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apparently, in pure mechanism. But if we look closely, we shall see that
the explanation is merely verbal, that we are again the dupes of words,
and that the trick of the solution consists in taking the term
"adaptation" in two entirely different senses at the same time.

If I pour into the same glass, by turns, water and wine, the two liquids
will take the same form, and the sameness in form will be due to the
sameness in adaptation of content to container. Adaptation, here, really
means mechanical adjustment. The reason is that the form to which the
matter has adapted itself was there, ready-made, and has forced its own
shape on the matter. But, in the adaptation of an organism to the
circumstances it has to live in, where is the pre-existing form awaiting
its matter? The circumstances are not a mold into which life is inserted
and whose form life adopts: this is indeed to be fooled by a metaphor.
There is no form yet, and the life must create a form for itself, suited
to the circumstances which are made for it. It will have to make the
best of these circumstances, neutralize their inconveniences and utilize
their advantages - in short, respond to outer actions by building up a
machine which has no resemblance to them. Such adapting is not
_repeating_, but _replying_, - an entirely different thing. If there is
still adaptation, it will be in the sense in which one may say of the
solution of a problem of geometry, for example, that it is adapted to
the conditions. I grant indeed that adaptation so understood explains
why different evolutionary processes result in similar forms: the same
problem, of course, calls for the same solution. But it is necessary
then to introduce, as for the solution of a problem of geometry, an
intelligent activity, or at least a cause which behaves in the same way.
This is to bring in finality again, and a finality this time more than
ever charged with anthropomorphic elements. In a word, if the adaptation
is passive, if it is mere repetition in the relief of what the
conditions give in the mold, it will build up nothing that one tries to
make it build; and if it is active, capable of responding by a
calculated solution to the problem which is set out in the conditions,
that is going further than we do - too far, indeed, in our opinion - in
the direction we indicated in the beginning. But the truth is that there
is a surreptitious passing from one of these two meanings to the other,
a flight for refuge to the first whenever one is about to be caught _in
flagrante delicto_ of finalism by employing the second. It is really
the second which serves the usual practice of science, but it is the
first that generally provides its philosophy. In any _particular_ case
one talks as if the process of adaptation were an effort of the organism
to build up a machine capable of turning external circumstances to the
best possible account: then one speaks of adaptation _in general_ as if
it were the very impress of circumstances, passively received by an
indifferent matter.

But let us come to the examples. It would be interesting first to
institute here a general comparison between plants and animals. One
cannot fail to be struck with the parallel progress which has been
accomplished, on both sides, in the direction of sexuality. Not only is
fecundation itself the same in higher plants and in animals, since it
consists, in both, in the union of two nuclei that differ in their
properties and structure before their union and immediately after become
equivalent to each other; but the preparation of sexual elements goes on
in both under like conditions: it consists essentially in the reduction
of the number of chromosomes and the rejection of a certain quantity of
chromatic substance.[22] Yet vegetables and animals have evolved on
independent lines, favored by unlike circumstances, opposed by unlike
obstacles. Here are two great series which have gone on diverging. On
either line, thousands and thousands of causes have combined to
determine the morphological and functional evolution. Yet these
infinitely complicated causes have been consummated, in each series, in
the same effect. And this effect, could hardly be called a phenomenon of
"adaptation": where is the adaptation, where is the pressure of external
circumstances? There is no striking utility in sexual generation; it
has been interpreted in the most diverse ways; and some very acute
enquirers even regard the sexuality of the plant, at least, as a luxury
which nature might have dispensed with.[23] But we do not wish to dwell
on facts so disputed. The ambiguity of the term "adaptation," and the
necessity of transcending both the point of view of mechanical causality
and that of anthropomorphic finality, will stand out more clearly with
simpler examples. At all times the doctrine of finality has laid much
stress on the marvellous structure of the sense-organs, in order to
liken the work of nature to that of an intelligent workman. Now, since
these organs are found, in a rudimentary state, in the lower animals,
and since nature offers us many intermediaries between the pigment-spot
of the simplest organisms and the infinitely complex eye of the
vertebrates, it may just as well be alleged that the result has been
brought about by natural selection perfecting the organ automatically.
In short, if there is a case in which it seems justifiable to invoke
adaptation, it is this particular one. For there may be discussion about
the function and meaning of such a thing as sexual generation, in so far
as it is related to the conditions in which it occurs; but the relation
of the eye to light is obvious, and when we call this relation an
adaptation, we must know what we mean. If, then, we can show, in this
privileged case, the insufficiency of the principles invoked on both
sides, our demonstration will at once have reached a high degree of
generality.

Let us consider the example on which the advocates of finality have
always insisted: the structure of such an organ as the human eye. They
have had no difficulty in showing that in this extremely complicated
apparatus all the elements are marvelously co-ordinated. In order that
vision shall operate, says the author of a well-known book on _Final
Causes_, "the sclerotic membrane must become transparent in one point of
its surface, so as to enable luminous rays to pierce it;... the cornea
must correspond exactly with the opening of the socket;... behind this
transparent opening there must be refracting media;... there must be a
retina[24] at the extremity of the dark chamber;... perpendicular to the
retina there must be an innumerable quantity of transparent cones
permitting only the light directed in the line of their axes to reach
the nervous membrane,"[25] etc. etc. In reply, the advocate of final
causes has been invited to assume the evolutionist hypothesis.
Everything is marvelous, indeed, if one consider an eye like ours, in
which thousands of elements are coördinated in a single function. But
take the function at its origin, in the Infusorian, where it is reduced
to the mere impressionability (almost purely chemical) of a pigment-spot
to light: this function, possibly only an accidental fact in the
beginning, may have brought about a slight complication of the organ,
which again induced an improvement of the function. It may have done
this either directly, through some unknown mechanism, or indirectly,
merely through the effect of the advantages it brought to the living
being and the hold it thus offered to natural selection. Thus the
progressive formation of an eye as well contrived as ours would be
explained by an almost infinite number of actions and reactions between
the function and the organ, without the intervention of other than
mechanical causes.

The question is hard to decide, indeed, when put directly between the
function and the organ, as is done in the doctrine of finality, as also
mechanism itself does. For organ and function are terms of different
nature, and each conditions the other so closely that it is impossible
to say _a priori_ whether in expressing their relation we should begin
with the first, as does mechanism, or with the second, as finalism
requires. But the discussion would take an entirely different turn, we
think, if we began by comparing together two terms of the same nature,
an organ with an organ, instead of an organ with its function. In this
case, it would be possible to proceed little by little to a solution
more and more plausible, and there would be the more chance of a
successful issue the more resolutely we assumed the evolutionist
hypothesis.

Let us place side by side the eye of a vertebrate and that of a mollusc
such as the common Pecten. We find the same essential parts in each,
composed of analogous elements. The eye of the Pecten presents a retina,
a cornea, a lens of cellular structure like our own. There is even that
peculiar inversion of retinal elements which is not met with, in
general, in the retina of the invertebrates. Now, the origin of molluscs
may be a debated question, but, whatever opinion we hold, all are agreed
that molluscs and vertebrates separated from their common parent-stem
long before the appearance of an eye so complex as that of the Pecten.
Whence, then, the structural analogy?

Let us question on this point the two opposed systems of evolutionist
explanation in turn - the hypothesis of purely accidental variations, and
that of a variation directed in a definite way under the influence of
external conditions.

The first, as is well known, is presented to-day in two quite different
forms. Darwin spoke of very slight variations being accumulated by
natural selection. He was not ignorant of the facts of sudden variation;
but he thought these "sports," as he called them, were only
monstrosities incapable of perpetuating themselves; and he accounted for
the genesis of species by an accumulation of _insensible_
variations.[26] Such is still the opinion of many naturalists. It is
tending, however, to give way to the opposite idea that a new species
comes into being all at once by the simultaneous appearance of several
new characters, all somewhat different from the previous ones. This
latter hypothesis, already proposed by various authors, notably by
Bateson in a remarkable book,[27] has become deeply significant and
acquired great force since the striking experiments of Hugo de Vries.
This botanist, working on the _OEnothera Lamarckiana_, obtained at the
end of a few generations a certain number of new species. The theory he
deduces from his experiments is of the highest interest. Species pass
through alternate periods of stability and transformation. When the
period of "mutability" occurs, unexpected forms spring forth in a great
number of different directions.[28] - We will not attempt to take sides
between this hypothesis and that of insensible variations. Indeed,
perhaps both are partly true. We wish merely to point out that if the
variations invoked are accidental, they do not, whether small or great,
account for a similarity of structure such as we have cited.

Let us assume, to begin with, the Darwinian theory of insensible
variations, and suppose the occurrence of small differences due to
chance, and continually accumulating. It must not be forgotten that all
the parts of an organism are necessarily coördinated. Whether the
function be the effect of the organ or its cause, it matters little; one
point is certain - the organ will be of no use and will not give
selection a hold unless it functions. However the minute structure of
the retina may develop, and however complicated it may become, such
progress, instead of favoring vision, will probably hinder it if the
visual centres do not develop at the same time, as well as several parts
of the visual organ itself. If the variations are accidental, how can
they ever agree to arise in every part of the organ at the same time, in
such way that the organ will continue to perform its function? Darwin
quite understood this; it is one of the reasons why he regarded
variation as insensible.[29] For a difference which arises accidentally
at one point of the visual apparatus, if it be very slight, will not
hinder the functioning of the organ; and hence this first accidental
variation can, in a sense, _wait for_ complementary variations to
accumulate and raise vision to a higher degree of perfection. Granted;
but while the insensible variation does not hinder the functioning of
the eye, neither does it help it, so long as the variations that are
complementary do not occur. How, in that case, can the variation be
retained by natural selection? Unwittingly one will reason as if the
slight variation were a toothing stone set up by the organism and
reserved for a later construction. This hypothesis, so little
conformable to the Darwinian principle, is difficult enough to avoid
even in the case of an organ which has been developed along one single
main line of evolution, _e.g._ the vertebrate eye. But it is absolutely
forced upon us when we observe the likeness of structure of the
vertebrate eye and that of the molluscs. How could the same small
variations, incalculable in number, have ever occurred in the same
order on two independent lines of evolution, if they were purely
accidental? And how could they have been preserved by selection and
accumulated in both cases, the same in the same order, when each of
them, taken separately, was of no use?

Let us turn, then, to the hypothesis of sudden variations, and see
whether it will solve the problem. It certainly lessens the difficulty
on one point, but it makes it much worse on another. If the eye of the
mollusc and that of the vertebrate have both been raised to their
present form by a relatively small number of sudden leaps, I have less
difficulty in understanding the resemblance of the two organs than if
this resemblance were due to an incalculable number of infinitesimal
resemblances acquired successively: in both cases it is chance that
operates, but in the second case chance is not required to work the
miracle it would have to perform in the first. Not only is the number of
resemblances to be added somewhat reduced, but I can also understand
better how each could be preserved and added to the others; for the
elementary variation is now considerable enough to be an advantage to
the living being, and so to lend itself to the play of selection. But
here there arises another problem, no less formidable, viz., how do all
the parts of the visual apparatus, suddenly changed, remain so well
coördinated that the eye continues to exercise its function? For the
change of one part alone will make vision impossible, unless this change
is absolutely infinitesimal. The parts must then all change at once,
each consulting the others. I agree that a great number of uncoördinated
variations may indeed have arisen in less fortunate individuals, that
natural selection may have eliminated these, and that only the
combination fit to endure, capable of preserving and improving vision,
has survived. Still, this combination had to be produced. And, supposing
chance to have granted this favor once, can we admit that it repeats the
self-same favor in the course of the history of a species, so as to give
rise, every time, all at once, to new complications marvelously
regulated with reference to each other, and so related to former
complications as to go further on in the same direction? How,
especially, can we suppose that by a series of mere "accidents" these
sudden variations occur, the same, in the same order, - involving in each
case a perfect harmony of elements more and more numerous and
complex - along two independent lines of evolution?

The law of correlation will be invoked, of course; Darwin himself
appealed to it.[30] It will be alleged that a change is not localized in
a single point of the organism, but has its necessary recoil on other
points. The examples cited by Darwin remain classic: white cats with
blue eyes are generally deaf; hairless dogs have imperfect dentition,
etc. - Granted; but let us not play now on the word "correlation." A
collective whole of _solidary_ changes is one thing, a system of
_complementary_ changes - changes so coördinated as to keep up and even
improve the functioning of an organ under more complicated
conditions - is another. That an anomaly of the pilous system should be
accompanied by an anomaly of dentition is quite conceivable without our
having to call for a special principle of explanation; for hair and
teeth are similar formations,[31] and the same chemical change of the
germ that hinders the formation of hair would probably obstruct that of
teeth: it may be for the same sort of reason that white cats with blue
eyes are deaf. In these different examples the "correlative" changes are
only _solidary_ changes (not to mention the fact that they are really
_lesions_, namely, diminutions or suppressions, and not additions, which
makes a great difference). But when we speak of "correlative" changes
occurring suddenly in the different parts of the eye, we use the word in
an entirely new sense: this time there is a whole set of changes not
only simultaneous, not only bound together by community of origin, but
so coördinated that the organ keeps on performing the same simple
function, and even performs it better. That a change in the germ, which
influences the formation of the retina, may affect at the same time also
the formation of the cornea, the iris, the lens, the visual centres,
etc., I admit, if necessary, although they are formations that differ
much more from one another in their original nature than do probably
hair and teeth. But that all these simultaneous changes should occur in
such a way as to improve or even merely maintain vision, this is what,
in the hypothesis of sudden variation, I cannot admit, unless a
mysterious principle is to come in, whose duty it is to watch over the
interest of the function. But this would be to give up the idea of
"accidental" variation. In reality, these two senses of the word
"correlation" are often interchanged in the mind of the biologist, just
like the two senses of the word "adaptation." And the confusion is
almost legitimate in botany, that science in which the theory of the
formation of species by sudden variation rests on the firmest
experimental basis. In vegetables, function is far less narrowly bound
to form than in animals. Even profound morphological differences, such
as a change in the form of leaves, have no appreciable influence on the
exercise of function, and so do not require a whole system of
complementary changes for the plant to remain fit to survive. But it is
not so in the animal, especially in the case of an organ like the eye, a
very complex structure and very delicate function. Here it is impossible
to identify changes that are simply solidary with changes which are also
complementary. The two senses of the word "correlation" must be
carefully distinguished; it would be a downright paralogism to adopt one
of them in the premisses of the reasoning, and the other in the
conclusion. And this is just what is done when the principle of
correlation is invoked in explanations of _detail_ in order to account
for complementary variations, and then correlation _in general_ is
spoken of as if it were any group of variations provoked by any
variation of the germ. Thus, the notion of correlation is first used in
current science as it might be used by an advocate of finality; it is
understood that this is only a convenient way of expressing oneself,
that one will correct it and fall back on pure mechanism when explaining
the nature of the principles and turning from science to philosophy. And
one does then come back to pure mechanism, but only by giving a new
meaning to the word "correlation" - a meaning which would now make
correlation inapplicable to the detail it is called upon to explain.

To sum up, if the accidental variations that bring about evolution are
insensible variations, some good genius must be appealed to - the genius
of the future species - in order to preserve and accumulate these
variations, for selection will not look after this. If, on the other
hand, the accidental variations are sudden, then, for the previous
function to go on or for a new function to take its place, all the
changes that have happened together must be complementary. So we have to
fall back on the good genius again, this time to obtain the
_convergence_ of _simultaneous_ changes, as before to be assured of the
_continuity of direction_ of _successive_ variations. But in neither
case can parallel development of the same complex structures on
independent lines of evolution be due to a mere accumulation of
accidental variations. So we come to the second of the two great
hypotheses we have to examine. Suppose the variations are due, not to
accidental and inner causes, but to the direct influence of outer
circumstances. Let us see what line we should have to take, on this
hypothesis, to account for the resemblance of eye-structure in two
series that are independent of each other from the phylogenetic point of
view.

Though molluscs and vertebrates have evolved separately, both have
remained exposed to the influence of light. And light is a physical
cause bringing forth certain definite effects. Acting in a continuous
way, it has been able to produce a continuous variation in a constant
direction. Of course it is unlikely that the eye of the vertebrate and
that of the mollusc have been built up by a series of variations due to
simple chance. Admitting even that light enters into the case as an
instrument of selection, in order to allow only useful variations to
persist, there is no possibility that the play of chance, even thus
supervised from without, should bring about in both cases the same
juxtaposition of elements coördinated in the same way. But it would be
different supposing that light acted directly on the organized matter so
as to change its structure and somehow adapt this structure to its own
form. The resemblance of the two effects would then be explained by the
identity of the cause. The more and more complex eye would be something
like the deeper and deeper imprint of light on a matter which, being
organized, possesses a special aptitude for receiving it.

But can an organic structure be likened to an imprint? We have already
called attention to the ambiguity of the term "adaptation." The gradual
complication of a form which is being better and better adapted to the
mold of outward circumstances is one thing, the increasingly complex
structure of an instrument which derives more and more advantage from
these circumstances is another. In the former case, the matter merely
receives an imprint; in the second, it reacts positively, it solves a
problem. Obviously it is this second sense of the word "adapt" that is
used when one says that the eye has become better and better adapted to
the influence of light. But one passes more or less unconsciously from
this sense to the other, and a purely mechanistic biology will strive to
make the _passive_ adaptation of an inert matter, which submits to the
influence of its environment, mean the same as the _active_ adaptation
of an organism which derives from this influence an advantage it can
appropriate. It must be owned, indeed, that Nature herself appears to
invite our mind to confuse these two kinds of adaptation, for she
usually begins by a passive adaptation where, later on, she will build
up a mechanism for active response. Thus, in the case before us, it is
unquestionable that the first rudiment of the eye is found in the
pigment-spot of the lower organisms; this spot may indeed have been
produced physically, by the mere action of light, and there are a great
number of intermediaries between the simple spot of pigment and a
complicated eye like that of the vertebrates. - But, from the fact that
we pass from one thing to another by degrees, it does not follow that
the two things are of the same nature. From the fact that an orator
falls in, at first, with the passions of his audience in order to make
himself master of them, it will not be concluded that to _follow_ is the
same as to _lead_. Now, living matter seems to have no other means of
turning circumstances to good account than by adapting itself to them
passively at the outset. Where it has to direct a movement, it begins by
adopting it. Life proceeds by insinuation. The intermediate degrees
between a pigment-spot and an eye are nothing to the point: however



Online LibraryHenri BergsonCreative evolution → online text (page 6 of 34)