In the first of the three preceding essays it was attempted to solve
the question whether the transformations of a given complex of
characters in a certain systematic group could be completely explained
by the sole aid of Darwinian principles. It was attempted to trace
the origin of the marking and colouring of the Sphinx-caterpillars to
individual variability, to the influences of the environment, and to
the laws of correlation acting within the organism. These principles
as applied to the origin of a certain well-defined, although narrowly
restricted range of forms, were tested in order to see whether they
were alone sufficient to explain the transformation of the forms.
It appeared that this was certainly the case. In all instances, or
at least where the facts necessary to obtain a complete insight were
available, the transformations could be traced to these known factors;
there remained no inexplicable residual phenomena, and we therefore had
no reason for inferring the existence of some still unknown modifying
cause lying concealed in the organism. In this region of the marking
and colouring of caterpillars, the assumption of a phyletic vital force
had to be abandoned, as being superfluous for the explanation of the
facts.
In the second essay the attempt was next made with reference to double
form-relationship, as presented for observation in metamorphic insects,
to draw conclusions as to the causes of the transformations. It
appeared here that form- and blood-relationship do not always coincide,
since the larvæ of a species, genus, or family, &c., may show quite
different form-relationships to their imagines. These facts alone told
very decisively against the existence of an internal developmental
power, so that the latter had likewise to be set aside by the method
of elimination, since the observed incongruences as well as the
congruences of form-relationship, found sufficient explanation in the
action of the environment on the organism.
This investigation thus also led to the denial of a phyletic vital
force.
In the third essay I finally sought to prove that the only case
of transformation of one species into another at present actually
observed[282], could not without further evidence be interpreted as
the result of the action of a phyletic vital force, but that more
probably we had here only an apparent case of new formation, which was
in reality but a reversion to a stage formerly in existence.
If this last investigation removes the only certain observation which
could have been adduced in favour of the hypothesis of a phyletic vital
force, so also do the two former essays show that this hypothesis, at
least in the case of insects, must be abandoned as inadequate.
The question now arises whether this conclusion, based on such a
limited range of inquiry, can also be applied to the other groups of
the organic world without further evidence.
The supporters of a principle of organic development will deny this in
each individual case, and will demand special proof for each group of
organisms; I believe this position, however, to be incorrect. Here,
if anywhere, it appears to me justifiable to apply the conclusions
inductively from special cases to general ones, since I cannot at all
see why a power of such pre-eminent and fundamental importance as a
phyletic vital force should have its activity limited to solitary
groups in the organic world. If such a power exists it must be the
inciting cause of organic development in general, and must be equally
necessary in every part of creation, as no advancement could take place
without it. In this case, however, the force would be recognizable and
demonstrable at every point; the phenomena should nowhere stand in
opposition to its admission, and should in no case be explicable or
comprehensible without it. The same laws and forces which caused the
development of one group of forms must underlie the development of the
whole organic world.
I therefore believe that we are correct in applying to the whole living
world the results furnished by the investigation of insects, and in
thus denying the existence of an innate metaphysical developmental
force.
There is, however, a quite distinct method which leads to the same
results, and to the preliminary, if not to the complete and definitive
rejection of such a principle; _the admission of this power is directly
opposed to the laws of natural science_, which forbid the assumption of
_unknown_ forces as long as it is not demonstrated that _known_ forces
are insufficient for the explanation of the phenomena. Now nobody will
assert that this has in any case been proved; the test of applying the
known factors of transformation has only just commenced, and wherever
it has been made they have proved sufficient as causal forces. Thus,
even without the foregoing special investigations we should deny a
phyletic vital force; the more so as its admission is fraught with
the greatest consequences, since it involves a renunciation of the
possibility of comprehending the organic world. We should, on this
assumption, at once cut ourselves off from all possible mechanical
explanation of organic nature, _i.e._ from all explanation conformable
to law. But this signifies no less than the renunciation of all further
inquiry; for what is investigation in natural science but the attempt
to indicate the mechanism through which the phenomena of the world
are brought about? Where this mechanism ceases science is no longer
possible, and transcendental philosophy alone has a voice.
This conception represents very precisely the well-known decision of
Kant:--“Since we cannot in any case know _à priori_ to what extent the
mechanism of Nature serves as a means to every final purpose in the
latter, or how far the mechanical explanation possible to us reaches,”
natural science must everywhere press the attempt at mechanical
explanation as far as possible. This obligation of natural science will
be conceded even by those who lay great stress upon the necessity for
assuming a designing principle. Thus, Karl Ernst von Baer states that
we have no right “to assert of the individual processes of Nature, even
when these evidently lead to a definite result, that some Mind has
originated them designedly. The naturalist must always commence with
details, and may then afterwards ask whether the totality of details
leads him to a general and final basis of intentional design.”[283]
But even if we are precluded on these grounds only from assuming
the existence of a directive power, _i.e._ a phyletic vital
force, for explaining detailed phenomena, and are at the same
time debarred from the possibility of arriving at a physical or
mechanical explanation--which amounts to no less than the abandoning
of the scientific position--it certainly cannot be asserted that
the development of the organic world is already conceived of as
a mechanical process. We rather acquiesce in the belief that the
processes both of organic and of inorganic nature depend most probably
upon purely causal powers, and that the attempt to refer these to
mechanical principles should not therefore be abandoned. There is no
ground for renouncing the possibility of a mechanical explanation, and
the naturalist _must not_ therefore resign this possibility; for this
reason he cannot be permitted to assume a phyletic power so long as it
is not demonstrated that the phenomena can never be understood without
such an assumption.
It cannot be raised as an objection that even for the explanation of
individual life a vital power was long ago admitted, as there was not
then sufficient material at hand to enable the phenomena of life to be
traced to physical forces. It is now no longer questionable that this
assumption was a useless error--a false method--at the time when made
certainly very excusable, since the aspect of the question was then,
owing to the imperfect basis of facts, very different to the present
analogous question as to the causes of derivative development. Thus,
although it is now easy to prove this assumption to be erroneous, it
was in the former sense correct, as it was in accordance with the
existing state of knowledge. At that time there was hardly one of the
numerous bridges which now connect inorganic with organic nature,
so that the supposition that life depended upon forces which had no
existence outside living beings was sufficiently near.
In any case the philosophers of that period cannot be blamed for
filling up the gaps in the existing knowledge by unknown powers,
and in this manner seeking to establish a finished system. The task
of philosophy is different to that of natural science; the former
strives at every period to set up a completely finished representation
of the universe in accordance with the existing state of knowledge.
Natural science on the other hand is only concerned in collecting
this knowledge; she need not therefore always finish off, and indeed
can never close her account, since she will never be in a position to
solve all problems.[284] But science must not for this reason pronounce
any question to be insoluble simply because it has not yet been
completely solved; this she does, however, as soon as she renounces
the possibility of a mechanical explanation by invoking the aid of a
metaphysical principle.
That this is the correct mode of scientific investigation is seen
by the abandoning of the (ontogenetic) vital force. The latter is
no longer admitted by anybody, now that we have turned from mere
speculation to the investigation of Nature’s processes; nevertheless
its non-existence has not been demonstrated, nor are we yet in a
position to prove that all the phenomena of life must be traced to
purely physico-chemical processes, to say nothing of our being actually
able to thus trace them. Von Baer also states “that the abolishment of
the vital force is an important advance; it is the reduction of the
phenomena of life to physico-chemical processes, although these indeed
still contain many gaps.” He points out how very far we are still
removed from being able to reduce to physical causes, the processes
through which the fertilized yelk of an egg becomes developed into a
chicken.
How comes it therefore that we all have a conviction that such a
complete reduction will in time become possible, or if not this, that
the development of the individual depends entirely upon the same forces
which are in operation without the organism? For what reason have we
rejected the “vital force”?
Simply because we see no reason for assuming that known forces are
insufficient for explaining the phenomena, and because we are not
justified in admitting directive forces as long as we have any hope of
one day furnishing a mechanical explanation.
But if it is not only permissible, but even necessary, to explain the
_ontogenetic_ vital power by known forces, and to commence to indicate
the mechanism which produces the individual life, why should it not
be equally necessary to abandon that assumption of a _phyletic_ vital
force which stifles any deeper inquiry, and to attempt to point out
that here also the co-operation of mechanical forces has brought about
the multitudinous and wonderful phenomena of the organic world?
The renunciation of the old vital force was certainly an immediate
consequence of the acquisition of new facts--of the knowledge that the
same compounds which compose organic bodies can be produced without the
latter. This discovery, due to Wöhler and his followers, showed that
organic products could be prepared artificially.[285] In brief, the
decline of the vital force followed from the knowledge that at least
one portion of the processes of life was governed by known forces.
But in the domain of the development of the organic world have we not
quite analogous proofs of the efficacy of known forces? Is not the
_variability_ of all types of forms a fact? and must not this under the
action of natural selection and heredity lead to permanent changes? Has
not the problem of explaining the subserviency of all organic form to
law as a _result_ without invoking its aid as a _principle_ been thus
successfully solved? It is true that we have not directly observed the
process of natural selection from beginning to end; neither has anybody
directly observed the mode in which the heat of the animal body is
generated by the processes of combustion going on in the blood and
in the tissues; nevertheless, this is believed as a certainty, and a
“vital force” is not invoked.
Now the above-mentioned Darwinian principles of transmutation are
certainly not simple forces of nature like those underlying the
development of the individual, _i.e._ chemico-physical forces, and it
cannot be said _à priori_ whether in one of these principles--perhaps
in variability or in correlation--there may not lie concealed a
metaphysical principle in addition to the physical forces. In fact it
has lately been asserted by Edward von Hartmann[286] that the theory
of selection is not a mechanical explanation, since it combines forces
which are only partly mechanical and in part directive.
It must therefore be next investigated whether this assertion is
tenable.