the germ-substance; he believes that he is justified in making the
assumption that acquired characters can be inherited, although he
admits that it 'appears to him puzzling in the highest degree'
how characters which developed in the most diverse organs of the
mother-being can exert any influence on the germ. That he may be able
to assume this he points to the interconnexion of all organs by means
of the nervous system; it is this that makes it possible that 'the
fate of one reverberates in the other, and that, when excitement takes
place at any point, some echo of it, however dull, penetrates to the
remotest parts.' To the delicate-winged communication by means of the
nervous system, which unites all parts among themselves, must be added
the general communication by means of the circulation of the fluids of
the body. According to Hering's view, the germ experiences, in some
degree, in itself all that befalls the rest of the organs and parts
of the organism, and these experiences stamp themselves more or less
upon its substance, just as sense-impressions or perceptions stamp
themselves upon the nerve-substance of the brain, and these experiences
are reproduced during the development of the germ, just as the brain
brings memory-pictures back to consciousness. He says, 'If something in
the mother-organism has so changed its nature, through long habit or
exercise repeated a thousand times, that the germ-cell resting in it is
also penetrated by it in however weakened a fashion, when the latter
begins a new existence, expands, and increases to a new being whose
individual parts are still itself and flesh of its flesh, it reproduces
what it experienced as part of a great whole. This is just as wonderful
as when an old man suddenly remembers his earliest childhood, but it is
not more wonderful than this.'
But I think it is more wonderful. There exist demonstrably in the
brain thousands upon thousands of nerve-elements, whose activity
is a definite and limited one, because each particular visual
impression, for instance, only excites to activity certain definite
nerve-elements, and can leave memory-pictures in these alone. According
to my conception of it, the germ-plasm is quite as complex in its
composition, and does not consist of homogeneous elements, but of
innumerable different kinds, which are not related to the parts of the
complete organism indiscriminately, but only to particular parts. But
is it allowable to assume that there are invisible nerve-connexions,
not only to every germ-cell, but also within the germ-plasm, to
every determinant, like the nerve-paths which lead from the eye
to the nerve-cells in the optic-area of the brain? For if it were
otherwise, how could we conceive of the modification of an organ--as,
for instance, the ear-muscles in Man--communicating itself to the
precise determinants of these muscles in the germ-plasm? I have often
been met with the reproach that my conception of the composition of
the germ-plasm is much too complex--but the complexity of Hering's
suggestion seems to me to go a long way beyond mine.
Hering's ideas, which are not only ingenious but very stimulating,
might be accepted as the first indication of an understanding of the
assumed inheritance of functional modifications, if it could be proved
that such inheritance is a fact; but, as we have seen, that is not
the case. The assumption might be permitted, perhaps, if it could be
shown that certain groups of phenomena left no other possibility of
explanation open except this assumption, but that also, as far as I
can see, is not the case. Of course, others hold a different opinion,
but chiefly because they have rejected without much reflection the
sole explanation which presents itself for numerous phenomena--I
mean the processes which we are about to study under the name of
'germinal selection.' But, in any case, Hering's ideas seem to me very
valuable, because they make it apparent that, however much we know of
the organism, we only know it in a general way, and that numberless
delicate processes go on in it which leave no trace for our microscope,
and that we can only recognize the final results of numerous invisible
and often, in their subtlety, also unimaginable factors. This ought to
be taken to heart, especially by all those who speak of simplicity in
reference to the germ-plasm. So much at least is certain: If there were
any inheritance of functional modifications, we should have another
proof that the germ-plasm is composed of determinants, for without them
there could be no possibility that the 'experiences' of an individual
organ would be transmitted to the germ in the way that the Lamarckian
principle implies. Something, and that something material, must be
modified in the germ-plasm if the vigorous use of a group of muscles,
or of a gland, or of a nerve-cell, is to be communicated to the germ,
and not to the whole germ-plasm, but only to so much of it as is
necessary to cause variation in the corresponding group of cells in the
child. It may perhaps be said that this still does not necessitate the
assumption of special determinants for these cell-groups, and that one
might, with Herbert Spencer, conceive of the germ-plasm as consisting
of homogeneous units which vary in the development in accordance with
the diverse regularly alternating influences to which it is exposed
from step to step, and that, therefore, in each of these units of very
complex structure only a single molecule, or perhaps only a single
atom, would need to vary in order that, in the course of development,
the resulting cell-group should appear in the rudiment in somewhat
altered strength.
But I do not believe that a chemical molecule, still less an atom,
is sufficient for this, for reasons which I have already stated--yet
we need not go into this now, but rather deduce the consequences of
this admission. It follows that the 'unit' is made up of numerous
'molecules' or 'atoms,' of which each, by dint of changes it has
undergone, causes particular parts of the body to vary in a definite
manner; in other words, we have here again a theory of determinants,
only they are on a much smaller scale, since each invisible little
vital particle or 'unit' contains all the determinants within itself,
while in my theory it is only the id, that is, the visible chromosome,
which includes the determinant complex. Such a theory would be far from
a simplification of mine, it would rather complicate it enormously,
and that without anything being gained. At most it would be made more
evident how inconceivably complex the nerve-paths must be which lead
from the part that has been modified by exercise to the germ-plasm,
and must also lead to all the innumerable 'molecules or atoms' of the
individual 'units.' But even on my theory of the composition of the
ids as aggregates of living determinants, such nervous transmission of
qualities would be a monstrosity which no one would accept, and I think
on this account that my argument as to the impossibility of conceiving
of the transmission of the modifications of the personal part to the
germinal part retains its force, notwithstanding Hering's interesting
analogy.
If the transmission of functional modifications were an indisputable
fact, I repeat, we should have to give in, and then we might regard the
'memory of organized material' as affording a hint of the possibility
of the unimaginable process. But as long as the occurrence of this
transmission cannot be proved either directly or indirectly, such
a vague possibility of explanation need not induce us to assume an
unproved process.
LECTURE XXV
GERMINAL SELECTION
On what does disappearance after disuse depend, if not on
the Lamarckian principle?--Panmixia--Romanes--Fluctuations
in the determinant-system of the germ-plasm due to unequal
nutrition--Persistence of germinal variations in a definite
direction--The disappearance of non-functioning parts--Preponderance
of minus germinal variations--Law of the retrogression of
useless parts--Variation in an upward direction--Artificial
selection--Influence of the multiplicity of ids and of sexual
reproduction--Personal selection depends on the removal of certain
id-variants--Range of influence of germinal selection--Self-regulation
of the germ-plasm, which is striving towards stability--Ascending
variation-tendencies may persist to excess--Origin of secondary sexual
characters--Significance of purely morphological characters--The
markings of butterflies.
Now that we have recognized that the assumption of a transmission of
functional modifications is not justifiable, let us discuss some of
the many phenomena to explain which many people believe the Lamarckian
principle to be indispensable, and let us inquire whether we are in a
position to give any other explanation of these. How has it come about
that the effects of use and disuse _appear_ to be inherited? Can we
find a sufficient explanation in the principle of selection, and in the
natural selection of Darwin and Wallace?
The answer to these two questions will be most quickly found if we
begin by seeking for an explanation of the disappearance of a part when
it ceases to be exercised.
That this cannot lie in the Lamarckian principle we have already learnt
from the fact that passively functioning parts, such as superfluous
wing-veins, also disappear, and that the loss of the wings and
degeneration of the ovaries has taken place in worker ants, which can
transmit nothing because they do not reproduce.
We might be inclined to regard this gradual disappearance and ultimate
elimination of a disused organ as a direct gain, on the ground that
the economy of material and space thus effected may be of decided
advantage to the individual animal and thereby also for the maintenance
of the species, and that those animals would have an advantage in the
struggle for existence in which the superfluous organ was reduced to
the smallest expression. But that would be far from supplying us with
a sufficient explanation of the phenomenon; the individual variations
in the size of an organ which is in process of degenerating are,
even in extreme cases, far too slight to have any selection-value,
and I cannot call to mind a single case in which the contrary could
be assumed with any degree of probability. What advantage can a newt
or a crustacean living in darkness derive from the fact that its eye
is smaller and more degenerate by one degree of variation than those
of its co-partners in the struggle for existence? Or, to use Herbert
Spencer's striking illustration, how could the balance between life and
death, in the case of a colossus like the Greenland whale, be turned
one way or another by the difference of a few inches in the length of
the hind-leg, as compared with his fellows, in whom the reduction of
the hind-limb may not have gone quite so far? Such a slight economy of
material is as nothing compared with the thousands of hundredweights
the animal weighs. As long as the limbs protrude beyond the surface
of the trunk they may prove an obstacle to rapid swimming, although
that could hardly make much difference, but as soon as the phyletic
evolution had proceeded so far that they were reduced to the extent of
sinking beneath the surface, they would no longer be a hindrance in
swimming, and their further reduction to their modern state of great
degeneration and absolute concealment within the flesh of the animal
cannot be referred even to negative selection.
Years ago I endeavoured to explain the degeneration of disused parts
in terms of a process which I called Panmixia. Natural selection not
only effects adaptations, it also maintains the organ at the pitch
of perfection it has reached by a continual elimination of those
individuals in which the organ in question is less perfect. The longer
this conservative process of selection continues, the greater must be
the constancy of the organ produced by it, and deviations from the
perfect organ will be of less and less frequent occurrence as time goes
on.
Now if this conservative action of natural selection secures the
maintenance of the parts and organs of a species at their maximum of
perfection, it follows that these will _fall below this maximum as soon
as the selection ceases to operate_. And it does cease as soon as an
organ ceases to be of use to its species, like the eye to the species
of crustacean which descends into the dark depths of our lakes, or to
the abyssal zones of the ocean, or into a subterranean cave-system.
In this case all selection of individuals ceases as far as the eye is
concerned; it has no importance in deciding survival in the struggle
for existence, because no individual is at a disadvantage through
its inferior eyes, for instance, by being in any way hindered in
procuring its food. Those with inferior organs of vision will, _ceteris
paribus_, produce as good offspring as those with better eyes, and the
consequence of this must be that there will be a general deterioration
of eyes, because the bad ones can be transmitted as well as the good,
and thus the selection of good eyes is made impossible.
The mixture thus arising may be compared to a fine wine to which a
litre of vinegar has been added; the whole cask is ruined because the
vinegar mingles with every drop of the wine. As deviations from the
normal are always occurring in every part of every species, and among
them some that lessen the value of the organ, rarely perhaps at first,
but after a time in every generation, a sinking of the organ from
the highest point of possible perfection becomes inevitable as soon
as the organ becomes superfluous. The functional uselessness of the
organ must go on increasing the longer it is disused, as will readily
be admitted if it be remembered that only the most perfect adaptation
of all the separate parts of an organ can maintain its functional
capacity, that all the parts of an organ are subject to variation, and
that every deviation from the optimum implies a further deterioration
of the whole. An eye, for instance, can no longer vary in the direction
of 'better' if it has already reached the highest possible point of
perfection; every further variation must deteriorate it.
Romanes gave expression to this idea, that the cessation of natural
selection alone must cause the degeneration of a part, a decade
before I did, but neither he nor the scientific world of his time
attached great importance to it, and it was forgotten again. This was
intelligible enough, for, at that time, the validity of the Lamarckian
principle had not been called in question, and therefore the need for
some other principle to explain the disappearance of disused parts had
not begun to be felt.
I found myself in quite a different position. As my doubts regarding
the Lamarckian principle grew greater and greater, I was obliged to
seek for some other factor in modification, which should be sufficient
to effect the degeneration of a disused part, and for a time I thought
I had found this in panmixia, that is, in the mingling of all together,
well and less well equipped alike. This factor does certainly operate,
but the more I thought over it the clearer it became to me that
there must be some other factor at work as well, for while panmixia
might explain the deterioration of an organ, it could not explain
its decrease in size, its gradual wearing away, and ultimate total
disappearance. Yet this is the path followed, slowly indeed, but quite
surely, by all organs which have become useless. If panmixia alone
guided the deterioration of the organ, and it was thus only chance
variations which were inherited through panmixia and gradually diffused
over the whole species, how could it come about that all the variations
were in the direction of smaller size? Yet this is obviously the case.
Why should no variations in the direction of larger size occur? And
if this were so, why should a useless organ not be maintained at its
original size, if it be admitted that an increase in size would be
prevented by natural selection? But this never occurs, and diminution
in size is so absolutely the rule that the idea of a 'vestigial or
rudimentary' organ suggests a 'small organ' almost more than an
'imperfect' one.
There must then be something else at work which causes the
minus-variations in a disused organ to preponderate persistently and
permanently over the plus-variations, and this something can lie
nowhere else than where the roots of all hereditary variations are
to be found--in the germ-plasm. This train of thought leads us to
the discovery of a process which we must call selection between the
elements of the germ-plasm, or, as I have named it shortly, _Germinal
Selection_.
If the substance of the germ-plasm is--as we assumed--composed of
heterogeneous living particles, which have dissimilar rôles in the
building up of the organism, there must of necessity be among them
a definite labile state of equilibrium, which cannot be disturbed
without modifying in some way the structure of the organism itself
which arises from the germ-plasm. But if our further view be correct,
that these individual and different living units of the germ-plasm are
'determinants,' that is, are the primary constituents of particular
parts of the organism, in the sense that these parts could not arise if
their determinants were absent from the germ-plasm, and that they would
be different if the determinants were differently composed, we can draw
far-reaching deductions.
It is true that we cannot learn _anything directly_ in regard to the
intimate structure of the germ-plasm, and even in regard to the vital
processes going on within it we can only guess a very little, but so
much we may say--that its living parts are nourished, and that they
multiply. But it follows from this that nourishment in a dissolved
state must penetrate between its vital particles, and that whether the
determinants grow, and at what rate they do so, depends mainly on the
amount of nourishment which reaches them. As long as the germ-cells
multiply by division the determinants have no other function but
to grow; a part of their substance undergoes oxidation and thereby
yields the supply of energy necessary to assimilation, that is, to the
formation of new living substance.
If each kind of determinant always secured the same quantity of
nourishment, all would grow in the same degree, that is, in exact
proportion to their power of assimilation. But we know that in less
minute conditions which we can observe more directly, there is
nowhere absolute equality, that all vital processes are subject to
fluctuations; any little obstacles in the current of the nutritive
fluid, or in its composition, may cause poorer nutrition of one
part, better of another. We may therefore assume that there are
similar irregularities and differences in the minute and unobservable
conditions of the germ-plasm likewise, and the result must be a slight
shifting of the position of equilibrium as regards size and strength in
the determinant system; for the less well-nourished determinants will
grow more slowly, will fail to attain to the size and strength of their
neighbours, and will multiply more slowly.
But the vigour of growth does not depend only on the influence of
nourishment; one cell grows quickly, another slowly in the same
nutritive fluid; it depends in great part on the cell's power
of assimilation. In the same way the assimilating power of the
determinants and their affinity for nourishment will vary with their
constitution, and a weaker determinant will remain smaller than a
stronger one, even when the stream of nourishment is the same.
It seems to me that it is upon the unequal nutrition of the
determinants conditioned by the chances of the food-supply that
individual hereditary variability ultimately depends. If, for instance,
the determinant _A_ receives poorer nourishment at a particular time
than the determinant _B_, it will grow more slowly, remain weaker, and
then, when the germ-cells develop into an animal, the part to which it
gives rise will be weaker than it usually is in other individuals.
These primary inequalities in the equipment of the determinants which
are caused by a passing inequality in the food-stream are, of course,
so slight that we are unable to observe their consequences. They must
persist for a considerable time before they become observable, but
they may persist for a long time, and their effect must then mount
up, because every diminution in the strength of the determinant also
signifies a lessened power of assimilation, and growth becomes slower
for the twofold reason that passive and active nutrition decrease
at the same time. In the less minute conditions observable in the
histological elements of the body we know that function strengthens
the organ, and that disuse weakens it, and we are justified in
applying this proposition also to these more intimate conditions and
minuter vital units. Thus, in the course of the multiplication of
the germ-cells, the less vigorously working determinant, _A_, will
gradually, but very slowly, become weaker, that is, of diminished
power of assimilation, presupposing of course that the intra-germinal
food-stream does not become stronger again at the same place--a
possibility to which I shall subsequently refer. But while one
determinant may be slowly becoming weaker, its neighbour, on the other
hand, may be varying on an ascending scale, just because the former is,
on account of its diminished power of assimilation, no longer able to
exhaust completely the food-stream which flows to it.
The determinants are thus in constant motion, here ascending, there
descending, and it is in these fluctuations of the equilibrium of the
determinant-system that I see the roots of all hereditary variation,
while in the fact that the variation-directions of particular
determinants must continue the same without limit as long as they meet
with no obstacle lies the possibility of the adaptation of the organism
to changing conditions, the increase and transformation of one part,
the degeneration and disappearance of another, in short, the processes
of natural selection. The reason why such variation movements must
continue until they meet some resistance is that every chance upward
or downward movement--due, that is, to mere passive fluctuation in the
food-supply, at the same time strengthens or weakens the determinant,
and makes it either more or less capable of attracting nourishment to
itself; in the former case an increasingly strong stream of food will
be directed towards it, in the latter more and more of the available
food-supply will be withdrawn from it by its neighbour-determinants
on all sides; in the former the determinant will go on increasing
in strength as long as it can go on attracting more nourishment, in
the latter it will continue to become weaker until it disappears
altogether. To the ascending progression, as is evident, there are
limits set, not only by the amount of food which can circulate through
the whole id, but also by the neighbour determinants, which will sooner
or later resist the withdrawal of nourishment from them; but for the
descending progression there are no limits except total disappearance,
and this is actually reached in all cases in which the determinants are
related to a part which has become useless. But both these movements,
the upward and the downward alike, are quite independent of natural
selection, i.e. of personal selection; they are processes of a unique
kind which run their course purely in accordance with intra-germinal
laws. Whether a determinant 'ascends' or 'descends' depends solely
upon the play of forces within the germ-plasm, not upon whether the
direction of the variation in question is useful or prejudicial, or
on whether the organ in question, the determinate, is of value or
otherwise. In this fact lies the great importance of this play of
forces within the germ-plasm, that it gives rise to variations quite
independently of the relations of the organism to the external world.
In many cases, of course, personal selection intervenes, but even then
it cannot directly effect the rising or falling of _the individual_
determinants--these are processes quite outside of its influence--but
it can, by eliminating the bearers of unfavourably varying
determinants, set a limit to further advance in such directions. This
we shall consider in more detail later on. Personal selection operates
by removing unfavourably varying individuals from the genealogical tree
of the species, but at the same time the determinants which are varying
unfavourably are also removed, and their variation is thus put a stop
to for all time.