Edward von Hartmann may justly claim that his views should be
considered and tested by naturalists.[287] He would be correctly
classed with those philosophers who have approached this question with
a many-sided scientific preparation. It can nevertheless be perceived
in his case how difficult, and indeed how impossible, it is to estimate
the true value of the facts furnished by the investigation of nature,
when we attempt to take up only the results themselves, without being
practised in the methods by which these are reached, _i.e._ without
being completely at home in one of the scientific subjects concerned
through one’s own investigations. It appears to me that the denial of
the purely mechanical value of the Darwinian factors of transformation
arises in most part from an erroneous classification of the scientific
facts with which we have to deal. There can certainly be no mistake
that the entire philosophical conception of the universe, as laid down
by Von Hartmann in his “Philosophy of the Unconscious,” is unfavourable
to an unprejudiced estimate of scientific facts and to their mechanical
valuation.
Variability, heredity, and above all correlation, would not be regarded
by Von Hartmann as purely mechanical principles, but he would therein
assume a metaphysical directive principle.
In the first place, as regards variability, Von Hartmann endeavours
to show that it is only a quite unlimited variability which suffices
for the explanation of necessary and useful adaptations by means
of selection and the struggle for existence. But this does not
exist--variation rather takes place in a fixed direction only (in
Askenasy’s sense), and this can be nothing else than the expression of
an innate law of development, _i.e._ a phyletic vital force.
This deduction appears to me in two ways erroneous. In the first place
it is incorrect that a quite unlimited variability is a postulate
of the theory of selection, and in the next place the admission of
variability, which is in a certain sense “fixed in direction,” does not
necessitate the assumption of a phyletic vital force.
A mere unsettled variability, uniform in all possible directions, is,
according to Von Hartmann, necessary for the theory of selection,
because only then does the variability offer a certain guarantee “that
under given conditions of life the variations necessary for complete
adaptation will not be wanting.” But it is hereby overlooked that the
new life conditions to which the adaptation must take place are as
little fixed and unchangeable as the organism itself. In such a case of
transformation we have not to deal with a type of organization which
was before fixed and immutable, and which has to be squeezed into new
life-conditions as into a mould. The adaptation is not one-sided, but
mutual; a species in some measure selects its new conditions of life,
corresponding with those possible to its organization, _i.e._ with the
variations actually occurring. I will choose an instance which will
even be conceded by Von Hartmann as being only explicable by natural
selection, viz., a case of mimicry.
Supposing that among the South American _Heliconiidæ_ there occurred
a species of _Pieris_ which had no resemblance to these protected
butterflies, either in form, marking, or colouring; who can deny
that it would be most useful to this species to acquire the form and
colouring of a Heliconide, and thus, by taking to new conditions of
life, to avoid the persecutions of its foes? But if the physical
nature of the Pieride concerned precluded the occurrence of Heliconoid
variations, would this incapability of insinuating itself into these
new conditions necessitate the decline of the species? Could not its
existence be secured in some other manner? could not the destruction of
numerous individuals by foes be compensated for by increased fertility?
to say nothing of the numerous other means through which the number
of surviving individuals might become increased, and the existence
of the species secured. This case is not arbitrarily chosen; in the
districts where the _Heliconiidæ_ occur there are actually a large
number of Whites which do not possess the protective colours of the
former nauseous family. In the adoption of these new life conditions
we have not to deal therefore with survival or extermination, but only
with amelioration. It is not every species of “White” that can become
adapted to these conditions, because every species does not give rise
to the necessary colour variations; those that do, become in this way
modified, because they are thus better protected than before. And so
it is throughout; wherever we find protected insects enjoying immunity
from foes we see also mimickers, sometimes only single, sometimes
several, and generally from very diverse groups of insects, according
to the general resemblance which existed before the commencement of
the process of adaptation, and to the variations made possible by the
physical nature of the species concerned.
In the first essay of the second part of this work it was shown that in
certain Lepidopterous larvæ a process of adaptation is at the present
time still in progress, this depending upon the fact that while the
young caterpillar is very well protected by the leaf-green colour of
its body, this colour becomes insufficient to conceal the insect as
soon as it exceeds the leaf in size. All such caterpillars--and there
is a whole series of species--as they increase in size acquire the
habit of concealing themselves on the earth by day, and of feeding
only at night. New conditions of life are thus imposed, and these
are even compulsory, _i.e._ they could not be abandoned without
risking the existence of the species. Now in accordance with these
new conditions, some individuals in these species have lost the green
colouring of the young stages, and have acquired the brown coloration
of the dark surroundings of the insects which conceal themselves
by day. In one species this change has now occurred in almost all
individuals, in others in only a larger or smaller proportion of
them. Now supposing that among these species there occurred one, the
physical nature of which did not admit of the production of brown
shades of colour, would the species for this reason succumb? Is it not
conceivable that the want of colour adaptation might be compensated
for by better concealment, _i.e._ by burrowing into the earth, or by
a greater fertility of the species, or by the development of warning
signals--supposing the species to be unpalatable--or finally, by the
acquisition of a terrifying marking? In other words, could not the
caterpillar itself modify the new condition of life--that of being
concealed by day--in accordance with variations made possible by its
physical nature?
As a matter of fact in one of these species the green colour remains
unchanged in spite of the altered mode of life, and this species,
wherever it occurs, notwithstanding the persecution of entomologists,
is always common (_Deilephila Hippophaës_); it conceals itself better
and deeper however than those other species which, like _Sphinx
Convolvuli_, are difficult to detect on account of their brown colour.
In another species the striking yellowish green colouring is likewise
retained in the majority of individuals, but this species buries
itself by day in the loose soil (_Acherontia Atropos_).
To this it may be objected that there are also compulsory changes in
the conditions of life from which the species cannot withdraw itself,
but in which adaptation must necessarily follow, or extermination would
take place.
Such compulsory conditions of life do most assuredly occur, and there
is indeed no doubt that many living forms have perished through not
becoming transformed. I believe, however, that such conditions occur
much more rarely than one is inclined to admit at first sight. As
a rule the alternative of immediate change or of extermination is
offered only by such changes in the conditions of life as occur very
rapidly. The sudden appearance of a new and dominant enemy, such as
man, has already caused the extinction of the Dodo (_Didus ineptus_),
and of Steller’s Sea Cow (_Rhytina Stelleri_), and of other vertebrate
animals, and constantly leads to the extermination of many other
species of different classes. When in America hundreds of thousands of
acres of primeval forest are annually destroyed, the conditions of life
of a numerous fauna and flora must be thereby suddenly changed, leaving
no choice but extermination.
Such abrupt changes in the conditions of life occur, however, but
seldom in nature unless caused by man, and must therefore have very
rarely happened in former epochs of the earth’s history. Even climatic
changes, which we might at first regard as of this character, and
which produce a modification in one fixed direction, occur always so
gradually that the species has time either to adapt itself to the
conditions in this or that direction, according to the variations
possible to its physical nature, or else to emigrate.
It thus appears to me erroneous to suppose that variability must be
“merely undetermined” in order to complete its part in Darwin’s theory
of selection, and its “illimitedness” seems to me also as little
necessary for this purpose. Von Hartmann imagines that it is only
unlimited variability that furnishes a guarantee that any type, to
whatever extent diverging from its point of departure, will be reached
by the Darwinian method of gradual transmutation by means of selection
and the struggle for existence.
But who has ever asserted that _any_ type can be reached from any
point? Or if anybody has said such nonsense, who can prove that its
admission is necessary for the theory of selection? Nowhere in systemy
do we see any point of support for such an assumption. But when Von
Hartmann imagines that the “unlimited” variability which he postulates
for Darwin “is in itself unlimited, the limits of its divergence in
a given direction being found, not in itself, but only in external
obstacles,” he conceives variability to be something independent of,
and in some way added to, the animal body, and not a mere expression
for the fluctuations in the type of the organism. If, however, we
conceive variability in this latter, the true scientific sense, it
is in no way “quantitatively unlimited,” nor are its limits even
determined by _external_ influences, but essentially by _internal_
influences, _i.e._ by the underlying physical nature of the organism.
Darwin has indeed already shown this in a most beautiful manner in his
investigations upon the correlations of organs and systems of organs
of the body. To make use of a metaphor, the forces acting within the
body are in equilibrium; if one organ becomes changed this causes a
disturbance in the forces, and the equilibrium must be restored by
changes in other parts, and these again entail other modifications,
and so forth. Herein lies the reason why the primary change cannot
exceed a certain amount if the restoration of the equilibrium is
not to be quite impossible. This is but a metaphor, and I do not
wish to assert that we are at present in a position to formulate and
demonstrate mathematically for any particular case, how much an organ
can become changed in any one species before an interruption of the
internal harmony of the body takes place. But such impossibility of
demonstration does not appear to me to furnish a sufficient reason
for regarding variability as the expression of a directive power--as
an “innate tendency to variation conformable to law.”[288] On the
contrary, it is to me easily conceivable that we only learn to analyse
the processes of nature in detail very slowly, because of their
necessary complexity. It thus appears to me quite useless when in this
sense Wigand makes use of the objection, that “the gooseberry has not
undergone any enlargement since 1852, although it is inconceivable
why it should not attain the size of a pumpkin if variability was
not internally limited.” It may well be that this is for the present
“inconceivable;” nevertheless, this does not justify us in setting
up a hypothetical “force of variation” which will not admit of the
gooseberry surpassing the pumpkin in size. We are bound to maintain
that it is the action and reaction of known forces which sets a limit
to the enlargement of this fruit.
In more simple instances the causes of such limitations to growth can
be well perceived. Several decades have passed since Leuckart proved
in how exact a relation the proportion of volume and surface stood to
the degree of organization of an animal. In animals of a spherical
form the surface is quite sufficient for respiration, so long as they
are of microscopic size. But such an organism cannot become enlarged
at pleasure, because the ratio of the surface to the volume would
become quite different. The surface increases as the square, whilst
the volume increases as the cube, so that very soon the surface of
the more rapidly increasing bodily mass can no longer suffice for
respiration.[289] This sort of limitation is in no way equivalent to
that purely external kind which, for instance, manifests itself in such
a manner as to prevent the indefinite lengthening of the tail feathers
of the Bird of Paradise. In this case feathers that were too long would
hinder flight, and such individuals would accordingly be eliminated by
natural selection. The cause is in the former case purely internal,
depending upon the equilibrium of the forces governing the organism.
Von Hartmann is entirely in the right when he asserts that variability
is neither qualitatively nor quantitatively unlimited. In both
senses it is limited (in direction as well as in amount) by the
physico-chemical forces acting in some contrary way in each specific
organism--by the physical nature of each living form. He errs,
however, both in making absolute illimitability a necessary postulate
of the theory of selection, as also in inferring the existence of
a directive principle from that limitation of variability which is
certainly present. “Tendencies to variation” do however exist, not in
the sense of a directive power, but as expressions of the different
physical constitutions of species, which necessarily cause unequal
reactions to the same external actions, as will be more clearly proved
below.[290]
This is, of course, a modification of Darwin’s original assumption of
an unbounded variability not limited in direction; but Darwin himself
has later coincided in the view that the quality of the variations is
essentially determined by the nature of the organism.[291]
I now turn to the consideration of the second factor of the theory of
selection--heredity. This also, according to Von Hartmann is not a
mechanical principle. Darwin himself has now become convinced how great
is the probability against the hereditary retention of modifications
which, whether feebly or strongly pronounced, appear _only in single
individuals_, _i.e._ of those so-called “fortuitous” variations which
are not the expression of a directive developmental principle. “But as
among the numberless possible directions of an indefinite variability,
useful modifications can only occur in single cases, Darwin has by this
supplementary admission himself retracted an inadmissible assumption of
his theory of selection,” and so forth. A “regular, designed tendency
to variation, acting from within and contemporaneously affecting a
large number of individuals,” must therefore be assumed “in order to
insure the by itself improbable inheritance.”
But even from the unbounded variability laid down by the author, it
by no means follows that useful variations can only occur in single
individuals. In the whole category of _quantitative_ variations the
reverse is always the case. Is it the lengthening of some part that
is concerned; so would a large number of individuals always possess
the useful variation, since we are not dealing with an _absolute_
enlargement, but only with the fact that the part concerned is longer
than in other individuals.[292]
But if _qualitative_ variations come into consideration, it may be
asked whether Darwin’s “supplementary admission” does not go too
far. Such calculations as those quoted by Darwin from the article in
the _North British Review_ of March 1867 are extremely deceptive,
since we have no means of measuring the amount of protection afforded
by a useful variation, and we can therefore hardly compute with
any certainty, in how great a percentage of individuals a change
must contemporaneously occur in order to have a chance of becoming
transferred to the following generation. If our blue rock-pigeon could
exist in a polar climate, and if we had the power of introducing it
gradually, but not suddenly, into these regions in a wild state, who
can doubt that it would assume the white colour of all polar animals?
Nevertheless, among wild rock-pigeons white varieties do not occur
more frequently than among swallows, crows, or magpies. Or must the
white colour of polar animals, the yellow colour of desert species, and
the green colour of leaf-frequenting forms, be always referred to a
“regular, designed, fixed tendency to variation acting from within,”
and causing a “large number of individuals” to vary in a similar manner?
There is, however, a grain of truth in the foregoing; variations which
occur singly have but little chance of becoming predominant characters,
and this is obviously what Darwin concedes. But this is by no means
equivalent to the assumption that only those variations which from the
first occur in numerous individuals have a chance of being perpetuated.
Let us keep to the facts. We have not the slightest reason either
for regarding the white colour of polar animals as the direct action
of cold, or for considering that the green colour of foliage-living
caterpillars depends upon direct action arising from the habit of
resting upon the leaves;[293] both these characters are explicable only
by natural selection, and there is nothing to favour the assumption
(which Von Hartmann postulates as necessary for success) that many
individuals varied into white at the same time. We know no single
extra-polar species of a dark colour which frequently, _i.e._ in many
individuals of every generation, varies into white, but we know many
species which from time to time produce single white individuals. Now
when, on the other hand, we find that all polar animals to which the
white coloration is advantageous, and indeed none but species of which
the nearest allies vary only individually into white, possess this
colour, must we not conclude from this alone that _single_ variations
can, under favourable conditions, become predominant characters?
It appears to me that in this question one weighty factor has been too
little regarded, even by the supporters of the selection theory, viz.,
the slowness of most, and especially of climatic changes, which I have
already insisted upon. If the transformation of a temperate into an
arctic climate occurred so rapidly that the species exposed to it had
the alternative either of becoming white in ten or twenty generations
or of being unable to exist, then the hasty intervention of a directive
power could alone save them from extermination by causing hundreds of
thousands of individuals to become similarly coloured with all speed.
But it is quite different if the change of climate takes place only in
the course of several thousand generations; and this, according to the
geological evidence, must have been the true state of the case.
Let us take a definite example--the well-known one of the hare. With
us this animal remains brown in the winter and but seldom produces
white varieties, whilst its ally the Alpine hare is white during
seven months of the year, the Norwegian hare during nine months, and
the Greenland hare throughout the whole year. If our climate became
transformed into an arctic one, after a given time there would arrive
a period when the older coloration no longer possessed any advantage
over the occasional and singly-appearing white variations; the winter
days during which the ground was covered with snow would have become so
numerous, that the protection afforded to the white animals would be
equal to the protection enjoyed by the brown individuals on the equally
numerous days free from snow. From this time forth the hares that
were white in winter would not be subjected to a greater decimation
by foxes, &c., than the brown individuals. This period must however
be represented as consisting of one or more centuries, and it would
be strange if from the individual white hares, which now had an equal
chance of existing, some white families did not become established.
But the state of affairs would gradually become reversed--the brown
hares would experience greater decimation, and wherever there were
white families these would possess an advantage in the struggle for
existence. It does not follow that the dark individuals would be
forthwith extirpated; on the contrary, the advantage in favour of the
white would be but small throughout a long period of time, and these
individuals would only gradually increase to a higher percentage of
the total population; nevertheless their numbers would constantly but
very slowly augment. In the course of time this increase would become
more rapid for two reasons--first, because even a very small advantage
in favour of the increasing number of individuals would always leave a
greater number of these victorious; and secondly, because on the whole
as the climate became more arctic, the advantage of being white would
continually become more decisive in determining which should live and
which should succumb.
Thus I see no reason why individual variations which do not appear only
once, but which frequently recur in the course of generations, should
not acquire predominance under favourable conditions. All facts are
in accord with this. Even the common hare shows us that it would be
quite capable of becoming coloured in a similar manner. In the museum
of Stuttgart there are three specimens of _Lepus timidus_, killed in
Wurtemburg, which are completely white, and several others which are
silver-grey or spotted with white. In eastern Russia the common hare
possesses a light grey, almost white, winter coat, and Seidlitz[294]
makes known the interesting observation that such light specimens occur
_singly_ in Livonia, where “the common hare has become naturalized
since the commencement of the century.”
As I have already insisted upon above, from the point of view
of the conditions of life there is no reason for assuming rapid
transformations; the change of conditions is almost always extremely
slow; and indeed in numerous instances no objective change occurs,
but simply a subjective one, if we may thus designate those cases in
which the alteration in the conditions of life depends upon a change
in the animal form which is undergoing transformation, and not in that
of the environment. This is the case in the above-mentioned instances
of mimicry, where the whole change in the conditions of life arises
from one species becoming similar to another. The process of natural
selection has here as long a period of time as it requires to perfect
its results. It is quite similar in all cases of special protective
adaptations of form and colour. In all these it is always _improvement_
that is concerned, and not the question “to be or not to be” with which
we have to deal.
It is just cases of this last kind, however, which are best fitted for
exposing the improbability and insufficiency of the assumption of a
variational tendency as a distinct directive power. We have only to
fix our attention upon some particular case of sympathetic colouring,
or, still better, of mimicry. A “tendency to variation” implies that a
large number of individuals produce varieties resembling the model to
be imitated, and this--at least according to Von Hartmann--must take
place in each of the successive generations, so that by this means,
combined with heredity, the useful variation becomes increased. But how
comes it that this “tendency to variation” coincides with the existence
of the model both in time and place? Can this be due to accident if
the two have not a common cause? The upholders of a directive power
will certainly not admit this; so that there remains only Leibnitz’s
assumption of a pre-established harmony contained in the first organic
germ, which, after innumerable transformations of the organic form
and after millions of years, gave rise in the midst of the Amazonian
region to an inedible Heliconide with certain yellow, black, and white
markings on the wings, and at precisely the same time developed the
tendency in a Pieride at the same spot on the globe to imitate this
Heliconide as a model!
In addition to this assumption, which is certainly but little worthy
of consideration, there is perhaps one other remaining, viz., that all
or many Pierides and other species of butterflies possessed the same
tendency to a Heliconoid variation and were always everywhere striving
to develop this type, but succeeded only where they accidentally
coincided in time and place with the model, the “tendency” being thus
furthered by natural selection. But the facts negative this assumption,
since such imitative variations have never been observed to a
perceptible extent in other species.[295]
All variations which are demonstrably useful can be similarly dealt
with if their origin is explained by variational tendencies.
We perceive that the objection which Von Hartmann brings against
heredity is only valid on the ground that this process affords no
security for the preservation of variations which occur singly. That
heredity itself is a mechanical process is not directly disputed; it is
simply assumed that new characters can be transferred by inheritance
only when they are produced by the metaphysical “developmental
principle,” and not when they arise “accidentally.” This critic does
not therefore direct his attack against heredity, but rather against
the mechanical origin of variability.
Von Hartmann might have said here that a reference of the phenomenon
of heredity to purely mechanical causes, _i.e._ a mechanical theory
of heredity, is up to the present time wanting. That he has not done
so proves on the one hand that he despised the dialectical art, but,
on the other hand, that he himself has not overlooked the subserviency
of the total phenomenon to law, and that he grants the possibility of
finding a mechanical explanation therefor. If, in fact, the power of
inheritance does not depend upon mechanical principles, I know not
what organic processes we are entitled to regard as mechanical, since
they are all dependent in essence upon heredity, with which process
they are at one, and from which they cannot be thought of as isolated.
Haeckel correctly designates reproduction as surplus individual
growth, and accordingly refers the phenomena of heredity to those of
growth. Conversely, growth may also be designated reproduction, since
it depends upon a continuous process of multiplication of the cells
composing the organism, from the germ-cell to the innumerable congeries
of variously differentiated cells of the highly developed animal body.
Who can fail to see that these two processes, the reproduction of the
germ-cell and its offspring in the economy of the individual, and the
reproduction of individuals and species in the economy of the organic
world, show an exact and by no means simply superficial analogy?[296]
But whoso grants this must also conceive both processes to depend upon
the same cause--he cannot assume for the one a causal power and for
the other a directive principle. If nutrition and cell-multiplication
are purely mechanical processes, so also is heredity. Although it
has not yet been possible to demonstrate the mechanism of this
phenomenon, it can nevertheless be seen broadly that by means of a
minimum of living organic matter (_e.g._ the protoplasm of the sperm
and germ-cell) certain motions are transferred, and these can be
regarded as directions of development, as I have already briefly laid
down in a former work.[297] The power of organisms to transmit their
properties to their offspring appears to me to be only conceivable in
such a manner “that the germ of the organism by its chemico-physical
composition together with its molecular structure, has communicated to
it a fixed direction of development--the same direction of development
as that originally possessed by the parental organism....” (_loc.
cit._ p. 24). This is confessedly nothing more than a hint, and we do
not learn therefrom the means by which developmental direction can be
possibly transferred to another organism.
Recently Haeckel, that indefatigable pioneer to whom we are indebted
for such a rich store of new ideas, has attempted to bridge over this
gap in his essay on “The Perigenesis of the Plastidule,” Berlin, 1876.
The basic idea, that heredity depends upon the transference of motion,
and variability upon a change of this motion, completely corresponds
with the conviction gained in the province of physical science, that
“all laws must finally be merged in laws of motion” (Helmholtz[298]).
I hold this view to be the more completely justifiable--although
certainly not in the remotest degree as proved--because I formerly
designated the acquired individual variations as the “diversion of the
inherited direction of development.” Haeckel’s hypothesis in so far
accomplishes more than Darwin’s pangenesis, in which a transference
of matter, and not of a species of motion peculiar to this matter, is
assumed. But although the germ of a mechanical theory of heredity may
be contained in Haeckel’s hypothesis, this nevertheless appears to me
to be somewhat remote from completely solving the problem. It brings
well into prominence one portion of the process of inheritance; under
the image of a molecular motion of the plastidule, which motion is
modifiable by external influences, we can well understand the fact of
a change gradually taking place in the course of generations. On the
other hand, the assumption of consciousness in the plastidule,--however
admissible philosophically--although only as a formula, scarcely
furnishes any deeper knowledge. In the light of a theory, detailed
instances which were formerly obscure should become comprehensible.
I fail to see, however, how the various forms of atavism, _e.g._ the
reversions which so commonly occur by crossing different races, become
more comprehensible by assuming consciousness in the plastidule. If
in both parents the plastidule long ago acquired different molecular
motions, why, in its rencounters in the germ, does it recollect past
times and reassume the older and long abandoned motion? That it does
acquire the latter is indeed a fact if we once refer the directional
development of the individual to molecular motion of the plastidule;
the wherefore does not appear to me, however, to become clearer by
assuming consciousness in the plastidule. A mechanical theory of
heredity must rather be able to show that the plastidule movements of
the male and female germ-cells, in their rencounter in the case of the
crossing of widely divergent forms, become mutually modified in such
a manner that the motion of the common ancestral form must occur as
the resultant. To such demonstration there is however as yet a long
step. Haeckel himself moreover points out that his hypothesis is by
no means a “mechanical theory of heredity,” but only an introduction
to this theory, which he hopes “will be capable of being elevated to
the rank of a genetic molecular theory” (_loc. cit._ p. 17). But
although we must also confess with the critic of the “Philosophy of the
Unconscious,” that “the facts of heredity have hitherto defied every
scientific explanation,”[299] this furnishes us with no excuse for
flying to a metaphysical explanation, “which is here certainly least
able to satisfy the inability to understand the connection arising from
natural laws.”
It is not to be wondered at that Von Hartmann, on the ground of the
“Unconscious” on which he takes his stand, speaks of the law of
correlation as an unconscious acknowledgment of a “non-mechanical
universal principle on the side of Darwinism.” By “correlation” he
understands something quite different to the idea which we attach to
this expression. He supposes that “Darwinism sees itself compelled
to acknowledge through empirical facts the uniform correlation of
characters pertaining to the specific type; but it thereby contradicts
its mechanical principles of explanation, all of which amount to the
same thing as conceiving the type as a mosaic, chequered, superficial,
and accidental aggregate of characters, which have been singly
acquired, contemporaneously or successively, by selection or habit.”
I do not believe, however, that any such conception has ever been
admitted either by Darwin or any one else. The admission that not all,
but only every deep-seated _physiological_ detailed modification, is
or may be bound up with a system of correlated changes, indeed implies
that we on our side also acknowledge an internal harmony of parts--an
equilibrium, as I have above expressed it.
But does this include the admission of a teleological principle, or
exclude a mechanical explanation? Do we thereby acknowledge a “specific
type” in the sense of an inseparably connected complex of characters,
none of which can be taken away without all the others becoming
modified? Does such a view agree generally with the empirical facts?
Neither of these views appears to me to represent the case.
I will first answer the second question. On all possible sides the
earlier view of the absolute nature of species is contradicted; there
is no boundary between species and varieties. But when Von Hartmann
assumes that by the transformation of one species “into another” the
“whole uniformly connected complex must become changed,” he falls
back into the old doctrine of the absolute nature of species, which
is sharply contradicted by multitudes of facts. We not unfrequently
observe varieties which differ from the parent-form by only a single
character, whilst others show numerous differences, and again others
may be seen in which the differences predominate. This last deviation
would then be designated by many systematists as a new species, but not
so by others.
The “specific type” is thus indeed a kind of mosaic-work, but it is
a structure to which all the single characters--the stones of the
mosaic--belong and build up one harmonious whole, and not a meaningless
confusion. Some of the stones or groups of stones can be taken away
and replaced by others differently coloured without the structure
being thereby necessarily distorted, _i.e._ destroyed as a structure;
but the larger the stones which are exchanged the more necessary will
corrections in the other parts of the structure become, in order that
the harmony of the whole may be preserved.
Still more weighty than those insensible transitions which in various
groups of animals so frequently connect species with species, appear
to me, however, the facts made known in the second essay of the
second part of this volume, which prove that the two forms in which
one species appears can change entirely independently of one another.
The caterpillar changes and becomes a new variety or even species
(according to the form-value of the change), whilst the butterfly
remains unaltered. How could this occur if some other law than that
of physiological equilibrium linked together the parts or characters
and permitted them to become severed? Must not the two stages become
changed with and through one another, like the parts of one body, since
they first together constitute the specific type? Is not the fact of
this not happening a proof that the whole “uniformly connected complex”
of the specific type is not bound and held together by a metaphysical
principle, but simply by natural laws?
Now when Von Hartmann comprises the relations of different species
to one another under the idea of correlation, such for instance as
the relation of dependence in which orchidaceous flowers stand with
respect to the insects which visit them, he completely abandons the
scientific conception which should be associated with this expression,
and compares together two heterogeneous things which have nothing in
common excepting that they are both considered by him as a result
of the “Unconscious.” The consequence which is then deduced from
this correlation of his own construction, viz., that an organic law
of correlation is only another expression for a “law of organic
development” in the sense of a metaphysical power, obviously cannot be
admitted.
By correlation we understand nothing more than the dependence of one
part of the organism upon the others and the mutual inter-relations
of these parts, which depend entirely upon a “physiological relation
of dependence,” as Von Hartmann himself has correctly designated it.
Herein is evidently comprised the total morphology of the organism--the
structure as a whole, the length, thickness and weight of the single
parts, as well as the histological structure of the tissues, since upon
all these depends the performance of the single parts. But when, under
correlation, Von Hartmann comprises “also a morphological, systematic,
inter-action of all the elements of the organism with reference both
to the typical ground-plan of the organization as well as to the
microscopic anatomical structure of the tissues,” he drags into the
idea something foreign to it, not on the ground of facts, but actually
in opposition to them, and supported only by a supposed “innate
developmental principle” which “is not of a mechanical nature.”
The living organism has already been often compared with a crystal, and
the comparison is, _mutatis mutandis_, justifiable. As in the growing
crystal the single molecules cannot become joined together at pleasure,
but only in a fixed manner, so are the parts of an organism governed
in their respective distribution. In the crystal where nothing but
homogeneous parts become grouped together their resulting combination
is likewise homogeneous, and it is obvious that they offer but very
little possibility of modification, so that the governing laws thus
appear restricted and immutable. In the organism, whether regarded
microscopically or macroscopically, various parts become combined, and
these therefore offer numerous possibilities of modification, so that
the governing laws are more complex, and appear less restricted and
unchangeable. In neither instance do we know the final causes which
always lead to a given state of equilibrium; in the case of a crystal
it has not occurred to anybody to ascribe the harmonious disposition
of the parts to a teleological power; why then should we assume such
a force in the organism, and thus discontinue the attempt, which has
already been commenced, to refer to its natural causes that harmony of
parts which is here certainly present and equally conformable to law?
On these grounds the assertion that the theory of selection is not an
attempt at a “mechanical” explanation of organic development appears to
me to be incorrect. Variability and heredity, as well as correlation,
admit of being conceived as purely mechanical, and must be thus
regarded so long as no more cogent reasons can be adduced for believing
that some force other than physico-chemical lies concealed therein.
But we certainly cannot remain at the purely empirical conception as
laid down by Darwin in his admirable work on the “Origin of Species.”
If the theory of selection is to furnish a method of mechanical
explanation, it is essential that its factors should be formulated in a
precise mechanical sense. But as soon as we attempt to do this it is
seen that, in the first enthusiasm over the newly discovered principle
of selection, the one factor of transformation contained in this
principle itself has been unduly pushed into the background, to make
way for the other more apparent and better known factors.
I have for many years insisted that the first, and perhaps most
important, or in any case the most indispensable, factor in every
transformation, is _the physical nature of the organism itself_.[300]
It would be an error to believe that it is entirely the external
conditions which determine what changes shall appear in a given
species; the nature of these changes depends essentially upon the
physical constitution of the species itself, and a modification
actually arising can obviously be only regarded as the resultant of
this constitution and of the external influences acting thereon.
But if an essential or perhaps even a preponderating share in
determining new characters is to be undoubtedly ascribed to the
organism itself, for a mechanical representation of organic
developmental processes everything depends upon our being able
to conceive this most important factor in a definite theoretical
manner, and to comprise under one common point of view its apparently
contradictory manifestations of constancy and variability.
Now every change of considerable extent is certainly considered by
Darwin to be the direct or indirect consequence of external actions;
but indirect action always presupposes a certain small variability
(individual variability), without which larger modifications cannot
be brought about. Empirically this small amount of variability is
doubtless present, but the question arises, upon what does it depend?
Can it be conceived as arising mechanically, or is it perhaps just at
this point that the metaphysical principle steps in and offers those
minute variations which make possible that course of development which,
according to this view, is immutably pre-determined? It is certainly
the absence of a theoretical definition of variability which always
leaves open a door for smuggling in a teleological power. A mechanical
explanation of variability must form the basis of this side of the
theory of selection.
This explanation is not difficult to find. All dissimilarities of
organisms must depend upon the individuals having been affected by
dissimilar external influences during the course of the development of
organic nature. If we ascribe to the organism the power of giving rise
by multiplication only to exact copies of itself, or, more correctly,
the power of transmitting unaltered to its successors the motion of its
own course of development, each “individual variation” must depend upon
the power of the organism to react upon external influences, _i.e._ to
respond by changes of form and of function, and consequently to modify
its original (inherited) developmental direction.
It has sometimes been insisted upon, that the “individuals of the same
species” or the offspring of one mother cannot be absolutely equal,
because, from the commencement of their existence, they have been
subjected to dissimilar actions of the environment. But this implies
that by perfectly equal influences they would become equal, _i.e._
it supposes that variability is not inseparably bound up with the
essence of the organism, but is only the consequence of developmental
tendencies which are in themselves equal being unequally influenced. As
a matter of fact the first germs of an individual certainly cannot be
supposed to be perfectly equal, because the individual differences of
the ancestors must be contained therein in different degrees according
to their constitution, and we should have to go back to the primordial
organism of the earth in order to find a perfectly homogeneous root,
a _tabula rasa_ from which the descendants would commence their
development. Whether such a homogeneous root ever existed is however
doubtful; it is much more probable that _numerous_ organisms first
arose spontaneously,[301] and these cannot be presumed to have been
absolutely equal, since the conditions under which they came into
life cannot have been perfectly identical. Let us, however, for the
sake of simplicity assume a single primordial organism; the first
generation which took its rise from this by reproduction could only
have possessed such individual differences as were produced by the
action of dissimilar external influences. But the third generation,
together with self-acquired, would also have shown _inherited_,
dissimilarities, and in each succeeding generation the number of
tendencies to individual difference imparted to the germ by heredity
must have increased to a certain degree, so that it may be said that
all germs, from their first origination, bear in themselves a tendency
to show individual peculiarities, and would develop these even if
they should not be again affected by dissimilar influences. This is
obviously the case, since the youngest egg-cells in the ovary of an
animal are, as can be demonstrated, always exposed to unequal external
conditions with respect to nutrition and pressure.[302] Hence, if it
were possible that two germs were exactly equal with respect to the
direction of development imparted to them by heredity, they would
nevertheless furnish two incongruent individuals; and if, conversely,
it were possible that two individuals could be exposed to absolutely
the same external influences from the formation of the embryo, these
also could not be identical, because the individual differences of the
ancestors would entail small differences, even in asexual reproduction,
in the direction of development transmitted to the egg. The differences
between individuals of similar origin thus finally depend entirely upon
the dissimilarity of external influences--on the one side upon those
which divert the development of the progenitors, and on the other side
upon those which divert the individual itself from its course, _i.e._
from the developmental direction transmitted hereditarily. Although
I thus essentially agree with Darwin and Haeckel in so far as these
authors refer the “universal individual dissimilarity” to dissimilar
external actions, I differ from Darwin in this, that I do not see an
essential distinction between the direct and indirect production of
individual differences, if by the latter is meant only the unequal
influencing of the germ in the parental organism. Haeckel is certainly
correct in referring the “primitive differences of the germs produced
by the parents” to the inequalities of nutrition to which the single
germs must inevitably have been exposed in the parent organism;
but another dissimilarity of the germs must evidently be added--a
dissimilarity which has nothing to do with unequal nutrition, but
which depends upon unequal inheritance of the individual differences
of the ancestors, a source of dissimilarity which must arise to a
greater extent in sexual than in asexual reproduction. Just as in
sexual propagation there occurs a blending of the characters (or
more precisely, developmental directions) of two _contemporaneous
individuals_ in _one_ germ, so in every mode of reproduction there meet
together in the same germ the characters of a whole _succession of
individuals_ (the ancestral series), of which the most remote certainly
make themselves but seldom felt in a marked degree.
The fact of individual variability can in this way be well understood;
the living organism contains in itself no principle of variability--it
is the _statical element_ in the developmental processes of the
organic world, and would always reproduce exact copies of itself if the
inequality of the external influences did not affect the developmental
course of each new individual; these influences are therefore the
_dynamical elements_ of the process.
From this conception of variability two important empirically
established facts can be theoretically deduced, viz. the limitability
of variation with respect to quality, which has already been previously
mentioned, and the origination of transformations by the direct action
of external conditions of life.
If the differences in individuals of the same origin depend upon
the action of unequal influences, variation itself is nothing else
than the reaction of the organism to a definite external inciting
cause, the quality of the variation being determined by the quality
of the inciting cause and by that of the organism. In the cases of
individual variation hitherto considered, the quality of the organism
is equal but that of the inciting cause is unequal, and in this way
there arise minute differences in organisms of an equal physical
constitution--variations of a different quality.
The same result, viz., different qualities of variation, may also arise
in a reverse manner by organisms of a different physical nature being
affected by equal external influences. The response of the organism to
the cause inciting change would be different according to its nature,
or, in other words, organisms of different natures react differently
when affected by equal modifying influences. The physical nature of
the organism plays the chief part with respect to the quality of the
variations; each specific organism can thus give rise to extremely
numerous, but not to all conceivable, variations; that is, only to
such variations as are made possible by its physical composition.
From this it follows further that the possibilities of variation in
two species are more widely different, the wider they diverge in
physical constitution (including bodily morphology)--that a cycle of
variation is peculiar to every species. In this manner we are led to
the knowledge that there must certainly exist a “fixed direction of
variation,” but not in the sense of Askenasy and Von Hartmann, as the
result of an unknown internal principle of development, but as the
necessary, _i.e._ mechanical, consequence of the unequal physical
nature of the species, which must respond even to the same inciting
cause by unequal variations.
The facts, as far as we know them, agree very well with this
conclusion. Allied species vary in a similar manner, whilst species
which are more distantly related vary in a different manner, even
when acted upon by the same external influences. Thus, in the first
part of these “Studies” I have remarked that many butterflies under
the influence of a warm climate acquire an almost black coloration
(_Polyommatus Phlæas_), whilst on the other hand others become lighter
(_Papilio Podalirius_).
We can thus understand why always certain courses of development are
followed, a fact which cannot be completely explained by the nature of
the conditions of life which induce the variations. But as soon as we
clearly perceive that the quality of the changes essentially depends
upon the physical nature of the organism itself, we arrive at the
conclusion that species of widely diverging constitutions must give
rise to different variations, whilst those of allied constitutions
would produce similar variations. But definite courses of development
are thus traced out, and we perceive that from any point of the organic
developmental series, it is impossible that any other point can be
attained at pleasure. Variation in a definite direction thus by no
means necessitates the acknowledgment of a metaphysical developmental
principle, but can be well conceived as the mechanical result of the
physical constitution of the organism.
The manner in which the dissimilar physical constitution of organisms
must arise can also be easily shown, although the first commencement of
the whole developmental series, _i.e._ the oldest living forms must be
assumed to have been almost homogeneous in their physical constitution.
The quality of the variation is, as said before, not merely the
product of the physical constitution, but the resultant of this and of
the quality of the changing external conditions. Thus from the first
“species” there proceeded, through the dissimilar influence of external
conditions of life, several new “species,” and as this took place the
former physical nature of the organism at the same time became changed,
necessitating also a new mode of reacting upon external influences,
_i.e._ another direction of variation. The difference from the primary
“species” must certainly be conceived as having been very minute, but
it must have increased with each new transformation, and must have
proceeded exactly parallel with the degree of physical change connected
with each transformation. Thus, hand in hand with the modifications,
the power of modification, or mode of reaction of the organism to
changing influences, must have continually become re-modified, and we
finally obtain an endless number of differently constituted living
forms, of which the variational tendencies are different in exact
proportion to their physical divergence, so that nearly allied forms
respond similarly, and widely divergent forms very differently, to the
same inciting causes.
Individual variation arises, as I have attempted to show, by each
individual having been continually affected by different, and indeed
by constantly changing, influences. Let us, however, imagine on the
contrary, that a large group of individuals is affected by the same
influences--in fact by such influences as the remaining individuals of
the species are not exposed to: this group of individuals would then
vary in a nearly similar manner, since both factors of variation, viz.
the external influence and the physical constitution, are equal or
nearly so. Such local variations would first become prominent when the
same external influence had acted upon a series of generations, and
the minima of variation produced in the individual by the once-exerted
action of the cause inciting change had become augmented by heredity.
Transformations of some importance (up to the form-value of species)
can thus arise simply by the direct action of the environment, in the
same way as that in which individual differences are produced--only
the latter fluctuate from generation to generation, since the inciting
influences continually change; whilst, in the former, the constant
external cause inciting modification always reproduces the same
variation, so that an accumulation of the latter can take place.
Climatic varieties can be thus explained.
A more efficacious augmentation of the variations arising in the single
individual is certainly brought about by the _indirect_ action of the
environment upon the organism. It is not here my intention to explain
once more the processes of natural selection; I mention this only in
order to point out that in these cases transformation depends upon a
_double action_ of the environment, since the latter first induces
small deviations in the organism by direct action, and then accumulates
by selection the variations thus produced.
By regarding variability in this manner--by considering each variation
as the reaction of the organism to an external action, as a diversion
of the inherited developmental direction, it follows that without a
change in the environment no advance in the development of organic
forms can take place. If we imagine that from any period in the earth’s
history the conditions of life remain completely unchanged, the species
present on the earth at this period would not, according to our view,
undergo any further modification. Herein is clearly expressed the
difference of this view from that other one according to which the
inciting principle of modification is not in the environment, but lies
in the organism itself in the form of a phyletic vital force.
I cannot here refrain from once more returning to the old (ontogenetic)
vital force of the natural philosophers, since the parallel between
this and its younger sister, the “phyletic vital force” which
appears in so many disguises, is indeed striking. Were the inciting
principle of the development of the individual actually an independent
vital force acting within the organism, the birth and growth of
the individual would be able to take place without the continuous
encroachment of the environment, such as occurs in nutrition and
respiration. Now this is known to be impossible, so that those who
support the existence of such a force, if any still exist, would be
driven to the obscure idea of a co-operation between the designing
power and the influences of the environment, just in the same manner
as such a co-operation is at present postulated by the defenders of
the phyletic vital force. I shall further on take the opportunity of
pointing out that this last idea is quite untenable; with respect to
the (ontogenetic) vital force any clearer proof cannot well be adduced,
but it will be admitted that the confused notion of the co-operation
and inter-action of teleological and causal powers is, from our
point of view, opposed to those very simple and clear ideas which
are in harmony with the views on phyletic development. As in racial
development each change of the organic type is entirely dependent upon
the action of the environment upon the organism, so in the development
of the individual, the totality of the phenomena of the personal life
must depend upon similar actions. Physiology, as is known, herein
entirely supports our view, since this shows that without the continual
alternating action of the environment and of the organism there can be
no life, and that vital phenomena are nothing but the reactions of the
organism to the influences of the environment.
It will be immediately perceived how exactly the processes of phyletic
and of ontogenetic development coincide, not merely in their external
phenomena but in their nature, if we trace the consequences of the
existing knowledge of the structure of the animal body. Although we
may not entirely agree with Haeckel’s doctrine of individuality in
its details, its correctness must on the whole be conceded, since it
cannot be disputed that the notion of individuality is a relative one,
and that several categories of morphological individuals exist, which
appear not only _singly_ as _physiological individuals_, _i.e._ as
independent living beings of lowest grade, but which can also _combine_
to form beings of a higher order.
But if we admit this, we should see with Haeckel nothing but
reproduction in the origination of a high organism from a single cell,
the egg; this reproduction being at the same time combined with various
differentiations of the offspring, _i.e._ with adaptations of the
latter to various conditions of life. Not even in the fact that the
tissues and organs of a single physiological individual stand in great
dependence upon one another through physical causes,[303] is there any
striking difference between this view and the phyletic composition of
the animal (and vegetable) kingdom out of physiological individuals
(Haeckel’s “_Bionten_”), since contemporaneous animals (individuals and
species) are known to influence one another in the most active manner.
Now if we further consider that the same units (cells) which, by
their reproduction and division of labour, at present compose the
body of the highest organism, must at one time have constituted as
independent beings the beginning of the whole of organic creation, and
that consequently the same processes (division of cells) which now
lead to the formation of a mammal, at that time led only to a long
series of different independent beings, it will be admitted that both
developmental series must depend upon the same inciting powers, and
that with reference to the causes of the phenomena it is not possible
that any great gap can exist between ontogeny and phylogeny, _i.e._
between the life-phenomena of the individual and those of the type.
According to our view both depend upon that co-operation of the same
material physical forces which admits of being briefly summarized
as the reaction of organized living matter to influences of the
environment.
Our opponents either cannot boast of such harmony in their conception
of nature, or else they must, together with the phyletic vital force,
re-admit into their theory the old ontogenetic vital force. I know not
indeed why they should not do so. Whoever inclines to the view that
organic nature is governed not merely by causal, but at the same time
by teleological, forces, may admit that the latter are as effective as
inciting causes of individual, as they are of phyletic, development.
According to my idea they are even bound to admit this, since it cannot
be perceived why the adaptations of the ontogeny should not depend upon
the same metaphysical principle assumed for each individual, as the
adaptations of the phylogeny; the latter are indeed only brought about
by the former. I believe therefore that the vital force (ontogenetic)
of the ancients stands or falls with the modern (phyletic) vital force.
We must admit both or neither, since they both rest on the same basis,
and are supported or opposed by the same arguments. Whoever feels
justified in setting up a metaphysical principle where complete proof
that _known forces_ are sufficient for the explanation of the phenomena
has not yet been adduced, must do the same with respect to individual,
as he does to phyletic, development, since this proof is in both cases
very far from being complete, and still contains large and numerous
gaps.[304]
The theoretical conception of variation as the reaction of the organism
to external influences has also not yet been experimentally shown to
be correct. Our experiments are still too coarse as compared with the
fine distinctions which separate one individual from another; and
the difficulty of obtaining clear results is greatly increased by
the circumstance that a portion of the individual deviations always
depends upon heredity, so that it is frequently not only difficult,
but absolutely impossible, to separate those which are inherited from
those which are acquired. Still further are we removed from being
able to refer variation to its final mechanical causes, _i.e._ from a
mechanical theory of reproduction, which would bring within the range
of mathematical calculation both the phenomena of stability (heredity)
and of change (variability).
But although sufficient proofs of the correctness of the views
here advocated cannot at present be adduced, these views are not
contradicted by any known facts--they are, on the contrary, supported
by many facts which they in turn make comprehensible (local forms,
different cycles of variation in heterogeneous species). These views
are finally completely justified by their furnishing the only possible
theoretical formulation of variability on which a mechanical conception
of organic development can be based. That such a conception is not only
admissible, but is unavoidable, at least to the naturalist, I have
already attempted to prove.