germ-plasm, Germinal Selection, because they are analogous to those
processes of selection which we already know in connexion with the
larger vital units, cells, cell-groups and persons. If the germ-plasm
be a system of determinants, then the same laws of struggle for
existence in regard to food and multiplication must hold sway among its
parts which hold sway between all systems of vital units--among the
biophors which form the protoplasm of the cell-body, among the cells of
a tissue, among the tissues of an organ, among the organs themselves,
as well as among the individuals of a species and between species which
compete with one another.
If this be the case, we have here ready to hand the explanation of
every heritable variation of a part, ascending and descending alike.
Let us consider for a little the latter category--that is, the
disappearance of functionless _or useless organs_. It is clear that,
from the moment in the life of a species that an organ, _N_, becomes
useless, natural selection withdraws her hand from it; individuals
with better or worse organs _N_ are now equally capable of life and
struggle, the state of panmixia is entered upon, and the organ _N_
of necessity falls somewhat below its previously attained degree of
perfection.
That this must be so will be admitted when it is remembered that each
organ of a species is only maintained at its highest level because
personal selection keeps ceaseless watch over it, and sets aside all
the less favourable variations by eliminating the individuals which
exhibit them. But this is no longer the case with a useless organ. When
a weaker variant of a disused organ arises through the intra-germinal
fluctuations of nutrition, this is transmitted to the descendants
just as well as the normally developed organ, and in the course of
generations will be inherited by a greater and greater number of
individuals, and must ultimately be inherited by all in some degree or
other. The objection has been urged from many sides that variations
upwards would be quite as likely to arise as those downwards, but
this is an error. Even if, at the beginning, the minus-variations
were rarer than the plus-variations, in the course of generations
the minus ones would preponderate because ascending variations of
disused organs are not indifferent for the organism but injurious to
it. Perhaps an increase in the size of the organ itself would do no
harm, but in that of its determinant it certainly would, because an
ascending determinant requires more nourishment than previously, and
withdraws it from its surroundings, and thus from the determinants in
its immediate neighbourhood; but these are those of functioning and
indispensable parts. Individuals in whose germ-plasm the determinants
of disused organs ascend, and thereby depress the determinants of
organs which are still active, are subject to personal selection,
and are eliminated. There thus remain only those with descending
determinants; in other words, the chance of variants in the direction
of weakness in useless determinants far outweighs that of variants in
the direction of increased strength; the latter will soon cease to
occur at all, for as soon as a determinant has fallen a little below
its normal level, it finds itself upon an inclined plane, along which
it glides very slowly but steadily downwards. This might be disputed if
it could be maintained that, at every stage of the descent, a change
of direction was possible. But this probably takes place rarely and
only in the case of individual ids, and will therefore not be permanent
because in general the stronger neighbour determinants will possess
themselves of the superfluous nourishment, and a lasting ascent will
thus be impossible to the weakened determinant. This is precisely what
I have called Germinal Selection. The determinant whose assimilating
power is weakened by ever so little is continually being robbed by
its neighbours of a part of the nourishment which flows towards it,
and must consequently become further weakened. As no more help will
be given to it by natural selection, since the organ is no longer of
any value to the species, the better among the weakened determinants
of _N_ are never selected out, and they must gradually give way in the
struggle with the neighbouring determinants which are necessary to the
species, becoming gradually weaker and ultimately disappearing.
This process can, of course, no more be proved mathematically than
any other biological processes. No one who is unwilling to accept
germinal selection can be compelled to do so, as he might be to accept
the Pythagorean propositions. It is not built up from beneath upon
axioms, but is an attempt at an explanation of a fact established by
observation--the disappearance of disused parts. But when once the
inheritance of functional modifications has been demonstrated to be a
fallacy, and when it has been shown that, even with the assumption of
such inheritance, the disappearance of parts which are only _passively_
useful, and of any parts whatever in sterile animal forms, remains
unexplained, he who rejects germinal selection must renounce all
attempt at explanation. It is the same as in the case of personal
selection. No one can demonstrate mathematically that any variation
possesses selection value, but whoever rejects personal selection gives
up hope of explaining adaptations, for these cannot be referred to
purely internal forces of development.
The total disappearance of a part which has become useless takes place
with exceeding slowness; the whales, which have existed as such since
the beginning of the tertiary period, have even now not completely lost
their hind-limbs, but carry them about with them as rudiments in the
muscular mass of the trunk, and the birds, which are even older, still
show in their embryonic primordia the five fingers of their reptilian
forefathers, although even their bird-ancestors of the Jurassic period,
if we may argue from _Archæopteryx_, had only three fingers like our
modern birds. A long series of similar examples might be given, and
modern embryology in particular has contributed much that, like this
example of birds' fingers, points to a certain orderliness in the
disappearance of the individual parts of an organ which has become
superfluous. Parts which, in the complete animal, have disappeared
without leaving a trace, appear again in each embryonic primordium, and
disappear in the course of the ontogeny. Speaking metaphorically, we
might express this on the basis of the determinant theory, by saying
that the determinants, as they become weaker, can only control an
increasingly short period of the whole ontogeny of the organ, so that
ultimately nothing more than its first beginning comes into existence.
But this is only a metaphor; we cannot tell what really happens as
long as we are ignorant of the physiological rôle of the determinants,
and even of the laws governing the degeneration of a useless organ.
In respect of the latter, much might still be achieved if comparative
anatomy and embryology were studied with this definite end in view, and
perhaps we should even be able to draw more definite conclusions in
regard to the composition and activity of the determinants in the germ.
In the meantime we must be content with the knowledge that, on the
determinant hypothesis, the disappearance of organs which have
become useless may be regarded as a process of intra-selection going
on between the 'primary constituents' (_Anlagen_) of the germ, and
depending on the same principle of the 'struggle of parts' which
William Roux introduced into science with such brilliant results. If a
struggle for food and space actually takes place, then every passive
weakening must lead to a permanent condition of weakness and a lasting
and irretrievable diminution in the size and strength of the primary
constituent concerned, unless personal selection intervenes, and
choosing out the strongest among these weakened primary constituents,
raises them again to their former level. But this never happens when
the organ has become useless.
This explains why not only parts with active function, like limbs,
muscles, tendons, nerves, and glands, disappear when they cease to
function, but also passive parts like the colouring of the external
surfaces of animals, the lifeless skeletal parts of Arthropods and the
exact adaptation of their thickness to the dwindling function, the
disappearance of superfluous wing-veins, and of the hard chitinous
covering of the abdomen when it is concealed in a protecting house,
as in the case of hermit-crabs, Phryganidæ, and Psychidæ. Here too we
find a sufficient explanation of the fact that parts which have become
functionless, such as the wings of ants, can disappear even in the case
of sterile workers.
* * * * *
The principle of germinal selection, however, can only be understood
in its full significance if we take the positive aspect also into
consideration. We had reached the conclusion that because of the
fluctuations of the food-supply one set of the homologous determinants
represented in the various ids may vary in a minus direction, and
another set in a plus direction, and that this direction will be
adhered to as long as no intra-germinal obstacles come in the way. As
long as this does not happen the determinant concerned will pursue the
path of variation it has once struck out, and indeed the tendency will
be strengthened, because every passive variation, upwards or downwards,
results in a strengthening or weakening of the determinant's power of
assimilation.
Let us take a case of positive variation of the determinants of an
organ _N_, which would be more useful to the species if it were
more highly developed than it had previously been. The variation in
an upward direction is at first purely passive, having arisen from
fluctuations in the food-supply, but it soon becomes active, since the
determinants that have become stronger will have a stronger affinity
for food and will attract more and more of the available supply. The
increased food-stream is thus maintained, and its gradual result is
such a strengthening of the determinants in the course of generations
of germ-cells, that the parts controlled by these determinants--the
determinates--must enter on a path of plus-variations. If to this
there be added personal selection, either natural or artificial, any
fluctuations of this primary constituent towards the minus side will be
effectually prevented, the direction of variation will remain positive,
and the continued intervention of personal selection may raise its
development to its possible maximum, that is, so far that further
development in the same direction would not make for greater fitness,
and personal selection must call a halt. This will always happen as
soon as further increase of the organ would be prejudicial to the
living power of the whole, and when the harmony of the bodily parts
would thereby be permanently disturbed.
That variation in an upward direction really can persist for a long
time is shown by artificial selection as practised by Man in regard
to his domesticated animals and cultivated plants. At first general
variability, or at least variability in many directions, sets in as
a result of the greatly altered conditions of life; the ordinary
fluctuations of the determinants are intensified by the greater
fluctuations in the nutritive stream, and it becomes possible for Man
consciously or unconsciously to select for breeding whatever he prefers
among the chance variations that arise in individual parts or in whole
complexes of parts, and he may thus give rise to a long-continued,
often apparently unlimited, augmentation of variations in the same
direction, although he cannot exercise _any direct_ influence upon
the germ-plasm or its determinants. When a determinant has assumed
a certain variation-direction it will follow it up of itself, and
selection can do nothing more than secure it a free course by setting
aside variations in other directions by means of the elimination of
those that exhibit them.
That artificial selection can cause the increase of a part has long
been established, but in what way this is possible, and how it can
be theoretically explained has hitherto been very obscure, for even
if we take the favourable case that both parents possess the desired
variation, it cannot be supposed that the characters of the parents
are, so to speak, added together in the child; all we can say is that
the probability that the children will also exhibit the character
in question--for instance, a long or crooked nose--becomes greater.
Certainly an increase of the character may result if in both parents
the determinants _K_ are present in excess as compared with the
heterodynamous determinants _K´_ and _K´´_, for in that case there
is an increased probability that, through reducing divisions and
amphimixis, there will again be a preponderance of the determinants
_K_ composing the germ-plasm of the child, and further, that these
determinants _K_ will dominate strongly as compared with the few
_K´_'s. It may thus happen that the long nose of the two parents
will give rise to a still longer nose in the child, or that parents
of considerable bodily size may have still bigger children, but such
increase would be confined to one generation, and would not lead to a
permanent increase of the character; permanent increase cannot depend
merely on the number of the determinants _K_ and on their supremacy
over their converse, the determinants _K´_; it must also depend on
their own variation, and this again can depend only on germinal
selection and not upon personal selection, although the former can be
materially assisted by the latter.
That inheritance from both parents is only a secondary consideration
in regard to the increase of a part by artificial selection is made
evident by the fact that _many secondary sexual characters_ have been
modified, although the breeder selected only in regard to one parent.
Nevertheless in this very domain the greatest results have been
achieved; witness the Japanese breed of cocks with tail-feathers six
feet long. This astonishing result has been reached by the strictest
selection of the cocks in which the feathers were a little longer
than those of other cocks, and the increase in the length of feathers
depended--according to our theory--simply on the fact that, by the
selection of the determinants which were already varying in the
direction of increased length, this process of increase was guarded
from interruption by chance unfavourable conditions of nutrition.
The continuance of variation in the upward direction in which it had
already started is not effected directly by personal selection, but
is so indirectly, for without this constant fresh intervention of
selection the increase would be apt to come to a standstill, or the
variation might even take a contrary direction. There are two other
factors operative to which we have not yet given sufficient attention.
They are, the multiplicity of the ids in every germ-plasm, and sexual
reproduction.
If--as we must assume--each germ-plasm is made up of several or many
ids, there must be several or many determinants of each part of the
organism, for each id contains potentially the whole organism, though
with some individuality of expression. The child is thus not determined
by the determinants of a single id, but by those of many ids, and the
variations of any part of the body do not depend on the variations of a
single determinant _X_, but on the co-operation of all the determinants
_X_ which are contained in the collective ids of the relevant
germ-plasm. Thus it is only when a majority of the determinants have
varied upwards or downwards that they dominate collectively the
development of the part _X´_ and cause it to be larger or smaller.
We have assumed passive fluctuations in nutrition to be the first
cause in individual variation, and it is obvious that the action of
this first cause of dissimilarity must be greatly restricted by the
multiplicity of the ids and the corresponding homologous determinants.
For although passive fluctuations in nutrition should occur continually
in the case of all determinants, this would not imply that they would
follow the same direction in all the determinants _X_ of all ids, for
some determinants _X_ might vary upwards, and others downwards, and
these might counteract each other in ontogeny; so that in many cases
the fluctuations of the individual determinants will not be felt in
their products at all. But since there are--as we shall see later--only
two directions of variation, upwards and downwards, plus and minus,
it must also sometimes happen that a majority take one direction, and
this affords the basis on which germinal selection can build further,
and on which it is materially supported by reducing division and the
subsequent amphimixis.
For reducing division removes half of the ids and thus of the
determinants from the mature germ-cell, and according as chance leaves
together or separates a majority of _X_-determinants varying in the
same direction, this particular germ-cell will contain the primary
constituents of a plus- or of a minus-variation of _X_, and it is
possible that the presence of a majority or a minority may be entirely
due to the reduction. The germ-plasm of the parent may contain,
for instance, the determinant _X_ in its twenty ids 12 times in
minus-variation form, 8 times in plus-variation form; and the reducing
division, according to our view, may separate these into two groups
of which one contains eight plus- and two minus-variations, the other
ten minus-variations, or the one six plus- and four minus-variations,
the other two plus- and eight minus-variations, and so on. Now every
germ-cell which contains a majority of plus- or minus-variations--and
this must be the case with most of them--may unite, if it attains to
amphimixis, with a germ-cell which also contains a majority of plus
or minus _X_-determinants, and if similar majorities let us say
plus--meet together, the plus-variation of _X_ must be all the more
sharply emphasized in the child.
Thus, although the individual determinants _X_ may not be incited to
further variation by their co-operation with others varying in the
same direction, the collective effect of the plus-determinants will
be greater, and adherence to the same direction of variation in the
following generation will be assured, for if in the germ-plasm of the
parent there be, for instance, sixteen out of twenty determinants
possessing the plus-variation, a minus-majority can no longer result
from reducing division.
It is upon this that the operation of natural selection, that is,
personal selection, must depend--that the germ-plasms in which the
favourable variation-direction is in the majority are selected for
breeding, for it is this and nothing else that natural selection does
when it selects the individuals which possess the preferred variations.
The ascending process is thus considerably advanced, because the
opposing determinants are more and more eliminated from the germ-plasm,
till the preferred variations of _X_ are left, and among these, as
ascent in the direction begun continues, the opposing variations
are again set aside by germinal selection, and so on. Reducing
divisions and amphimixis are thus powerful factors in furthering
the transformations of the forms of life, although they are not the
ultimate causes of these.
Now that we have made ourselves familiar with the idea of germinal
selection we shall attempt to gain clearness as to what it can do,
and how far the sphere of its influence extends, and, in particular,
whether it can effect lasting transformations of species without the
co-operation of personal selection, and what kind of variations we may
ascribe to it alone.
First, I must return for a moment to the question we have already
briefly discussed--whether the variation of a determinant upwards or
downwards must so continue without limit. We might be inclined to
think that the great constancy which many species exhibit was a plain
contradiction of this, for if every minute variation of a determinant
necessarily persisted without limit in the same direction, we should
expect to find all the parts of the organism in a state of continual
unrest, some varying upwards, some downwards, always ready to break the
type of the species. Must there not be some internal self-regulation
of the germ-plasm which makes it impossible that every variation
which crops up can persist unlimitedly? Must there not be some kind
of automatic control on the part of the germ-plasm, which is always
striving to re-establish the state of equilibrium that has once been
attained by the determinant system whenever it is disturbed?