HOW IS ORGANIC EVOLUTION CAUSED?
§ 143. Already it has been necessary to speak of the causes of organic
evolution in general terms; and now we are prepared for considering them
specifically. The task before us is to affiliate the leading facts of
organic evolution, on those same first principles conformed to by evolution
at large.
Before attempting this, however, it will be instructive to glance at the
causes of organic evolution which have been from time to time alleged.
§ 144. The theory that plants and animals of all kinds were gradually
evolved, seems to have been at first accompanied only by the vaguest
conception of cause--or rather, by no conception of cause properly so
called, but only by the blank form of a conception. One of the earliest who
in modern times (1735) contended that organisms are indefinitely
modifiable, and that through their modifications they have become adapted
to various modes of existence, was De Maillet. But though De Maillet
supposed all living beings to have arisen by a natural, continuous process,
he does not appear to have had any definite idea of that which determines
this process. In 1794, in his _Zoonomia_, Dr. Erasmus Darwin gave reasons
(sundry of them valid ones) for believing that organized beings of every
kind, have descended from one, or a few, primordial germs; and along with
some observable causes of modification, which he points out as aiding the
developmental process, he apparently ascribes it, in part, to a tendency
given to such germ or germs when created. He suggests the possibility "that
all warm-blooded animals have arisen from one living filament, which THE
GREAT FIRST CAUSE endued with animality, with the power of acquiring new
parts, attended with new propensities, directed by irritations, sensations,
volitions, and associations; and thus possessing the faculty of continuing
to improve by its own inherent activity." In this passage we see the idea
to be, that evolution is pre-determined by some intrinsic proclivity. "It
is curious," says Mr. Charles Darwin, "how largely my grandfather, Dr.
Erasmus Darwin, anticipated the erroneous grounds of opinion, and the views
of Lamarck." One of the anticipations was this ascription of development to
some inherent tendency. To the "plan général de la nature, et sa marche
uniforme dans ses opérations," Lamarck attributes "la progression évidente
qui existe dans la composition de l'organisation des animaux;" and "la
_gradation_ régulière qu'ils devroient offrir dans la composition de leur
organisation," he thinks is rendered irregular by secondary causes.
Essentially the same in kind, though somewhat different in form, is the
conception put forth in the _Vestiges of Creation_; the author of which
contends "that the several series of animated beings, from the simplest and
oldest up to the highest and most recent, are, under the providence of God,
the results, _first_, of an impulse which has been imparted to the forms of
life, advancing them, in definite times, by generation, through grades of
organization terminating in the highest dicotyledons and vertebrata;" and
that the progression resulting from these impulses, is modified by certain
other causes. The broad contrasts between lower and higher forms of life,
are regarded by him as implying an innate aptitude to give birth to forms
of more perfect structures. The last to re-enunciate this doctrine has been
Prof. Owen; who asserts "the axiom of the continuous operation of creative
power, or of the ordained becoming of living things." Though these words do
not suggest a very definite idea, yet they indicate the belief that organic
progress is a result of some in-dwelling tendency to develop,
supernaturally impressed on living matter at the outset--some ever-acting
constructive force which, independently of other forces, moulds organisms
into higher and higher forms.
In whatever way it is formulated, or by whatever language it is obscured,
this ascription of organic evolution to some aptitude naturally possessed
by organisms, or miraculously imposed on them, is unphilosophical. It is
one of those explanations which explain nothing--a shaping of ignorance
into the semblance of knowledge. The cause assigned is not a true
cause--not a cause assimilable to known causes--not a cause that can be
anywhere shown to produce analogous effects. It is a cause unrepresentable
in thought: one of those illegitimate symbolic conceptions which cannot by
any mental process be elaborated into a real conception. In brief, this
assumption of a persistent formative power inherent in organisms, and
making them unfold into higher types, is an assumption no more tenable than
the assumption of special creations: of which, indeed, it is but a
modification; differing only by the fusion of separate unknown processes
into a continuous unknown process.
§ 145. Besides this intrinsic tendency to progress which Dr. Darwin
ascribes to animals, he says they have a capacity for being modified by
processes which their own desires initiate. He speaks of powers as "excited
into action by the necessities of the creatures which possess them, and on
which their existence depends;" and more specifically he says that "from
their first rudiment or primordium, to the termination of their lives, all
animals undergo perpetual transformations; which are in part produced by
their own exertions, in consequence of their desires and aversions, of
their pleasures and their pains, or of irritations, or of associations; and
many of these acquired forms or properties are transmitted to their
posterity." While it embodies a belief for which much may be said, this
passage involves the assumption that desires and aversions, existing before
experiences of the actions to which they are related, were the originators
of the actions, and therefore of the structural modifications caused by
them. In his _Philosophie Zoologique_, Lamarck much more specifically
asserts "le _sentiment intérieur_," to be in all creatures that have
developed nervous systems, an independent cause of those changes of form
which are due to the exercise of organs: distinguishing it from that simple
_irritability_ possessed by inferior animals, which cannot produce what we
call a desire or emotion; and holding that these last, along with all "qui
manquent de système nerveux, ne vivent qu'à l'aide des excitations qu'ils
reçoivent de l'extérieur." Afterwards he says--"je reconnus que la nature,
obligée d'abord d'emprunter des milieux environnants la _puissance
excitatrice_ des mouvements vitaux et des actions des animaux imparfaits,
sut, en composant de plus en plus l'organisation animale, transporter cette
puissance dans l'intérieur même de ces êtres, et qu'à la fin, elle parvint
à mettre cette même puissance à la disposition de l'individu." And still
more definitely he contends that if one considers "la _progression_ qui se
montre dans la composition de l'organisation," ... "alors on eût pu
apercevoir comment les _besoins_, d'abord réduits à nullité, et dont le
nombre ensuite s'est accru graduellement, ont amené le penchant aux actions
propres à y satisfaire: comment les actions devenues habituelles et
énergiques, ont occasionné le développement des organes qui les exécutent."
Now though this conception of Lamarck is more precisely stated, and worked
out with much greater elaboration and wider knowledge of the facts, it is
essentially the same as that of Dr. Darwin; and along with the truth it
contains, contains also the same error more distinctly pronounced. Merely
noting that desires or wants, acting directly only on the nervo-muscular
system, can have no immediate influence on very many organs, as the
viscera, or such external appendages as hair and feathers; and observing,
further, that even some parts which belong to the apparatus of external
action, such as the bones of the skull, cannot be made to grow by increase
of function called forth by desire; it will suffice to point out that the
difficulty is not solved, but simply slurred over, when needs or wants are
introduced as independent causes of evolution. True though it is, as Dr.
Darwin and Lamarck contend, that desires, by leading to increased actions
of motor organs, may induce further developments of such organs; and true,
as it probably is, that the modifications hence arising are transmissible
to offspring; yet there remains the unanswered question--Whence do these
desires originate? The transference of the exciting power from the exterior
to the interior, as described by Lamarck, begs the question. How comes
there a wish to perform an action not before performed? Until some
beneficial result has been felt from going through certain movements, what
can suggest the execution of such movements? Every desire consists
primarily of a mental representation of that which is desired, and
secondarily excites a mental representation of the actions by which it is
attained; and any such mental representations of the end and the means,
imply antecedent experience of the end and antecedent use of the means. To
assume that in the course of evolution there from time to time arise new
kinds of actions dictated by new desires, is simply to remove the
difficulty a step back.
§ 146. Changes of external conditions are named, by Dr. Darwin, as causes
of modifications in organisms. Assigning as evidence of original kinship,
that marked similarity of type which exists among animals, he regards their
deviations from one another, as caused by differences in their modes of
life: such deviations being directly adaptive. After enumerating various
appliances for procuring food, he says they all "seem to have been
gradually produced during many generations by the perpetual endeavour of
the creatures to supply the want of food, and to have been delivered to
their posterity with constant improvement of them for the purposes
required." And the creatures possessing these various appliances are
considered as having been rendered unlike by seeking for food in unlike
ways. As illustrating the alterations wrought by changed circumstances, he
names the acquired characters of domestic animals. Lamarck has elaborated
the same view in detail: using for the purpose, with great ingenuity, his
extensive knowledge of the animal kingdom. From a passage in the
_Avertissement_ it would at first sight seem that he looks upon direct
adaptation to new conditions as the chief cause of evolution. He says--"Je
regardai comme certain que le _mouvement des fluides_ dans l'intérieur des
animaux, mouvement qui c'est progressivement accéléré avec la composition
plus grande de l'organisation; et que _l'influence des circonstances_
nouvelles, à mesure que les animaux s'y exposèrent en se répandant dans
tous les lieux habitables, furent les deux causes générales qui ont amené
les différents animaux à l'état où nous les voyons actuellement." But
elsewhere the view he expresses appears decidedly different from this. He
asserts that "dans sa marche, la nature a commencé, et recommence encore
tous les jours, par former les corps organisés les plus simples;" and that
"les premières ébauches de l'animal et du végétal étant formées dans les
lieux et les circonstances convenables, les facultés d'une vie commençante
et d'un mouvement organique établi, ont nécessairement développé peu à peu
les organes, et qu'avec le temps elles les ont diversifies ainsi que les
parties." And then, further on, he puts in italics this proposition:--"_La
progression dans la composition de l'organisation subit, çà et là, dans la
série générale des animaux, des anomalies opérées par l'influence des
circonstances d'habitation, et par celle des habitudes contractées._"
These, and sundry other passages, joined with his general scheme of
classification, make it clear that Lamarck conceived adaptive modification
to be, not the cause of progression, but the cause of irregularities in
progression. The inherent tendency which organisms have to develop into
more perfect forms, would, according to him, result in a uniform series of
forms; but varieties in their conditions work divergences of structure,
which break up the series into groups: groups which he nevertheless places
in uni-serial order, and regards as still substantially composing an
ascending succession.
§ 147. These speculations, crude as they may be considered, show much
sagacity in their respective authors, and have done good service. Without
embodying the truth in definite shapes, they contain adumbrations of it.
Not directly, but by successive approximations, do mankind reach correct
conclusions; and those who first think in the right direction, loose as may
be their reasonings, and wide of the mark as their inferences may be, yield
indispensable aid by framing provisional conceptions and giving a bent to
inquiry.
Contrasted with the dogmas of his age, the idea of De Maillet was a great
advance. Before it can be ascertained how organized beings have been
gradually evolved, there must be reached the conviction that they _have_
been gradually evolved; and this conviction he reached. His wild notions
about the way in which natural causes acted in the production of plants and
animals, must not make us forget the merit of his intuition that animals
and plants _were_ produced by natural causes. In Dr. Darwin's brief
exposition, the belief in a progressive genesis of organisms is joined with
an interpretation having considerable definiteness and coherence. In the
space of ten pages he not only indicates several of the leading classes of
facts which support the hypothesis of development, but he does something
towards suggesting the process of development. His reasonings show an
unconscious mingling of the belief in a supernaturally-impressed tendency
to develop, with the belief in a development arising from the changing
incidence of conditions. Probably had he pursued the inquiry further, this
last belief would have grown at the expense of the first. Lamarck, in
elaborating this general conception, has given greater precision both to
its truth and to its error. Asserting the same imaginary factors and the
same real factors, he has traced out their supposed actions in detail; and
has, in consequence, committed himself to a greater number of untenable
positions. But while, in trying to reconcile the facts with a theory which
is only an adumbration of the truth, he laid himself open to the criticisms
of his contemporaries; he proved himself profounder than his contemporaries
by seeing that natural genesis, however caused, has been going on. If they
were wise in not indorsing a theory which fails to account for a great part
of the facts; they were unwise in ignoring that degree of congruity with
the facts, which shows the theory to contain some fundamental verity.
Leaving out, however, the imaginary factors of evolution which these
speculations allege, and looking only at the one actual factor which Dr.
Darwin and Lamarck assign as accounting for some of the phenomena; it is
manifest, from our present stand-point, that this, so far as it is a cause
of evolution, is a proximate cause and not an ultimate cause. To say that
functionally-produced adaptation to conditions originates either evolution
in general, or the irregularities of evolution, is to raise the further
question--why is there a functionally-produced adaptation to
conditions?--why do use and disuse generate appropriate changes of
structure? Neither this nor any other interpretation of biologic evolution
which rests simply on the basis of biologic induction, is an ultimate
interpretation. The biologic induction must itself be interpreted. Only
when the process of evolution of organisms is affiliated on the process of
evolution in general, can it be truly said to be explained. The thing
required is to show that its various results are corollaries from first
principles. We have to reconcile the facts with the universal laws of the
re-distribution of matter and motion.