanessa prorsa_, or
where they have to be cemented together in a little pillar, so that
they bear a deceptive resemblance to the green flower-buds of the
food-plant (the stinging-nettle).
It is certainly astonishing how exactly the stimulus in these cases is
specialized to the liberation of the instinct. In general the smell
of the food-plant of the caterpillar is enough for most butterflies,
and this attracts the female ready to deposit its eggs, but complete
liberation of the instinct is only effected by the visual impression
of the under side of the leaf. We cannot but be astonished that there
is room for such finely graded nerve-mechanisms in the little brain
of a butterfly, and yet it would be easy enough to adduce still more
complex instincts connected with oviposition in insects. The large
water-beetle, _Hydrophilus piceus_, lays its eggs on a floating raft
made by itself; the gall-wasps must first pierce with their ovipositor
into a particular part of a particular plant to be able to lay the
eggs in the proper place, and this in no haphazard way but with great
carefulness and in a perfectly definite manner. But there is no
necessity to refer here to many or to the most complicated cases of
egg-laying; I only wish to show that, even in the simple cases, such
as that of the butterflies just referred to, there is a precisely
regulated combination of actions which is executed mechanically, and
which cannot be interpreted as inherited habit, because it never was a
habit in any individual of any generation.
It is thus placed beyond the possibility of doubt that very many
instincts, at least, must depend on selection, and it would be useless
to go further in this direction by extending our survey to other
groups of instincts. I shall, however, return later on to the study of
instincts, and, after we have become acquainted with the main features
of the laws of inheritance, it will then be seen that, even among
higher animals, instincts can never be interpreted in terms of the
Lamarckian principle.
LECTURE IX
ORGANIC PARTNERSHIPS OR SYMBIOSIS
Hermit-crabs and sea-anemones--Hermit-crabs and hydroid polyps--Fishes
and sea-anemones--Green fresh-water polyps--Green Amœba--Sea-anemones
and yellow Algæ--Cecropia trees and ants--Lichens--Root fungi--Origin
of Symbiosis--Nostoc and Azolla apparently contradict the origin
through natural selection.
WE have already seen, by means of many examples, to what a great degree
animals and plants are able to adapt themselves to new conditions
of life; how animals imitate their surroundings in colour and form,
how instincts have varied in all directions, how plants have made
use of the chance of frequent contact with little animals to obtain
nourishment from them, and have developed contrivances adapted for
bringing as many of these as possible into their power and causing them
to yield them the largest possible amount of food. A great many of
these could only be interpreted in terms of natural selection, and in
others it seemed at least very probable that selection was one of the
factors in bringing them about.
Particularly clear proof of the reality of natural selection is
afforded by those cases where one form of life associates itself with a
very different one so intimately that they are dependent on one another
and cannot live without one another--at least in extreme cases--and
that new organs, and, indeed, new dual organisms, are sometimes
produced by this interdependence of life. This phenomenon--so-called
'Symbiosis'--was discovered by two sharp-sighted botanists, Anton de
Bary and Schwendener. But Symbiosis occurs not only between plants;
it occurs also between plants and animals and between two species
of animal, and we understand by it a life of partnership depending
on mutual benefits, so that each of the two species affords some
advantage to the other, and makes existence easier for it. In this
respect Symbiosis differs from Parasitism, in which one species is
simply preyed upon by another without receiving any benefit from it in
return, and also from the more innocent Commensalism of Van Beneden,
the table-companionship in which one species depends for its existence
on the richly-spread table of another. Symbiosis is particularly
interesting, because, in addition to extreme cases with marked
adaptations, many occur which are of great simplicity, and which seem
to have brought about almost no change in the two associated species.
We shall take our first examples from the Animal Kingdom.
The partnership between certain sea-anemones (Actiniæ) and hermit-crabs
(Paguridæ) had been noticed long before any particular attention
was devoted to it. Many species of hermit-crab frequently carry a
large sea-anemone about with them on the mollusc shell which they
use as a protecting-house; indeed, two or three of these beautiful
many-tentacled polyps are often attached to them, and this is not at
all a matter of chance, but depends upon instinct on the part of both
animals; they have the feeling of belonging to each other. If the
sea-anemone be taken away from the hermit-crab and put in a distant
part of the aquarium, the crab seeks about till it finds it, then
seizes it with its claws and sets it on its house again. The instinct
to cover itself with Actiniæ is so strong within it that it loads
itself with as many of these friends as it can procure, sometimes with
more than there is room for on the shell. The sea-anemone on its part
calmly submits to the crab's manipulations--a fact very surprising to
any one who is aware of the anemone's ordinarily extreme sensitiveness
to contact, and knows how it immediately draws itself together on any
attempt to detach it from the ground, and will often let itself be
torn in pieces rather than give way. The mutual instincts of the two
creatures are thus adapted to each other; but it does not at first
sight seem as if any structural changes had taken place in favour of
the partnership. This is true, indeed, as regards the hermit-crab, but
not as regards the sea-anemone, although the nature of the adaptation
on the sea-anemone's part only becomes apparent when the two animals
are closely observed in their life together.