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THE OPEN COURT PUB. CO., CHICAGO.
* * * * *
ON
GERMINAL SELECTION
AS A
SOURCE OF DEFINITE VARIATION
BY
AUGUST WEISMANN
TRANSLATED FROM THE GERMAN BY
THOMAS J. McCORMACK
* * * * *
SECOND EDITION
* * * * *
CHICAGO
THE OPEN COURT PUBLISHING COMPANY.
LONDON AGENTS:
KEGAN PAUL, TRENCH, TRUEBNER & CO., LTD.
1902.
* * * * *
COPYRIGHT BY
THE OPEN COURT PUBLISHING CO.
1896
* * * * *
{3}
PREFACE.
The present paper was read in the first general meeting of the
International Congress of Zooelogists at Leyden on September 16, 1895.
Several points, which for reasons of brevity were omitted when the paper
was read, have been re-embodied in the text, and an Appendix has been added
where a number of topics receive fuller treatment than could well be
accorded to them in a lecture. The address was first printed in _The
Monist_ for January, 1896, and afterwards in a German pamphlet.
The basal idea of the essay--the existence of Germinal Selection--was
propounded by me some time since,[1] but it is here for the first time
fully set forth and tentatively shown to be the necessary complement of the
process of selection. Knowing this factor, we remove, it seems to me, the
patent contradiction of the assumption that the general fitness of
organisms, or the adaptations _necessary_ to their existence, are produced
by _accidental_ variations--a contradiction which formed a serious
stumbling-block to the theory of selection. Though still assuming that the
_primary_ variations are "accidental," I yet hope to have demonstrated that
an interior mechanism exists which compels them to go on increasing in a
definite direction, the moment selection intervenes. _Definitely directed
{4} variation exists_, but not predestined variation, running on
independently of the life-conditions of the organism, as Naegeli, to
mention the most extreme advocate of this doctrine, has assumed; on the
contrary, the variation is such as is elicited and controlled by those
conditions themselves, though indirectly.
In basing my proof of the doctrine of Germinal Selection on the fundamental
conceptions of my theory of heredity, a few words of justification are
necessary, owing to the fact that the last-mentioned theory has been widely
and severely assailed since its first emergence into light and even
repudiated as absolutely futile and erroneous.
In the first place, many critics have characterised it as a "pure creation
of the imagination." And to a certain extent it is such, as every theory
is. But is it on that account necessarily wrong? Can not its fundamental
ideas still be quite correct, and it itself therefore perfectly justified
as a means of further progress?
Surely my critics cannot be ignorant of the prominent part which
imagination has recently played in the exactest of all natural
sciences--physics? Are they unaware that the English physicist Maxwell
"constructed from liquid vortices and friction-pulleys enclosed in cells
with elastic walls, a wonderful mechanism, which served as a mechanical
model for electromagnetism"?[2] He hoped "that further research in the
domain of theoretical electricity would be promoted rather than hindered by
such mechanical {5} fictions." And so it actually happened, for Maxwell
found by means of them "the very equations, whose singular and almost
incomprehensible power Hertz has so beautifully portrayed in his lecture on
the relations between light and electricity." "Maxwell's formulae were the
direct outcome of his mechanical models." "These ideal mechanisms"--so
relates Boltzmann in the same interesting essay--"were at first widely
ridiculed, but gradually the new ideas worked their way into all fields.
They were themselves more convenient than the old hypotheses. For the
latter could be maintained only in the event of everything's proceeding
smoothly; whereas now little inconsistencies were fraught with no peril,
for no one can take amiss a slight hitch in a mere analogy.--Ultimately
Maxwell's ideas were philosophically generalised as the theory that all
knowledge consists in the disclosure of analogies."
But not only does it seem that there is little appreciation among
biologists for the scientific import of imagination, they also appear to
have little sense for the significance of theory. It is a favorite attitude
nowadays to look upon theory as a sort of superfluous ballast, as a
worthless survival from the epoch of decrepit "nature-philosophies." People
pronounce with pride the miscomprehended utterance of Newton, _Hypotheses
non fingo_, and place the value of the slightest new fact infinitely higher
than that of "the most beautiful theory."[3] And yet theory originally {6}
fashions science out of facts and is the indispensable precondition of
every important scientific advance.
Heinrich Hertz,[4] the discoverer of electric undulations, had the same
thought in mind when he said: "We form inward representations or constructs
of outward objects, so constituted that the results that follow logically
and necessarily from the constructs are in turn always constructs of the
results flowing naturally and necessarily from the objects." "These
constructs or mental images copied after familiar objects possessed of
familiar properties, so constituted that from their manipulation effects
result similar to those which we observe in the objects to be explained.
Experience teaches us that the requirements here made can be fulfilled and
that consequently such 'correspondences' between reality and the supposed
images [or, as Hertz says, between nature and mind] actually exist. Having
succeeded in extracting from the accumulated experience of the past,
representative images or constructs fulfilling all these necessary
requirements, we can then reproduce by them in a short space of time, as we
might by models, results that in the outward world require a long space of
time for their actualisation or can be produced only through our personal
intervention," etc.
{7}
Such representative models, or constructs, now, in my theory of heredity,
are the _determinants_, which may be conceived as indefinitely fashioned
packages of units (biophores) which are set into activity by definite
impressions and put a distinctive stamp upon some small part of the
organism, on some cell or group of cells, evoking definite phenomena
somewhat as a piece of fireworks when lighted produces a brilliant sun, a
shower of sparks, or the glowing characters of a name.
The _ids_, also, are such representative models, and may be compared to a
definitely ordered but variously compounded aggregate of fireworks, in
which the single pieces are so connected as to go off in fixed succession
and to produce a definite resultant phenomenon like a complete inscription
surrounded by a hail of fire and glowing spheres.
Owing to the greater complexity of the phenomena in biology we can never
hope to reach the same distinctness in our constructs and models as in
physics, and the attempt to derive from them mathematical formulae by the
independent development of which research could be continued, would at
present be utterly fruitless. In the meantime it seems preferable to have
some sort of adequate model to which the imagination can always resort and
with which it can easily operate, rather than to have to revert, in
considering every special problem of heredity, to the mutual actions of the
molecules of living substance and outward agents--processes which we know
only in their roughest outlines. Or is any one presumptuous enough to
believe we can infer from our slight knowledge of the chemical and physical
constitution of the germs of a trout and a salmon the real cause {8} of the
one's becoming a trout and of the other's becoming a salmon?
The fact is, we can make no show of accounting for the complex phenomena of
heredity with mere _material_ units; we can never reach these phenomena
from below, but must begin farther up and make the assumption of _vital_
units and _hereditary_ units, if there is to be any advance in this field.
It is undoubtedly a splendid aim which the newly founded science of
developmental mechanics has set itself of laying bare the entire causal
line leading from the egg to the finished organism; yet, however much we
may wish to see the success of this plan realised, we cannot disguise the
fact that little or nothing is to be accomplished by it in the settlement
of the problems of heredity. It is impossible to suspend the study of
heredity until this mechanics is completed, and even if we could it would
help us little, for the riddles of heredity are not concealed in the
ontogenesis of types, or, to give an example, in the developmental history
of man _as a race_, but in the ontogenesis of _individuals_, in that of a
_definite and particular_ man. This last ontogenesis exhibits the phenomena
of variation, of reversion, of the predominance of the one or the other
parent, etc., and no one is likely to believe that inductive evolutional
inquiry alone will ever afford us knowledge of these minute and delicate
processes, which, in their bearing on the total resultant development,
phylogenesis, are after all the most important of all.
There is, accordingly, no choice left. If we are really bent on
scientifically investigating the question of heredity, we are obliged
perforce to form from the observed facts of heredity a highly detailed and
{9} elaborate theory, on the basis of which we can propound new questions,
which will give rise in turn to new facts, and thus will exercise a
retroactive influence on the theory, improving and transforming it.
This is precisely what I have sought to accomplish by my theory of
Germ-plasm, as I stated in the Preface to the book bearing that name. It
was never intended as a theory of life, nor, indeed, primarily, as a theory
of evolution, but first and above all as a theory of heredity. I cannot
understand, therefore, the animadversion, that my theory in no way furthers
our insight into the mechanics of development. That is not its purpose; in
fact, it takes the ultimate physical and chemical processes which make up
the vital processes for granted; and inevitably it is constrained to do so.
Its aim is to put into our hands a serviceable formula by means of which we
can go on working in the field of heredity at any rate, and, if I am not
mistaken, also in that of evolution. To me, at least, the newest results of
developmental mechanics do not seem so widely at variance with the theory
of determinants as might appear at first sight; so far as I can see, they
can be quite readily made to harmonise with the theory, provided only the
initial stage of the disintegration of the germ-plasm in the determinant
groups be not invariably placed at the beginning of the process of
segmentation, but be transferred according to circumstances to a subsequent
period. The exact state of things cannot as yet be determined, so long as
the mass of facts is still in constant flux.
In any event I still hold fast to the hope which I expressed in the Preface
to my _Germ-plasm_, that despite the unavoidable uncertainties in its
foundation my theory would yet prove more than a mere work {10} of
imagination, and that the future would find in it some durable points which
would outlive the mutations of opinion. It is possible that one of these
durable gains is my much impugned idea of determinants, and in fact not
only will the present essay be made to rest on this idea, but it will also
defend it on new grounds, although primarily only as a representation of
something which we do not as yet exactly know, but which still exists and
on which we can reckon, leaving it to the future to decide the greater or
less resemblance of our hypothetical construct to nature.
The real aim of the present essay is to rehabilitate the principle of
selection. If I should succeed in reinstating this principle in its
emperilled rights, it would be a source of extreme satisfaction to me; for
I am so thoroughly convinced of its indispensability as to believe that its
demolition would be synonymous with the renunciation of all inquiry
concerning the causal relation of vital phenomena. If we could understand
the adaptations of nature, whose number is infinite, only upon the
assumption of a teleological principle, then, I think, there would be
little inducement to trouble ourselves about the causal connexion of the
stages of ontogenesis, for no good reason would exist for excluding
teleological principles from this field. Their introduction, however, means
the ruin of science.
AUGUST WEISMANN.
FREIBURG, Nov. 18, 1895.
* * * * *
{11}
GERMINAL SELECTION.
* * * * *
Numerous and varied are the objections that have been advanced against the
theory of selection since it was first enunciated by Darwin and
Wallace--from the unreasoning strictures of Richard Owen and the acute and
thoughtful criticisms of Albert Wigand and Naegeli to the opposition of our
own day, which contends that selection cannot create but only reject, and
which fails to see that precisely through this rejection its creative
efficacy is asserted. The champions of this view are for discovering the
motive forces of evolution in the _laws_ that govern organisms--as if the
norm according to which an event happens were the event itself, as if the
rails which determine the direction of a train could supplant the
locomotive. Of course, from every form of life there proceeds only a
definite, though extremely large, number of tracks, _the possible
variations_, whilst between them lie stretches without tracks, _the
impossible variations_, on which locomotion is impossible. But the actual
travelling of a track is not performed by the track, but by the locomotive,
and on the other hand, the choice of a track, the decision whether the
destination of the train shall be Berlin or Paris, is not made by the
locomotive, the cause of the variation, but by the driver of the
locomotive, who directs the engine on the right track. In the theory of
selection the engine-driver is represented by utility, for with utility
rests the decision {12} as to what particular variational track shall be
travelled. The cogency, the irresistible cogency, as I take it, of the
principle of selection is precisely its capacity of explaining why fit
structures always arise, and that certainly is the great problem of life.
Not the fact of change, but the _manner_ of the change, whereby all things
are maintained capable of life and existence, is the pressing question.
It is, therefore, a very remarkable fact, and one deserving of
consideration, that to-day (1895), after science has been in possession of
this principle for something over thirty years and during this time has
steadily and zealously busied itself with its critical elaboration and with
the exact determination of its scope, that now the estimation in which it
is held should apparently be on the decrease. It would be easy to enumerate
a long list of living writers who assign to it a subordinate part only in
evolution, or none at all. One of our youngest biologists speaks without
ado of the "pretensions of the refuted Darwinian theory, so called,"[5] and
one of the oldest and most talented inquirers of our time, a pioneer in the
theory of evolution, who, unfortunately, is now gone to his rest, Thomas
Huxley, implicitly yet distinctly intimated a doubt regarding the principle
of selection when he said: "Even if the Darwinian hypothesis were swept
away, evolution would still stand where it is." Therefore, he, too,
regarded it as not impossible that this hypothesis should disappear from
among {13} the great explanatory principles by which we seek to approach
nearer to the secrets of nature.
I am not of that opinion. I see in the growth of doubts regarding the
principle of selection and in the pronounced and frequently bitter
opposition which it encounters, a transient depression only of the wave of
opinion, in which every scientific theory must descend after having been
exalted, here perhaps with undue swiftness, to the highest pitch of
recognition. It is the natural reaction from its overestimation, which is
now followed by an equally exaggerated underestimation. The principle of
selection was not overrated in the sense of ascribing to it too much
explanatory efficacy, or of extending too far its sphere of operation, but
in the sense that naturalists imagined that they perfectly understood its
ways of working and had a distinct comprehension of its factors, which was
not so. On the contrary, the deeper they penetrated into its workings the
clearer it appeared that something was lacking, that the action of the
principle, though upon the whole clear and representable, yet when
carefully looked into encountered numerous difficulties, which were
formidable, for the reason that we were unsuccessful in tracing out the
actual details of the individual process, and, therefore, in _fixing_ the
phenomenon as it actually occurred. We can state in no single case how
great a variation must be to have selective value, nor how frequently it
must occur to acquire stability. We do not know when and whether a desired
useful variation really occurs, nor on what its appearance depends; and we
have no means of ascertaining the space of time required for the fulfilment
of the selective processes of nature, and hence cannot calculate the exact
number of such {14} processes that do and can take place at the same time
in the same species. Yet all this is necessary if we wish to follow out the
precise details of a given case.
But perhaps the most discouraging circumstance of all is, that in scarcely
a single actual instance in nature can we assert whether an observed
variation is useful or not--a drawback that I distinctly pointed out some
time ago.[6] Nor is there much hope of betterment in this respect, for
think how impossible it would be for us to observe all the individuals of a
species in all their acts of life, be their habitat ever so limited--and to
observe all this with a precision enabling us to say that this or that
variation possessed selective value, that is, was a decisive factor in
determining the existence of the species.
In many cases we can reach at least a probable inference, and say, for
example, that the great fecundity of the frog is a property having
selective value, basing our inference on the observation that in spite of
this fertility the frogs of a given district do not increase.
But even such inferences offer only a modicum of certainty. For who can say
precisely how large this number is? Or whether it is on the increase or on
the decrease? And besides, the exact degree of the fecundity of these
animals is far from being known. Rigorously viewed, we can only say that
great fecundity must be advantageous to a much-persecuted animal.
And thus it is everywhere. Even in the most indubitable cases of
adaptation, as, for instance, in that of the striking protective coloring
of many butterflies, {15} the sole ground of inference that the species
upon the whole is adequately adapted to its conditions of life, is the
simple fact that the species is, to all appearances, preserved
undiminished, and the inference is not at all permissible that just this
protective coloring has selective value for the species, that is, that if
it were lacking, the species would necessarily have perished.
It is not inconceivable that in many species today these colorings are
actually unnecessary for the preservation of the species, that they
formerly were, but that now the enemies which preyed on the resting
butterflies have grown scarce or have died out entirely, and that the
protective coloring will continue to exist by the law of inertia[7] only
for a short while till panmixia or new adaptations shall modify it.
Discouraging, therefore, as it may be, that the control of nature in her
minutest details is here gainsaid us, yet it were equivalent to sacrificing
the gold to the dross, if simply from our inability to follow out the
details of the individual case we should renounce altogether the principle
of selection, or should proclaim it as only subsidiary, on the ground that
we believe the protective coloring of the butterfly is not a protective
coloring, but a combination of colors inevitably resulting from internal
causes. The protective coloring remains a protective coloring whether at
the time in question it is or is not necessary for the species; and it
arose as protective coloring--arose not because it was a constitutional
necessity of the animal's organism that here a red and there a white,
black, or yellow spot should be produced, but because it was {16}
advantageous, because it was necessary for the animal. There is only one
explanation possible for such patent adaptations and that is selection.
What is more, no other natural way of their originating is conceivable, for
we have no right to assume teleological forces in the domain of natural
phenomena.
I have selected the example of the butterfly's wing, not solely because it
is so widely known, but because it is so exceedingly instructive, because
we are still able to learn so much from it. It has been frequently asserted
that the color-patterns of the butterfly's wings have originated from
internal causes, independently of selection and conformably to inward laws
of evolution. Eimer has attempted to prove this assertion by establishing
in a division of the genus Papilio the fact that the species there admit of
arrangement in series according to affinity of design. But is a proof that
the markings are modified in definite directions during the course of the
species's development equivalent to a definite statement as to the _causes_
that have produced these gradual transformations? Or, is our present
inability to determine with exactness the biological significance of these
markings and their modifications, a proof that the same have no
significance whatever? On the contrary, I believe it can be clearly proved
that the wing of the butterfly is a tablet on which nature has inscribed
everything she has deemed advantageous to the preservation and welfare of
her creatures, and nothing else; or, to abandon the simile, that these
color-patterns have not proceeded from inward evolutional forces, but are
the result of selection. At least in all places where we do understand
their biological significance these patterns are constituted and
distributed over the wing exactly as utility would require. {17}
I do not pledge myself, of course, to give an explanation of every spot and
every line on a wing. The inscription is often a very complicated one,
dating from remote and widely separated ages; for every single existing
species has inherited the patterns of its ancestral species and that again
the patterns of a still older species. Even at its origin, therefore, the
wing was far from being a _tabula rasa_, but was a closely written and
fully covered sheet, on which there was no room for new writing until a
portion of the old had been effaced. But other parts were preserved, or
only slightly modified, and thus in many cases gradually arose designs of
almost undecipherable complexity.
I should be far from maintaining that the markings arose unconformably to
law. Here, as elsewhere, the dominance of law is certain. But I take it,
that the laws involved here, that is, the physiological conditions of the
variation, are without exception subservient to the ends of a higher
power--utility; and that it is utility primarily that determines the kind
of colors, spots, streaks and bands that shall originate, as also their
place and mode of disposition. The laws come into consideration only to the
extent of conditioning the quality of the constructive materials--the
variations, out of which selection fashions the designs in question. And
this also is subject to important restrictions, as will appear in the
sequel.
The meaning of formative laws here is that definite spots on the surfaces
of the wings are linked together in such a manner by inner, invisible
bonds, as to represent the same spots or streaks, so that we can predict
from the appearance of a point at one spot the appearance of another
similar point at another, and {18} so on. It is an undoubted fact that such
relations exist, that the markings frequently exhibit a certain symmetry,
that--to use the words of the most recent observer on this subject,
Bateson[8]--a meristic representation of equivalent design-elements occurs.
But I believe we should be very cautious in deducing laws from these facts,
because all the rules traceable in the markings apply only to small groups
of forms and are never comprehensive nor decisive for the entire class or
even for the single sub-class of diurnal butterflies, in fact, often not so
for a whole genus. All this points to special causes operative only within
this group.
If internal laws controlled the marking on butterflies' wings, we should
expect that some general rule could be established, requiring that the
upper and under surfaces of the wings should be alike, or that they should
be different, or that the fore wings should be colored the same as or
differently from the hind wings, etc. But in reality all possible kinds of
combinations occur simultaneously, and no rule holds throughout. Or, it
might be supposed that bright colors should occur only on the upper surface
or only on the under surface, or on the fore wings or only on the hind
wings. But the fact is, they occur indiscriminately, now here, now there,
and no one method of appearance is uniform throughout all the species. But
the fitness of the various distributions of colors is apparent, and the
moment we apply the principle of utility we know why in the diurnal
butterflies the upper surface alone is usually variegated and the under
surface protectively colored, or why in the nocturnal {19} butterflies the
fore wings have the appearance of bark, of old wood, or of a leaf, whilst
the hind wings, which are covered while resting, alone are brilliantly
colored. On this theory we also understand the exceptions to these rules.
We comprehend why Danaids, Heliconids, Euploids, and Acracids, in fact all
diurnal butterflies, offensive to the taste and smell, are mostly brightly
marked and equally so on both surfaces, whilst all species not thus exempt
from persecution have the protective coloring on the under surface and are
frequently quite differently colored there from what they are on the upper.
In any event, the supposed formative laws are not obligatory. Dispensations
from them can be issued and are issued _whenever utility requires it_.
Indeed, so far may these transgressions of the law extend, that in the very
midst of the diurnal butterflies is found a genus, the South American
Ageronia, which, like the nocturnal butterfly, shows on the entire _upper_
surface of both wings a pronounced bark-coloration, and concerning which we
also know (and in this respect it is an isolated genus and differs from
almost all other diurnal butterflies), that it spreads out its wings when
at rest like the nocturnal butterfly, and does not close them above it as
its relatives do. Therefore, entirely apart from cases of mimicry, which
after all constitute the strongest proof, the facts here cited are alone
sufficient to remove all doubt that not inner necessities or so-called
formative laws have painted the surface of the butterflies' wings, but that
the conditions of life have wielded the brush.
This becomes more apparent on considering the details. I have remarked that
the usually striking colorations of exempt butterflies, as of the
Heliconids, {20} are the same on both the upper and the lower surfaces of
the wings. Possibly the expression of a law might be seen in this fact, and
it might be said, the coloration of the Heliconids _runs through_ from the
upper to the under surface. But among numerous imitators of the Heliconids
is the genus Protogonius, which has the coloration of the Heliconids on its
upper surface, but on its lower exhibits a magnificent leaf-design. During
flight it appears to be a Heliconid and at rest a leaf. How is it possible
that two such totally different types of coloration should be combined in a
single species, if any sort of _inner_ rigorous necessity existed,
regulating the coloration of the two wing-surfaces? Now, although we are
unable to prove that the Protogonius species would have perished unless
they possessed this duplex coloration, yet it would be nothing less than
intellectual blindness to deny that the butterflies in question are
effectively protected, both at rest and during flight, _that their
colorations are adaptive_. We do not know their primitive history, but we
shall hardly go astray if we assume that the ancestors of the Protogonius
species were forest-butterflies and already possessed an under surface
resembling a leaf. By this device they were protected when at rest.
Afterwards, when this protection was no longer sufficient, they acquired on
their upper surface the coloration of the exempt species with which they
most harmonised in abode, habits of life, and outward appearance.
At the same time it is explained why these butterflies did not acquire the
coloration of the Heliconids on the under surface. The reason is, that in
the attitude of repose they were already protected, and that in an
admirable manner. {21}
That _exempt_ diurnal butterflies should be colored on the upper and under
surfaces alike, and should never resemble in the attitude of repose their
ordinary surroundings, is intelligible when we reflect that it is a much
greater protection to be despised when discovered than to be well, or very
well, but never absolutely, protected from discovery.
It has been so often reiterated that diurnal butterflies, as a rule, are
protectively colored on the under surfaces, that one has some misgivings in
stating the fact again. And yet the least of those who hold this to be a
trivial commonplace know how strongly its implications militate against the
inner motive and formative forces of the organism, which are ever and anon
appealed to. No less than sixty-two genera are counted today in the family
of diurnal butterflies known as the Nymphalidae. Of these by far the
largest majority are sympathetically colored underneath, that is, they show
in the posture of rest the colorings of their usual environment. In a large
number of the species belonging to this group the entire surface of the
hind wings possesses such a sympathetic coloration, as does also the
distant apex of the fore wings. Why? The reason is obvious. This part only
of the fore wing is visible in the attitude of repose. Here, then,--as a
zealous opponent of the theory of selection once exclaimed,--there is
undoubted "correlation" between the coloring of the surface of the hind
wing and of the apex of the fore wing. Correlation is unquestionably a fine
word, but in the present instance it contributes nothing to the
understanding of the problem, for there are near relatives and often
species of the same genera in which this correlation is not restricted to
the apex of the {22} fore wings, but extends to a third or even more of
their wings, and these species are also in the habit of drawing back their
wings less completely in the state of rest, thus rendering a larger portion
of them visible. There are species, too, like the forest-butterflies of
South America just mentioned, the Protogonius, Anaea, Kallima species,
etc., which have nearly the _whole_ of the under surfaces of their fore
wings marked according to the same pattern with their hind wings, and these
butterflies when at rest hold their fore wings free and uncovered by their
hind wings. Where are the formative laws in such cases?
Or, perhaps some one will say: "The covering by the hind wings hinders the
formation of scales on the wing, or impedes the formation of the colors in
the scales." Such a person should examine one of these species. He will
find that the scales are just as dense on the covered as on the uncovered
surface of the wing, and in many species, for example, in Katagramma, the
scales of the covered surface are colored most brilliantly of all.
But the facts are still more irresistible, when we consider _special
adaptations_; for example, the imitation of leaves, which is so often
cited. It is to be noted, first, that this sort of imitation is by no means
restricted to a few genera, still less to a few species. All the numerous
species of the genus Anaea, which are distributed over the forests of
tropical South America, exhibit this imitation in pronounced and varied
forms, as do likewise the American genera Hypna and Siderone, the Asiatic
Symphaedra, the African Salamis, Eurypheme, etc. I have observed
fifty-three genera in which it is present in one, several, or in many
species, but there are many others. {23}
These genera, now, are by no means all so nearly allied that they could
have inherited the leaf-markings from a common ancestral form. They belong
to different continents and have probably for the most part acquired their
protective colorings themselves. But one resemblance they have in
common--they are all _forest-butterflies_. Now what is it that has put so
many genera of forest-butterflies and no others into positions where they
could acquire this resemblance to leaves? Was it directive formative laws?
If we closely examine the markings by which the similarity of the leaf is
determined, we shall find, for example, in Kallima Inachis, and Parallecta,
the Indian leaf-butterflies, that the leaf-markings are executed _in
absolute independence of the other uniformities governing the wing_.
From the tail of the wing to the apex of the fore wings runs with a
beautiful curvature a thick, doubly-contoured dark line accompanied by a
brighter one, representing the midrib of the leaf. This line cuts the
"veins" and the "cells" of the wing in the most disregardful fashion, here
in acute and here in obtuse angles, and in absolute independence of the
regular system of divisions of the wing, which should assuredly be the
expression of the "formative law of the wing," if that were the product of
an internal directive principle. But leaving this last question aside, this
much is certain with regard to the markings, that they are dependent, not
on an _internal_, but on an _external_ directive power.
Should any one be still unconvinced by the evidence we have adduced, let
him give the leaf-markings a closer inspection. He will find that the
midrib is composed of two pieces of which the one belongs to the {24} hind
wing and the other to the fore wing, and that the two fit each other
exactly when the butterfly is in the attitude of repose, but not otherwise.
Now these two pieces of the leaf-rib do not begin on corresponding spots of
the two wings, but on absolutely non-identical spots. And the same is also
true of the lines which represent the lateral ribs of the leaf. These lines
proceed in acute angles from the rib; to the right and to the left in the
same angle, those of the same side parallel with each other. Here, too, no
relation is noticeable between the parts of the wings over which the lines
pass. The venation of the wing is utterly ignored by the leaf-markings, and
its surface is treated as a _tabula rasa_ upon which anything conceivable
can be drawn. In other words, we are presented here with a _bilaterally
symmetrical_ figure engraved on a surface which is essentially _radially
symmetrical_ in its divisions.
I lay unusual stress upon this point because it shows that we are dealing
here with one of those cases which cannot be explained by mechanical, that
is, by natural means, unless natural selection actually exists and is
actually competent to create new properties; for the Lamarckian principle
is excluded here _ab initio_, seeing that we are dealing with a formation
which is only passive in its effects; the leaf-markings are effectual
simply by their existence and not by any function which they perform; they
are present in flight as well as at rest, during the absence of danger, as
well as during the approach of an enemy.
Nor are we helped here by the assumption of _purely internal motive
forces_, which Naegeli, Askenasy, and others have put forward as supplying
a _mechanical_ force of evolution. It is impossible to regard the {25}
coincidence of an Indian butterfly with the leaf of a tree now growing in
an Indian forest as fortuitous, as a _lusus naturae_. Assuming this
seemingly mechanical force, therefore, we should be led back inevitably to
a teleological principle which produces adaptive characters and which must
have deposited the directive principle in the very first germ of
terrestrial organisms, so that after untold ages at a definite time and
place the illusive leaf-markings should be developed. The assumption of
pre-established harmony between the evolution of the ancestral line of the
tree with its pre-figurative leaf, and that of the butterfly with its
imitating wing, is absolutely necessary here--a fact which I pointed out
many years ago,[9] but which is constantly forgotten by the promulgators of
the theory of internal evolutionary forces.
For the present I leave out of consideration altogether the question as to
the conceivable extent of the sphere of operation of natural selection; I
am primarily concerned only with elucidating the process of selection
itself, wholly irrespective of the comprehensiveness or limitedness of its
sphere of action. For this purpose it is sufficient to show, as I have just
done, _that cases exist wherein all natural explanations except that of
selection fail us_. But let us now see how far the principle of selection
will carry us in the explanation of such cases--natural selection, I mean,
as it was formulated by Darwin and Wallace.
There can be no doubt but the leaf-markings readily admit of production in
this manner, slowly and with a gradual but constant increase of fidelity,
provided a single condition is fulfilled: _the occurrence of the {26} right
variations at the right place_. But just here, it would seem, is the
insurmountable barrier to the explanatory power of our principle, for who,
or what, is to be our guarantee that dark scales shall appear at the exact
spots on the wing where the midrib of the leaf must grow? And that later
dark scales shall appear at the exact spots to which the midrib must be
prolonged? And that still later such dark spots shall appear at the places
whence the lateral ribs start, and that here also a definite acute angle
shall be accurately preserved, and the mutual distances of the lateral ribs
shall be alike and their courses parallel? And that the prolongation of the
median rib from the hind wing to the fore wing shall be extended exactly to
that spot where the fore wing is not covered by the hind wing in the
attitude of repose? And so on.
If I could go more minutely into this matter, I should attempt to prove
that the markings, as I have just assumed, have not arisen suddenly, but
were perfected very, very gradually; that in one species they began on the
fore wing and in another on the hind wing; and that in many they never
until recently proceeded beyond one wing, in other species they went only a
little way, and in only a few did they spread over the entire surface of
both wings.
That these markings advanced slowly and gradually, but with marvelous
accuracy, is no mere conjecture. But it follows that the right variations
at the right places must never have been wanting, or, as I expressed it
before: _the useful variations were always present_. But how is that
possible in such long extensive lines of dissimilar variations as have
gradually come to constitute markings of the complexity here presented?
Suppose that the useful colors had not {27} appeared at all, or had not
appeared at the right places? It is a fact that in constant species, that
is, in such as are not in process of transformation, the variations of the
markings are by no means frequent or abundant. Or, suppose that they had
really appeared, but occurred only in individuals, or in a small percentage
of individuals?
Such are the objections raised against the theory of selection by its
opponents, and put forward as insurmountable obstacles to the process. Nor
are such objections relevant only in the case of protective colorings; they
are applicable in all cases where the process of selection is concerned.
Take the case of instincts that are called into action only once in life,
as, for example, the pupal performances of insects, the artificial
fabrication of cocoons, etc. How is it that the useful variations were
always present here? And yet they must have been present, if such
complicated spinning instincts could have taken their rise as are
observable in the silk-worm, or in the emperor-moth. And they have been
developed, and that in whole families, in forms varying in all species, and
in every case adapted to the special wants of the species.
Particularly striking is the proof afforded of this constant presence of
the useful variations by cases where we meet with the development of highly
special adaptations that are uncommon even for the group of organisms
concerned. Such a case, for example, is the apparatus designed for the
capture of small animals and their digestion, found in widely different
plants and widely separated families. On the other hand, very common
adaptations, such as the eyes of animals, show distinctly that in all cases
where it was necessary, the useful variations for the formation of {28} an
eye were presented, and were presented further exactly at spots at which
organs of vision could perform their best work: thus, in Turbellaria and
many other worms that live in the light, at the anterior extremity of the
body and on the dorsal surface; in certain mussels, on the edge of the
mantle; in terrestrial snails, on the antennae; in certain tropical marine
snails inhabiting shallow waters, on the back; and in the chitons even on
the dorsal surface of the shell!
But even taking the very simplest cases of selection, it is impossible to
do without this assumption, that the useful variations are always present,
or that _they always exist in a sufficiently large number of individuals
for the selective process_. You know the thickness and power of resistance
of the egg-shells of round-worms. The eggs of the round-worms of horses
have been known to continue their course of development undisturbed even
after they had been thrown into strong alcohol and all other kinds of
injurious liquids--much to the vexation of the embryologists, who wished to
preserve a definite stage of development and sought to kill the embryo at
that stage. Indeed, think of the result, if in the course of their
phylogenesis stout and resistant variations of egg-shells had not been
presented in these worms, or had not always been presented, or had not been
presented in every generation and not in sufficient quantities.
The cogency of the facts is absolutely overpowering when we consider that
practically no modification occurs _alone_, that every primary modification
brings in its train secondary ones, and that these induce forced
modifications in many parts of the body, frequently of the most
diversified, or even self-contradictory, forms. Recently Herbert Spencer
has drawn {29} fresh attention to these secondary modifications, which must
always occur in harmony with the primary one, and has, as he thinks,
advanced in this set of facts, a convincing disproof of the contention that
such coadaptive modifications of numerous cofunctioning parts can rest on
natural selection. Now, although I deem his conclusion precipitate, yet the
very fact of a simultaneous, functionally concordant, yet essentially
diversified modification of numerous parts, points conclusively to the
circumstance that _something is still wanting to the selection of Darwin
and Wallace, which it is obligatory on us to discover, if we possibly can_,
and without which selection as yet offers no complete explanation of the
phyletic processes of transformation. There is a hidden secret to be
unriddled here before we can obtain a satisfactory insight into the
phenomena in question. _We must seek to discover why it happens that the
useful variations are always present._