TRANSFORMISM
The species is therefore a group of organisms all of which exhibit
the same morphological characters. This sameness is not absolute, for
the individuals composing the species may vary from each other with
respect to any one character. But the range of these variations is
limited. They fluctuate about an imaginary mean value which remains
constant in the case of a species which is not undergoing selection,
and is therefore nearly the same throughout a series of generations.
The formal characters which we regard as diagnostic of the species are
these imaginary mean ones.
It is possible to breed from stock a very great number of animals, all
of which are connected by a tie of blood-relationship, that is, all
have descended from the same ancestor or ancestors. Such an assemblage
of animals would resemble those assemblages living in the wild which
we call species, in that a certain morphological similarity would be
exhibited by all the individuals. If the breeding were conducted so as
to avoid selection, the range of variability would be very much the
same as that observed in the wild race. The two groups of animals--that
bred artificially, and that observed in natural conditions--would be
very much alike, and it is impossible to resist the conclusion that the
natural race, like the artificial one, is a family in the human sense,
that is, all the individuals composing it are connected together by a
tie of common descent.
Let us extend this reasoning to categories of organisms of higher
orders than species. We can associate together groups of species in
the same way that we associate together the individuals of the same
species. There are certain morphological characters which are common
to all the species in the category, but there are also differences
between specific group and specific group, and these differences may
be regarded as variations from the generic morphological type. All
the Cats, for instance, have certain characters in common: fully
retractile claws, a certain kind of dentition, certain cranial
characters, and so on. We postulate a feline type of structure, and
we then regard the characters displayed by the cat, lion, tiger,
leopard, etc., as deviations from this feline morphological type. Thus
we establish the Family Felidæ. But again we find that the Felidæ
together with the Canidæ, and many other species of animals, also
display common characters, dental and osteological chiefly, and we
express this resemblance by assembling all these families in one Order,
the Carnivora. The Carnivores, however, are only one large group of
Quadrupeds: there are many others, such as the Rodents, Ungulates,
Cetacea, etc., and all of these possess common characters. In all
of them the integument is provided with hairs, or other similarly
developed structures; all breathe by means of a diaphragm; in all,
the young are nourished by suckling the mammæ of the mother; and
all develop on a placenta. We therefore group them all in the Class
Mammalia. Now the Mammals possess an internal skeleton of which the
most fundamental part is an axial rod--the notochord--developing
to form a vertebral column; and this notochordal skeleton is also
possessed by the Birds, Reptiles, Amphibia, and Fishes. There are
also some smaller groups in which the notochord is present but does
not develop to form segmented vertebræ. Including these, we are able
to form a large category of animals--the Chordata--and this phylum is
sharply distinguished from all other cognate groups.
All animals and plants may be classified in a similar way. Insects,
Spiders, and Crustacea, for instance, are all animals in which the
body is jointed, each joint or segment being typically provided with
a pair of jointed appendages or limbs. Because of this similarity of
fundamental structure we include all these animals, with some others,
in one phylum, the Arthropoda. So also with the rest of the animal
kingdom, and similar methods may be extended to the classification of
the plants. A few small groups in each of the kingdoms are difficult to
classify, but it has been possible to arrange most living organisms in
a small number of sub-kingdoms or phyla, and even to attempt to trace
relationships between these various categories.
The mere systematic description of the organic world would have
resulted in such a reasoned classification apart altogether from any
notions of an evolutionary process. But the classification, originally
a conventional way of making a list of organisms, would at once
suggest morphological similarities. It would suggest that all the
Cats were Carnivores, that all the Carnivores were Mammals, and that
all the Mammals were Chordates. It would suggest that all Wasps were
Hymenoptera, that all Hymenoptera were Insects, and that all Insects
were Arthropods. It would establish a host of _logical_ relations
between animals of all kinds.
It would show us a number of groups of animals separated from each
other by morphological dissimilarities. But let us also consider all
those animals which lived in the past of the earth, and the remains of
which are found in the rocks as fossils. Including all the forms of
life known to Palæontology, we should find that the dissimilarities
between the various groups would tend to disappear. The gaps between
existing Birds and Reptiles, for instance, would become partially
bridged. Palæontology would also supplement morphology in another way.
The study of the structure of animals leads us to describe them as
“higher” and “lower”--higher in the sense of a greater complexity of
structure. Thus the body of a Carnivore is more complex than that of
a Fish, inasmuch as it possesses the homologues of the truly piscine
gills, but it also possesses a four-chambered heart instead of a
two-chambered one; and it possesses the mammalian lungs, diaphragm, and
placenta, structures which are not present in the Fish. Now, so far as
its imperfect materials go, palæontology shows us that the higher forms
of life appeared on the earth at a later date than did the lower forms.
The remains of Mammals, for instance, are first found in rocks which
are younger than (that is, they are superposed upon) those rocks in
which Reptiles first appear; and so also Reptiles appear later in the
rock series than do Fishes. Palæontology thus adds to the logical order
suggested by morphology a _chronological_ order of this nature: higher,
or more complex forms of life appeared at a later date in the history
of the earth than did lower or less complex ones.
A parallel chronological sequence would also be suggested by the
results of embryology. This branch of biology shows us that
all animals pass through a series of stages in their individual
development, or ontogeny. The earlier stages represent a simple type of
structure, usually a hollow ball of cells, but as development proceeds,
the structure of the embryo becomes more and more complex. The process
of development is continuous in many animals, but in others (perhaps in
most) larval stages appear, that is, development is interrupted, and
the animal may lead for a time an independent existence similar to that
of the fully developed form. Often these larval stages suggest types
of structure lower than that of the fully developed animal into which
they transform. Even if larval stages may not appear in the ontogeny,
it is very often the case that the developing embryo exhibits traces,
or at least reminiscences, of the types of morphology characteristic
of the animals which are lower or less complex than itself; thus the
piscine gills appear during the development of the tailed Amphibian,
and even in that of the Mammal, and then vanish, or are converted into
organs of another kind. The individual thus passes through a series
of developmental stages of increasing complexity: it repeats, in its
ontogeny, the palæontological sequence in a distorted and abbreviated
form.
It is true that the evidence afforded by palæontology is very meagre.
The preservation of the remains of organisms in the stratified rocks is
a very haphazard process, and it depends for its success on a series
of conditions that are not always present. As the surface of the earth
becomes better known, our knowledge of the life of the past will become
fuller, but there can be little doubt that whole series of organisms
must have existed in the past, and that no recognisable traces of
these are known to us. There is also no doubt that the sequences
indicated by palæontology are very incomplete: they are obscured and
shortened by many conditions. The earlier embryologists entertained
hopes that the study of embryology would reveal the direction of the
evolutionary process in many groups of animals: if the organism repeats
in its ontogeny the series of stages through which it passed in its
phylogenetic development, then a close study of the embryological
process ought to disclose these stages. Although these hopes have
not been realised, there is yet sufficient truth in the doctrine of
recapitulation to enable us to state that there is a rough parallelism
between the palæontological and embryological sequences.
We therefore state a plausible hypothesis when we assert that
different species may be related to each other in the same way that
the individuals of the same species are related, that is, by a tie
of blood-relationship; and that different genera, families, orders,
and so on are also so related. Morphological studies enable us to
arrange numbers of species in such a way that series, in each of which
there is an increasing specialisation of structure, are formed. Both
palæontology and embryology show, to some extent at least, that these
stages of ever-increasing specialisation of structure occurred one
after the other. Now, stated briefly and baldly as we have put it, this
argument may not appear to the general reader to possess much force,
but it is almost impossible to over-state the strength of the appeal
which it makes to the student of biology. To such a one a belief in a
process of transformism will appear to be inseparable from a reasoned
description of the facts of the science.
But it would be no more than a belief, not even a hypothesis, if we
did not attempt to verify it experimentally. It is merely logical
relationships that we establish, and the chronological succession of
forms of life, higher forms succeeding lower ones, does not itself do
more than suggest an evolutionary process. All that we have said is
compatible with a belief in a process of special creation. But if we
cling to such a belief, if we suppose that the organisms inhabiting
the earth, now and in the past, are the manifestations of a Creative
Thought, we must still accept the notion of logical and chronological
relationships between all these forms of life. If we permit ourselves
to speculate on the working of the Creative Thought, we seem to
recognise that the ideas of the different species must have generated
each other, and that the genesis of living things must have occurred
in some such order as is indicated by a scientific hypothesis of
transformism. An evolutionary process must have occurred somewhere, but
the kinships so established between organisms would be logical and not
material ones.
Science must not, of course, describe the mode of origin of species
in this way. So long as it investigates living things by the same
methods which it uses in the investigation of inorganic things, it
must hold that the concepts of physical science are also adequate for
the description of organic nature. It must assume that matter and
energy and natural law are given; and that, even in the conditions of
our world, life must have originated from lifeless matter; must have
shaped itself, and undergone the transformations that are suggested
by the results of biology. It must assume, in spite of the formidable
difficulties that the assumption encounters, that cosmic physical
processes are reversible and cyclical; and that worlds and solar
systems are born, evolve, and decay again. Every stage in such a cosmic
process, as well as every stage in the evolution of living things,
must have been inevitably determined by the stages preceding it. Such
a mechanistic explanation must assume that a superhuman intellect,
but still a finite intellect like our own, such a calculator as that
imagined by Laplace or Du Bois-Reymond, would be able to deduce any
state of the world, or universal system, from any other state, by means
of an immense system of differential equations. It would be able, as
Huxley says, to calculate the fauna of Great Britain from a knowledge
of the properties of the primitive nebulosity with as much certainty
as we can say what will be the fate of a man’s breath on a frosty day.
Such a fine notion as that of an universal mathematics must ever remain
as the ideal towards which science strives to approximate.
Or we may suppose that a plan or design has been superposed on nature,
is immanent in matter and energy, and works itself out, so to speak.
Such a teleological explanation of inorganic and organic evolution
inevitably forces itself upon us if we reject the notion of radical
mechanism. We think of an universal system of matter and energies as
consisting of elements which, when assembled together, interact in a
certain way, and with results which are definite and calculable. The
assembling together of the elements of the system would be the result
of the previous phases of the system. That is radical mechanism.
But let us think of the elements of the system as being differently
assembled--thus involving the idea of an agency, external to the
system, which rearranges them--then the same energies inherent in
this system, as in that previously imagined, will also work out by
themselves. But the result will be different, and will depend on the
manner in which the elements were originally arranged. That would be
radical finalism.
Science must reject this notion as it rejects that of special creation,
since it introduces indeterminism into the evolutionary process. It
must regard the organism and its environment as a physico-chemical
system studied from without. It must avoid all attempts to acquire an
intuitive knowledge of the actions of the organism, for the latter, and
the things which environ it, are only bodies moving in nature. In the
systems studied by it time must be the independent variable, and there
must be a strict functionality between the parts of the organism and
the parts of the reacting environment, so that any change in the one
must necessarily be dependent on a change in the other. Such a system
and series of interactions is that which is described in a mechanistic
hypothesis of transformism.
All this is indeed suggested to ordinary and aided methods of
observation. The plant or animal acts upon, and is acted on by, the
environment, though it is usually the modification of the organism to
which we attend. A man’s face becomes reddened by wind and sun and
rain; manual labour roughens his hands and develops callosities; in the
summer he sweats and loses heat; in the winter the blood-vessels of his
skin contract and heat is economised. In the winter months the fur of
many animals becomes more luxuriant and may change in colour. Fishes
which inhabit lightly coloured sand are lightly pigmented, but their
skins become dark when they move on to darkly coloured sea-bottoms;
prawns which are brown when they live on brown weed, become green when
they are placed on green weed. Birds migrate into warmer countries,
and _vice versa_, when the seasons change. Such are instances of the
adaptations of the morphology and functioning of organisms consequent
on changes of environment.
What is an adaptation? The term plays a great part in biological
speculation, but it is often used in a loose and inaccurate manner,
and not always in the same sense. It suggests that the organism is
_contained_ by the environment, and that its form becomes adapted to
that of the latter, just as the metal which the ironfounder pours into
the mould takes the form of the cavity in the sand. “We see once more
how plastic is the organism in the grasp of its environment”--such
a quotation from morphological literature is perhaps a typical one.
Over and over again this passive change in the organism as the result
of the action of something rigid which presses upon it is what is
understood by an adaptation. No doubt the organism may be so affected,
and often the change which it experiences is of the same order as the
environmental change. In the winter many animals become sluggish and
may hibernate; their heart-beats slow down; their respiratory movements
become less frequent, and generally the rate of metabolism, that is
the rapidity with which chemical reactions proceed in their tissues,
becomes lessened. All these changes become reversed in sign when the
temperature again rises. The time of year at which a fish spawns
depends on the nature of the previous season. The rate of development
of the egg of a cold-blooded animal varies with the temperature.
The quantity of starch formed in a green leaf depends on certain
variables--the intensity of light, the temperature, and the quantity
of carbonic acid contained in the medium in which it is placed. In
all these cases the rate at which certain metabolic processes go on
in the body of an organism varies according to the conditions of the
environment. In general they are cases of van’t Hoff’s law, that is,
the rapidity at which a chemical reaction proceeds varies according to
the temperature.
They are changes of functioning passively experienced by the organism
as the result of environmental changes, and we must clearly distinguish
between them and such changes as are the result of some activity or
effort on the part of the organism. A flounder which lives in a river
migrates out to sea when the first of the winter snows melt and flood
the estuary with ice-cold water. Brown or striped prawns living on
brown or striped weeds become green when they are placed on green
weed, changing their pigmentation to match that of the alga. A kitten
brought up in a cold-storage warehouse develops a sleeker and more
luxuriant coat than does its sister reared in a well-warmed house. An
animal which recovers from diphtheria forms an antitoxin which enables
it to resist, for a time at least, repeated infection. A man who goes
exploring in polar seas puts on warmer clothing than he wears in the
tropics.
It is not necessary that an environmental change should occur in
order that an adaptation should be evoked, for the organism may react
actively and purposefully to a change in itself. The athlete acquires
by running or rowing a more powerful heart; the blacksmith develops
more muscular shoulders and arms; and the professional pianist more
supple wrists and fingers. If one kidney is removed by operation,
or if one lung becomes diseased, the organ on the other side of the
body becomes hypertrophied. Aphasia, which is due to a lesion in the
unilateral speech-centre, may pass away if the previously unused centre
on the other side of the brain should become functionally active. In
general, the continued use of an organ leads to its increase in size
and efficiency, and conversely disuse leads to a decrease of size and
even to atrophy.
The essence of an adaptation is that it is an active, purposeful change
of behaviour, or functioning, or morphology, by which the organism
_responds_ to some change in its physical environment, or to some
other change in its own behaviour, or functioning, or morphology.
It is also a change which remains as a permanent character in the
organisation of the animal exhibiting it. It does not matter even if
the change of behaviour is one which is willed in response to some
change of environment actually experienced, or whether it anticipates
some change that is foreseen. A changed mode of behaviour adapted
intelligently leaves, at the least, a memory which becomes a permanent
part of the consciousness of the animal, and may influence its future
actions; or if it is evoked by a process of education it must involve
the establishment of a “motor habit.” The education of a singer sets
up, in the cortex and lower centres of the brain, a nervous mechanism
which controls and co-ordinates the muscles of the chest and larynx,
and which did not exist prior to the process of education. Adaptations
are therefore acquired changes of some kind or other by means of which
the organism is able to exert a greater degree of mastery over its
environment, including in the latter both the inert matter of inorganic
nature and the other organisms with which the animal competes.
They are acquirements because of which the organism deviates from
the morphological structure characteristic of the species to which
it belongs. Do they affect the entire organisation of the animal
exhibiting them, that is, may an acquired change of structure be so
fundamental that it affects not only the body of the animal in which
it occurs but also the progeny of this animal? Let us suppose that
this is the case; let us suppose that quite a large proportion of
all the individuals of a species inhabiting a restricted part of the
earth’s surface acquire the same change of character simultaneously
and that they transmit this deviation of structure to their progeny.
Then we should have an adequate means whereby the specific type becomes
modified--a means of transformism.
This is the hypothesis which is associated with the name of Lamarck,
and its essential postulate is that characters which are acquired by
an organism during its own lifetime are transmitted to its offspring.
It seems reasonable to suppose that this transmission of acquired
characters should occur--how reasonable we should note when we see that
de Vries tacitly assumes that fluctuating variations due to the action
of the environment may be inherited by the offspring of organisms which
exhibit them. That transmutation of species might occur in this way was
a popular and widespread belief in England and Germany throughout the
greater part of the nineteenth century; and it was a belief entertained
by Darwin himself, and confidently, and even dogmatically affirmed at
one time by the majority of biologists in both countries.
How was it, then, that a very general change of opinion with regard
to this question occurred both in England and Germany during the
last two decades of the last century? Certainly many botanists and
zoologists continued to adhere to the older hypothesis, and most
physiologists still do not appear to make any clear distinction between
morphological characters which are inherited and those which are
acquired; but the majority of biologists did not hesitate to conclude
that not only was the transmission of acquired characters an unproved
conjecture, but that it was even theoretically inconceivable. At the
beginning of the nineteenth century this belief had almost become a
doctrine dogmatically asserted, and one cannot fail to notice a tone
of irritation and impatience on the part of the spokesmen of zoology
when the contrary opinions are expressed. “Nature,” says Sir E. Ray
Lankester, “(and there’s an end of it) does not use acquired characters
in the making and sustaining of species for the very simple reason that
she cannot do so.”
There can be little doubt that the interrogation of nature with regard
to this question was not a very thorough process. The dogmatic denial
of the transmission of acquired characters was not the result of
exhaustive experiment and observation, but was due rather to the very
general acceptance in England and Germany of Darwin’s hypothesis of
the transmutation of species by means of natural selection, and of
Weismann’s hypothesis of the continuity of the germ-plasm.
The newer hypothesis of transmutation was one which seemed adequate to
account for the diversity of forms of life, so that it was unnecessary
to invoke the older one; though Darwin himself admitted that the
individual acquirement of structural modifications might be a factor
in the evolutionary process; and for more than twenty years after the
publication of the “Origin of Species” Lamarck’s hypothesis was not
strenuously denied by naturalists. Early in the ’eighties, however,
Weismann published his book on the germ-plasm, and the brilliancy and
constructive ability of the speculations contained in this remarkable
work, as well as the analogies which they suggested between organic
and inorganic phenomena, compelled the attention of biologists. The
essential parts of Weismann’s hypothesis, as it was first presented to
the world, are as follows: very early in the evolution of living from
non-living matter many kinds of life-substance came into existence.
These were chemical compounds of great complexity, able to accumulate
and expend energy, and capable of indefinite growth and reproduction.
They were able to exist in an environment which was hostile to them and
which tended always to their dissolution, and which was able to modify
their nature and their manner of reacting, though it could not destroy
them. These elementary life-substances were very different from those
which we know in the world of to-day. They were _naked_ protoplasmic
aggregates, undifferentiated into cellular or nuclear plasmata, much
less into somatic and germinal tissues. All of their parts were
similar, or rather their substance was homogeneous. But even with the
evolution of the unicellular organism a profound change was initiated,
for henceforth one part of the living entity, the nucleus, became
charged with the function of reproduction, although it still continued
to exercise general control over the functions of the extra-nuclear
part of the cell. When the multi-cellular plant and animal became
evolved, the heterogeneity of the parts of the organism became greater
still. All the cells of the metazoan animal do indeed contain nuclei,
but these structures are only the functional centres of the cells: some
of the latter are sensory, others motor, others assimilatory, others
excretory, and so on. Only in the nuclei which form the essential parts
of the reproductive organs does the reproductive function persist in
all its entire potentiality: there only does the protoplasm retain all
the properties which were possessed by the primitive life-substance
before it became heterogeneous, that is, before nucleus and cytoplasm
evolved. When part of the primitive life-substance became secluded in a
nuclear envelope, it became, to that extent, shielded from the action
of the physical environment, and when the organism became composed of
multicellular tissues this seclusion became more complete. Clothed
in the garments of the flesh, it was henceforth protected from the
shocks of the environment, and it became the immutable germ-plasm.
But for a very long time before this evolution of tissues the naked
life-substance had been exposed to the action of external physical
agencies, and it had been modified by these into very numerous
forms of protoplasmic matter. When multicellular plants and animals
had been evolved there were, therefore, not one, but many kinds of
life-substance in existence, and these have persisted until to-day as
the unchanging germ-plasmata of the existing organisms.
The Weismannian hypothesis of to-day, supported and amplified, as it
is, by subsidiary hypotheses, does not make the same appeal to the
student as did the pristine and altogether attractive speculation
of thirty years ago. The analogy which it then presented with the
matured chemical theory of matter must have been almost irresistible.
Just as the indefinitely numerous compounds of chemistry are only the
permutations and combinations of some of eighty-odd different kinds
of matter, so all the forms of life are combinations and permutations
of some of the many different kinds of life-substance which came into
existence before the evolution of the multicellular organism. And just
as the chemical elements were regarded (in 1883) as immutable things,
preserving their individuality even when they were associated together
as compounds, so Weismann and his followers looked upon the different
kinds of life-substance contained in the chromatic matter of the
nucleus as immutable and immortal living entities. Associated together
in indefinitely numerous ways by sexual conjugation, they may build up
indefinitely variable living structures, but they remain individualised
and lying side by side in the germ-plasmata of organisms, just as the
atoms were supposed to lie side by side in the chemical molecule of the
inorganic compound.[31]
[31] We know now that this statement is not quite accurate.
If these speculations were true, a change of morphology or functioning,
acquired by the body, or somatoplasm, could not possibly be transmitted
to the progeny of the organism, for by hypothesis the germ-plasm
cannot be affected by external changes, and it is only the germ-plasm
contained in the spermatozoon of the male parent, or in the ovum of
the female, that shapes and builds the body of the offspring. As if
this were not enough, Weismann and his followers argued that the
transmissibility of a somatic change to the germ was inconceivable.
Why? Because the germ-cells are apparently simple: they are only
semi-fluid protoplasmic cell bodies and nuclei, not differing
appreciably from the cell bodies and nuclei of the somatoplasm (by
hypothesis, it should be noted, the difference is profound). There are
no structural connections--no nerves, for instance--which join together
the cells of the bodily tissues with the parts of the germ and transmit
changes in the former to the latter. How, then, could a somatic change
affect the germ so that when the latter developed into an organism
this particular change became reproduced? Now this may have seemed a
conclusive argument in 1883, but is it so conclusive to-day? We know
that the cells and tissues are not isolated particles, but that all are
connected together by protoplasmic filaments. We know that specialised
nervous tissues are not necessary for the transmission of an impulse
from a sensory to a motor surface, but that such an impulse may be
transmitted by undifferentiated protoplasm. We know that nerve-cells
and nerve-fibres are not structurally continuous with each other but
that the impulse leaps across gaps, so to speak. We know that events
that occur in one part of the body of the mammal may affect other parts
by means of the liberation of a chemical substance, or hormone, into
the blood stream. It would be strange indeed if a logical hypothesis
capable of accounting for the transmission of a particular change from
the soma to the germ could not be elaborated.
But acquired characters were not really transmitted after all. So
those who clung to Weismannism argued--an unnecessary task surely
if this transmissibility were inconceivable. We cannot discuss the
evidence here, and it is unnecessary that we should do so, since it is
all considered in the popular books on heredity. There is an apparent
consensus of opinion in these books which should not influence the
reader unfamiliar with zoological literature, nor obscure the fact
that many zoologists and botanists accept the opposite conclusion.
The discussion is all very tiresome, but we may glean some results of
positive value from it. It is unquestionable that very few conclusive
and adequate investigations have been made: one cannot help noticing
that the literature contains an amount of controversy out of all
proportion to the amount of sound experimental and observational
work actually carried out. Most of the experiments deal with the
consideration of traumatic lesions or mutilations, and it seems to be
proved that such defects are not transmitted, or at least are very
rarely transmitted. The tails of kittens have been cut off; the ears
of terrier-dogs have been lopped; and the feet and waists of Chinese
and European ladies have been compressed, and all throughout very
numerous generations, yet these defects are not transmitted from parent
to offspring. This kind of evidence forms the bulk of that which
orthodox zoological opinion has adduced in favour of the belief in
the non-inheritability of acquired characters, but does it all really
matter? What might be transmitted is a useful, purposeful modification
of morphology, or functioning, or behaviour, induced by the environment
throughout a number of generations--an adaptation rather than a harmful
lesion. There is little conclusive evidence that such adaptations
are inherited, though anyone who carefully studies the evidence in
existence will not be likely to say that they are certainly not
transmitted. Does, for instance, the blacksmith transmit his muscular
shoulders and arms to his sons, or the pianiste her supple wrists and
fingers to her daughters? There are no observations and experiments in
the literature worthy of the importance attaching to the question at
issue.
It should be noted also that the germ-plasm is certainly not the
immutable substance that the hypothesis originally postulated. Changes
in the outer physical environment may certainly affect it; thus the
larvæ bred from animals which live in abnormal physical conditions
(temperature, moisture, etc.) may differ morphologically from the larvæ
bred from animals belonging to the same species but living in a normal
environment. The latter must therefore react on the germ-plasm, but the
environment formed by the bodily tissues which surround the germ-cells
may also so react: thus the germ-cells may be affected by such bodily
changes as differences in the supply of nutritive matter, for instance.
The offspring may deviate from the parental structure as the result
of structural modifications acquired by the parent during its own
lifetime, and, even if the filial deviation were not of the same nature
as the parental modification, its inheritance would be an adequate
cause of _some_ degree of transmutation.
It is, however, certainly difficult to prove that organisms transmit
to their progeny the _same_ kinds of deviation from the specific
structure that they themselves acquire as the result of the action of
the environment. Even if they did transmit such acquired deviations,
it does not seem clear that this kind of inheritance alone would
be a sufficient cause of the diversity of forms of life that we do
actually observe in nature. Change of morphology would indeed occur,
but we should expect to find insensible gradations of form and not
individualised species. Let us suppose that Lamarckian inheritance
acts for a considerable time on two or three originally distinct
species inhabiting an isolated tract of land, and let us suppose that
we investigate the variations occurring among all the organisms which
are accessible to our observation with respect to some one variable
character.
The diagram _A_ represents what would seem to be the result of this
process of transmutation. The numbers along the horizontal line are
proportional to their distance from o, the origin, and represent the
magnitude of the variation considered; and the height of the vertical
lines represents the number of organisms exhibiting each degree of
variation. We should expect to find that all the variations were
equally frequent in their occurrence, but this is not what a study of
variability in such a case as we have supposed--that of the animals
inhabiting an isolated part of land--does actually indicate. What we
should find would be the conditions represented by the diagram _B_.
There would be two or more _modes_, that is, values of the variable
character which are represented by a greater number of individuals than
any other value of the variation. The environmental conditions _favour_
the individuals displaying this variation to a greater extent than they
favour the rest.
[Illustration: FIG. 24.]
That is to say, the environment selects some kinds of variations among
the many that are exhibited, and this is, of course, the essential
feature of the hypothesis of the transmutation of species by means of
natural selection of variable characters. Organisms enter the world
differently endowed with the power of acting on the medium in which
they live, or on the environment consisting of their fellow-organisms.
Those that are most favourably endowed live longest and have a more
numerous progeny than those that are less favourably endowed, and
they transmit this favourable endowment to their offspring. Among
the progeny of the progeny there may be some in which the favourable
variation is still more favourable than it was when it first appeared.
Thus the variations which are selected increase in amount. Elimination
of the weakest occurs. The idea is eminently clear and simple, and
possesses a great degree of generality: it is self-evident, says
Driesch, meaning that it cannot be refuted, for it was certainly not
clearly obvious to the naturalists before Darwin and Wallace. But,
unless we choose to be dogmatic, we can hardly claim that it is an
all-sufficient cause for the evolutionary process, and it is useless
to attempt to minimise the difficulties of the hypothesis. It is not
easy to make it account for the origin of instincts or tropisms, or for
restitutions and regenerations of lost parts, or for the appearance
of the first non-functional rudiments of organs which later become
functional and useful. It is, indeed, possible to devise plausible
hypotheses accounting for all these things in terms of natural
selection, but each such subsidiary hypothesis loads the original one
and weakens it to that extent.
Natural selection does not, of course, induce or evoke variations;
these are given to its activity, and they are the material on which it
operates. What, then, is the nature of the deviations from the specific
types of morphology that are selected or eliminated? Not those induced
by the environment, and transmitted in their nature and direction to
the progeny of the organisms first displaying them. It is not unproved
that such variations do occur, and it is even probable that they do
occur. But we may conclude that the frequency of their occurrence is
not great enough to afford sufficient material for natural selection.
It is also clear that the ordinarily occurring variations that we
observe in any large group of organisms collected at random are not
alone the material for selection; for we have seen that experimental
breeding from such variations does not lead to the establishment of a
stable race or “variety.” Nevertheless some effect is produced, and
this may be accounted for by supposing that the observed variations are
really of two kinds--fluctuating variations, which are not inherited,
and mutations, which are inherited. The small observed effect is due to
the selection of the mutations alone: it is a real effect of selection,
an undoubted transmutation of the specific form, but experimental
and statistical investigations seem to show that selection from the
variations that we usually observe is too slow a process to account for
the existing forms of life.
Natural selection acts, therefore, on mutations. Now it seems that we
are forced to recognise the existence of two categories of mutations,
(1) those stable modifications of an “unit-character” which we term
“Mendelian characters,” and (2) those groups of stable modifications to
which de Vries applied the term mutations. It seems at first difficult
to see how permanent modifications of the specific form can be brought
about by the transmission of Mendelian characters, for these characters
are always transmitted in pairs. Let us take a concrete case--that of
a man who has six fingers on his right hand, and let us suppose that
this was a real, spontaneously appearing character or mutation which
had not previously occurred in the ancestry of the man. Two contrasting
characters would then be transmitted, (1) the normal five-fingered
hand, and (2) the six-fingered hand. Both of these characters are
supposed to be present at the same time in the organisation of the men
and women of the family originating in this individual, but one of them
is always latent or recessive. There would, however, be individuals in
which only one of the characters would be present--either the normal
or abnormal number of digits, but intermarriage with individuals
belonging to the other pure strain would immediately lead again to
the transmission of the contrasting characters, or allelomorphs,
although marriage with an individual belonging to the same pure strain
would carry on the normal or abnormal unmixed character into another
generation. But if the possession of six fingers conveyed an undoubted
advantage, and if natural selection did really act in civilised man as
regards the transmission of morphological characters, then a stable
variety (_Homo sapiens hexadactylus_, let us say) might be produced
by its agency. The mutations which we consider in the investigation
of the inheritance of alternating characters are therefore just as
much the material for natural selections as the mutations which occur
among the ordinary variations displayed by organisms in general: but
since only one or two characters appear to be subject to this mode of
transmission, the process would be so slow as to be inadmissible as an
exclusive cause of evolution.
If we assume that de Vries’ mutations are the material on which
selection works, this difficulty is immediately removed, for we now
have to deal with _groups_ of stable deviations: not one or two, but
_all_ the characters of the organism appear to share in the mutability.
But another difficulty now arises. A species of plant or animal may
have got along very well with its ordinary structural endowment, and
then a number of individuals begin to mutate. Some of the deviations
from the specific type may be of real advantage, but others may not:
we can, indeed, imagine an in-co-ordination between the mutating parts
or organs which would be fatal to the animal; on the other hand,
there might be complete co-ordination, with the result that great
advantage might be conferred upon the individual. It is easy to see
how co-ordination of mutating parts is absolutely essential. An animal
which preserves its existence by successful avoidance of its enemies
would not be greatly benefited by a more transparent crystalline lens
if the vitreous humour of its eye were slightly opaque; and even if all
the parts of the eye were perfectly co-ordinated, increased acuity of
vision would not greatly help it if its limbs were not able to respond
all the more quickly to the more acute sensation. Un-co-ordinated
mutations would therefore tend to become eliminated, while co-ordinated
ones would become selected and would become the characters of new
species.
We must now ask why some groups of variations are co-ordinated while
others are not, and it is here that we encounter the most formidable of
the difficulties of any hypothesis of transformism which depends on the
concept of natural selection. If we assume that the environment induces
the appearance of variations, it seems to follow that these variations
are likely to be co-ordinated, but we then invoke the principle of the
acquirement of characters and their transmission by heredity. If, on
the other hand, we assume that variations appear spontaneously, and
quite irresponsibly, so to speak, in the germ-plasm of the organism,
the selection, or elimination, by the environment will not occur until
the co-ordinated or un-co-ordinated variations appear. It is far more
likely that a large number of simultaneously appearing variations will
be un-co-ordinated than that they will be co-ordinated. Merely as a
matter of probability the progressive modification of a species will
take place slowly--too slowly to account for what we see.
Two examples will make it easier to appreciate this difficulty.
Evolution has undoubtedly proceeded in definite _directions_.
There are two dominant groups of fishes, the Teleosts and the
Elasmobranchs, and both must have originated from a common stock.
All the characters in each kind of fish must have been useful (since
they were selected), and all must have been modifications of the
characters of the common stock. The latter became modified along two
main lines, or directions, which are indicated by the characters of the
existing Teleosts and Elasmobranchs. The whole skeleton, the gills,
the circulatory system, and the brain differ in certain respects in
these groups. Therefore a modification of the brain in the primitive
Elasmobranchs was associated with a modification of the cranium, and
therefore with the jaw-apparatus, and so with the branchial skeleton
and the gills, and therefore also with the heart, and so on. Suppose
that the evolutionary process included ten useful and co-ordinated
variations--not an unlikely hypothesis--and suppose that each of these
ten useful variations was associated with nineteen useless ones. The
chance that any one of them did occur was therefore one in twenty; and
if they all occurred independently, that is, if the occurrence of any
one of them was compatible with the occurrence of any other one, or of
all the others, then the chance that all the ten variations occurred
simultaneously was 20^{-10} that is, one in the number 20 followed by
10 cyphers, a rather great improbability.
Most biological students are familiar with the similarity of the
so-called eye of the mollusc Pecten and that of the vertebrate. The
resemblance is one of general structure: in each of these organs
there is a _camera obscura_, a transparent cornea, and behind that
a crystalline lens. On the posterior wall of the camera there is a
receptor organ, or retina, and this is composed of several layers of
nervous elements. The actual nerve-endings are on the surface of
the retina, which is turned away from the light, that is, the optic
nerve runs towards the anterior surface of the retina, and then its
fibres turn backwards. This “inversion of the retinal layers” occurs
in all vertebrate animals, but it is exceptional in the invertebrates.
The above general description applies equally well to the eye of the
vertebrate and to that of Pecten.
Let us admit that these mantle organs in Pecten _are_ eyes, for
there is no conclusive experimental evidence that they really are
visual organs, and plausible reasoning suggests that they may
subserve other functions. Let us assume that the minute structure of
the Pecten eye is similar to that of the vertebrate, and that its
development is also similar: as a matter of fact both histology and
embryology are different. Then we have to explain, on the principles
of natural selection, the parallel evolution of similar structures
along independent lines of descent; for mollusc and vertebrate have
certainly been evolved from some very remote common ancestor in which
the eye could not have been more than a simple pigment spot with a
special nerve termination behind it. In each case the organ was formed
by a very great number of serially occurring variations, yet these
two sets of variations must have been the same at each stage in two
independently occurring processes. On any reasonable assumption as to
the number of co-ordinated variations required, and their chances of
occurrence, the mathematical improbability that these two series of
variations did occur is so great as to amount to impossibility so far
as our theory of transformism is concerned. Natural selection could
not, therefore, have produced these two organs.
This argument of Bergson’s fails, of course, in the particular
instance chosen by him, but this is because the case is an unfortunate
one. Probably a morphologist could find a very much better case of
convergent evolution--the parallelism between the teeth of some
Marsupials and some Rodents, for instance. If detailed histological and
embryological investigation should show a similarity of structure and
development, in such compared organs Bergson’s argument would retain
all its force. We should then have to assume that there was a directing
agency, or tendency in the organism, co-ordinating, or perhaps actually
producing, variations.
Mechanistic biology can suggest no means whereby simultaneously
occurring variations are co-ordinated: let us therefore think of
these variations as occurring independently of each other, and let us
ignore the difficulty of the infrequency of occurrence of suitably
co-ordinated variations. Variations _are_ exhibited by the evolving
organism, and the selection of co-ordinated series is the work of the
environment. But the environment is merely a passive agency, and it
has to confer direction on the innumerable variations presented to it
by the organism, rejecting most but selecting some. Let us think of
the environment, says a critic of Bergson, as a blank wall against
which numerous jets of sand are being projected. The jets scatter
as they approach the wall: each of them represents the variations
displayed by some organ or organ-system of an animal. Let us think of
a pattern drawn on the wall in some kind of adhesive substance: where
the wall is blank the sand would strike, but would fall off again, but
it would adhere to the parts covered by the adhesive paint. The sand
grains strike the wall from all sides, that is, their directions are
un-co-ordinated. The wall is passive, yet a pattern is imprinted upon
it. From passivity and un-co-ordination come symmetry and order.
This argument withstands superficial examination, but to accept it
is truly to be “fooled by a metaphor.” _For what is the pattern on
the wall?_ It is the environment, says the critic. But what is the
environment? Inevitably we think of it as something that makes or
moulds the organism, a way of regarding it that drags after it all
the confusion of thought implied in the above analogy. Clearly the
environment is made by the organism. Its _form_, that is, space,
is only the mode of motion possible to the organism; it is clear
that whether the space perceived by an organism is one-, two-, or
three-dimensional, space depends upon its mode of motion. Its universe
is whatever it can act upon, actually or in contemplation. Atoms and
molecules, planets and suns are its environment because it can in some
measure act upon these bodies, or at least they can be made useful to
it. Chloroform or saccharine, or methyl-blue and all the dye-stuffs
prepared from coal-tar by the chemists, are part of our environment
because we have _made them_. They existed only in potentiality prior to
the development of organic chemistry. They were possible, but man had
to assemble their elements before they became actual. In _making_ them,
he conferred _direction_ on inorganic reactions.
Surely the organism itself selects the variations of structure and
functioning that are exhibited by itself. If we hesitate to say
that these modifications are creations, let us say that they are
permutations of elements of structure, and that they were potential
in the organisation of the creature exhibiting them. They occur in
the latter if we must not say that they are produced. If they are
detrimental, the organism is the less able to live and reproduce, and
if it does reproduce, its progeny are subject to the same disability.
If, as is usual, they simply do not matter, they may or may not affect
the direction of evolution. If they are of advantage, that is, if they
confer increased mastery over the environment, over the inert things
with which the organism comes into contact, the latter enlarges its
universe or environment, lives longer, and transmits to its progeny its
increased powers of action. Indefinite increase of power over inert
matter is potential in living things, and variation converts this
potentiality into actuality.
This discussion is all very formal, but two conclusions emerge from it:
(1) the insufficiency of the mechanistic hypotheses of transformism
to account for all the diversity of life that has appeared on the
earth during the limited period of time which physics allows for the
evolutionary process. There does not appear to be any possibility of
meeting this objection if we continue to adhere to the hypothesis
of transformism already discussed: it faces us at every turn in our
discussion. How great a part is played, for instance, by “pure chance”
in the elimination of individual organisms during the struggle for
existence! Let us think of a shoal of sprats on which sea-birds are
feeding: it is chance which determines whether the birds prey on one
part of the shoal rather than another. Or let us think of the millions
of young fishes that are left stranded on the sea-shore by the receding
tide: it is chance that determines whether an individual fish will be
left stranded in a shallow sandpool which dries up under the sun’s
rays, rather than in a deeper one that retains its water until the tide
next flows over it. It is no use to urge that there is no such thing
as “pure chance,” and that what we so speak of is only the summation
of a multitude of small independent causes. Let us grant this, and
it still follows that the alternative of life or death to multitudes
of organisms depends not upon their adaptability but upon minute
un-co-ordinated causes which have nothing to do with their morphology
or behaviour. These are instances among many others which will occur
to the field naturalist: they shorten still further the time available
for natural selection in the shaping of species, for they reduce the
material on which this factor operates.
The other result of our discussion is to indicate that the problem
of transformism of species is in reality the problem of organic
variability. Let us assume that all the hypotheses of evolution
are true: that the environment may induce changes of morphology
and functioning in animals and plants, and that these changes
themselves--the actual acquirements themselves, that is--are
transmissible by heredity. Let us assume that the germ-cells may be
affected by the environment, either the outer physical environment, or
the inner somatic environment, and that mutations may thus arise. Let
us assume that mutations may be selected in some way, so that specific
discontinuities of structure--“individualised” categories of organisms,
or species--may thus come into existence. Even then transformism is
still as great a problem as ever, for the question of the mode of
origin of these variations or modifications still presses for solution.
The simplest possible cases that we can think of present the most
formidable difficulties. The muscles of the shoulders and arms of the
blacksmith become bigger and stronger as the result of his activity.
Why? We say that the increased katabolism of the tissues causes a
greater output of carbonic acid and other excretory substances, and
that these stimulate certain cerebral centres, which in turn accelerate
the rate of action of the heart and respiratory organs. An increased
flow of nutritive matter and oxygen then traverses the blood-vessels in
the muscles of the shoulders and arms, and the latter _grow_. Probably
processes of this kind do occur, but to say that they do is not to give
any real explanation of the hypertrophy of the musculature of the man’s
body, for what essentially occurs is the division of the nuclei and the
formation of new muscle fibres. How precisely does an increased supply
of nutritive matter cause these nuclei to divide and grow? This is a
relatively simple example of the adaptability of a single tissue-system
to a change in the general bodily activity, that is to say it is a
variation of structure induced by an environmental change.
In most cases, however, the variations of structure that form the
starting-points of transmutation processes cannot clearly be related to
environmental changes. Some fishes produce very great numbers of ova in
single broods--a female ling, for instance, is said to spawn annually
some eighteen millions of eggs. If we examine these ova we shall find
that there is considerable variation in the diameter and in other
measureable characters. We may attempt to correlate these deviations
from the mean characters with environmental differences. All the eggs
“mature,” that is, they absorb water and swell, while various parts,
such as the yolk, undergo chemical changes, during the month or so
before the fish spawns. This process of maturation takes place in the
closed ovarian sac; and the eggs lie practically free in this sac, and
are bathed in a fluid which exudes from the blood-vessels in its walls.
It may indeed be the case that there are variations in the composition
of this fluid in the different parts of the sac; but these variations
cannot be great; the fluid is not really a nutritive one; and the
process of maturation is not hurried. We can hardly believe that
the differences in morphology are due to these minute environmental
differences. We may indeed say that we do not really study the germ
cells when we measure the diameter of the egg or investigate any other
measurable character, for the real germ-plasm is the chromatic matter
of the nucleus. But this obviously begs the whole question: all the
parts of the egg that are accessible to observation do vary, and ought
we to conclude that the parts which are not accessible do not vary?
They _must_ vary: the germ-plasm of each egg _must_ be different from
that of all the others, for the organisms which develop from these
germs show inheritable differences. Further, can we contend that such
minute environmental differences as we have indicated affect the
germ-plasm? Is it so susceptible to external changes? A high degree of
stability of the germ-plasm is postulated in the mechanistic hypothesis
that we have considered, and indeed everything indicates that the
specific organisation is very stable. Can it then be upset by such
minute differences in the somatic environment?
But the germ-plasm is not really simple, says Weismann; it is a
complex mixture of ancestral germ-plasms. The individual fish that we
were considering arose from an aggregate of determinants, and half of
these determinants were received from the male parent and half from
the female one. But each of these parents also arose from a similar
aggregate of determinants, which again were received from both parents,
and so on throughout the ancestry of the fish. It is true that the
germ-plasms contributed by the ancestors were not quite different,
but they differed to some extent. Then there must have been as many
permutations of determinants in the ovum from which the fish developed
as there were permutations of characters in the eighteen millions of
ova produced by it. Does not the hypothesis collapse by its own weight?
It could only have been such difficulties as are here suggested that
led Weismann to formulate his hypothesis of germinal selection. All
those eighteen millions of eggs arose from the division of relatively
few germ cells. Each of these original cells contained the specific
assemblage of determinants, and the elements of the latter are of
course the biophors. The biophors, it will be remembered, are either
very complex chemical molecules, or aggregates of such. When the germ
cells of the germinal epithelium divide to form those cells which are
going to become the ova, the biophors must divide and grow to their
former size, and again divide--it is really a chemical hypothesis that
we are stating, though we have to employ language which seems to do
violence to all sound chemical notions! Now while the biophors were
dividing and growing they were “competing” for the food matter which
was in the liquid bathing them, and some got less, while others got
more than the average quantity. In this way their characters became
different, so that the eggs, on the attainment of maturity, became
different from each other. Now, apart altogether from the impossibility
of applying any test as to the objective reality of this hypothesis, it
must be rejected, for it confers on bodies which belong to the order of
molecules properties which are really those of aggregates of molecules.
The typical properties of a gas, for instance, are not the properties
of the molecules of which the gas is composed, but are statistical
properties exhibited by aggregates of molecules. On the hypothesis of
germinal selection the properties of the animals which develop from
the biophors are extended to the biophors themselves. It was surely a
desperate plight which evoked this notion! It is, as William James said
about Mr Bradley’s intellectualism, mechanism _in extremis_!
We seem forced to the conclusion--and this is the result to which all
this discussion is intended to approximate--that variations, heritable
variations at least, arise spontaneously. That is, there are organic
differences which have no causes, a conclusion against which all our
habits of reasoning rebel. Yet it may be possible to argue that the
problem of the causes of variations is really a pseudo-problem after
all, and that there is no logical reason why we should be compelled to
postulate such causes. When we think of organic variability, do we not
think, surreptitiously it may be, of something that varies, that is,
something that ought to be immutable but which is compelled to deviate?
But what is given to our observation is simply the variations among
organisms.
Let us think of the crude minting machines of Tudor times which
produced coins which were not very similar in weight and design. From
that time onward minting machines have continually been improved,
each successive engine turning out coins more and more alike in every
respect, so that we now possess machines which stamp out sovereigns
as nearly as possible identical with each other. Yet they are not
quite alike, and this is because the action of the engine, in all its
operations, is not invariably the same. In imagination, however, we
make a minting machine which does work perfectly, and turns out coins
absolutely alike, but this ideal engine is only the conceptual limit
to a series of machines each of which is more nearly perfect than was
the last one. It is unlikely that matter possesses the rigidity and
homogeneity which would enable us to obtain this perfect identity of
result; nevertheless this identity has a very obvious utility, and we
strive after it, so that the result of our activity is the conception
of a perfect mechanism, and of products which are identical. We assume
that the reasons why our early and cruder machines were imperfect are
also the reasons why our later and more perfect ones do not produce the
results that we desire.
We are artisans first of all, and then philosophers, and so we extend
this ingrained mechanism of the intellect into our speculations. To
the biologist the organism is a mechanism which, in reproduction,
_ought_ to turn out perfect replicas of itself. It does not do so.
Now, if biology shows us anything, it shows us that living matter is
essentially “labile,” that is, something fluent, while lifeless matter
is essentially rigid, or nearly so. Yet, ignoring this difference, we
expect from the organism that identity of result and operation that we
conceptualise, but do not actually obtain from the artificial machine.
We regard the organism, not only as a mechanism like the minting
machine, but as the conceptual limit to a series of mechanisms. The
reproductive apparatus of our fish does not turn out ova which are
identical, but which differ from each other. Some of this variation, we
say, is due to the action of the environment; and some of it is due to
the condition that each ovum receives a slightly different legacy of
characters from the multitude of ancestors. The rest we conceive as due
to the imperfect working of the reproductive machinery.
It is useful that science should so regard the working of the organism,
for in the search for the causes of variation our analysis of the
phenomena of life becomes more penetrating. But does any result of
investigation or reasoning justify us in assuming, as a matter of
pure speculation, that deviations from the specific type of structure
are physically determined in all their extent? Have we not just as
much justification for the belief that these deviations are truly
spontaneous, and that they arise _de novo_? So we approach, from the
point of view of experimental biology, Bergson’s idea of Creative
Evolution.