itself, for that must depend not on a mere exchange of ids, but rather
upon a variation of the ids. The ids of a worm of the primitive
world could not without variation now make up the germ-plasm of an
elephant, even if it be true that mammals are descended from worms.
The ids must have been meanwhile transformed times without number by
the modification, degeneration, and new formation of determinants.
Amphimixis, that is, the union of two germ-plasms, does not of itself
cause variation of the determinants, it only arranges the ids (the
ancestral plasms) in ever-new combinations. If the origin of variation
were limited to that alone, a transmutation of species and genera
would only be possible on a very limited scale; there could at most be
a narrow circle of variations, just as in the example already given
of the packs of cards; even if the taking away and mixing up of the
halves were repeated a thousand times, a definite though undoubtedly
large number of card-combinations would in the long run recur again and
again. But the case is different with the germ-plasm and amphimixis,
where there is an infinitely more varied series of results, because
the individual cards--the ids--are variable, even between one time of
sifting and shuffling and another, and therefore infinitely productive
of variety in the course of numerous repetitions of the shuffling.
I have been frequently and persistently credited with maintaining that
the germ-plasm is invariable--a misunderstanding of my position, due
perhaps to a somewhat too brief and terse statement which I made at an
earlier period (1886). I had described the germ-plasm as 'a substance
of great power of persistence,' and as varying with difficulty and
slowly, basing this statement upon the age-long persistence of many
species in which the specific constitution of the germ-plasm must have
remained unchanged. The idea of 'germinal selection,' of a ceaseless
struggle between the 'primary constituents' of the germ, and of
the resulting continual slight and invisible rising and falling of
individual characters, had not yet dawned upon me, nor had I at that
time formulated the conception of 'determinants.' I was even doubtful
at that time whether development, heredity, and variation were not
interpretable on the assumption of an undifferentiated substance
without primary constituents. But at no time was I unaware that the
whole phyletic evolution of the organic world is only conceivable
on the assumption of continual variation of the germ-plasm, that it
actually depends upon this, even if these variations come about with
exceeding slowness, and are thus in a certain sense 'difficult.'
Now that I understand these processes more clearly, my opinion is
that the roots of all heritable variation lie in the germ-plasm,
and furthermore, that the determinants are continually oscillating
hither and thither in response to very minute nutritive changes, and
are readily compelled to _variation in a definite direction_, which
may ultimately lead to considerable variations in the structure of
the species, if they are favoured by personal selection, or at least
if they are not suppressed by it as prejudicial. But selection is
continually keeping watch over both kinds of variation, and if the
conditions of life do not further the variation or do not even allow it
to persist, selection eliminates everything that lessens the purity of
the specific type, everything that transgresses the limits of utility,
or that might endanger the existence of the species. Thus we understand
how the germ-plasm may be variable, and yet at the same time remain
unvaried for thousands of years, how it is ready and able to furnish
any variation that is possible in a species if that is required by
external circumstances, and yet is able to preserve the characters of
the species in almost absolute constancy through whole geological ages;
in short, how it can be at once readily variable and yet slow to vary.
The importance which amphimixis thus has in connexion with the
adaptation of organisms lies, if I mistake not, in the necessity
for co-adaptation, that is, in the fact that in almost all
adaptations it is not merely a question of the variation of a single
determinant, but of the correlated variations of many--often very
numerous--determinants, of 'harmonious adaptation,' as we have already
said. Many-sided adaptation of this kind seems to me impossible without
a continually recurrent sifting and recombining of the germ-plasms, and
this can only be effected by amphimixis.
It may be objected that, apart from amphimixis, variation can be
brought about in many parts of an organism, as in purely asexual
reproduction. A plant, for instance, may vary when it is transferred
to a strange soil or climate; and even in that case the variations
seem to be harmonious, at least the harmony of the parts is so far
maintained that the plant continues to flourish, at any rate under
cultivation. A plant species may be incited by abundant nourishment
to gigantic growth, and caused to vary in many of its parts, and the
abundant food may even directly affect the germ-plasm so that all or
some of these variations may become hereditary; and yet this is far
from being a case of adaptation, it is merely a case of simultaneous
variations, and it is questionable whether they will make the continued
existence of the plant under the new conditions possible or not. It
might easily happen, for instance, that the plant, though it became
larger and bore more abundant blossoms, would be sterile, and therefore
unfitted for continued existence in a natural state. Variations are not
necessarily adaptations; the latter can never come about solely through
direct influence upon the germ-plasm. What direct influence upon the
germ-plasm could, for instance, make the hind-legs of a mammal long and
strong and the fore-legs short and weak? Obviously neither an increase
nor decrease in the food-supply, nor a higher or lower temperature--in
short, no direct influence, because all these affect the germ-plasm as
a whole, and therefore cannot possibly influence two homologous groups
of determinants in opposite directions.
This, it seems to me, is only possible when amphimixis brings about
in one individual a favourable coincidence of the chance germinal
variations of the determinants of the fore- and hind-limbs; and just
as it is with the two variations in this simple hypothetical case,
so it will be in the actual processes of adaptation where there are
involved numerous--we know not how numerous--variations essential to a
'harmonious adaptation.'
It need not be objected that the very number of variations necessary to
a 'harmonious adaptation' makes its occurrence impracticable; for it
is _the complete_ harmony of the parts that makes the adaptation, and
without this the individual was only imperfectly adapted, and therefore
incapable of survival. It is certainly not mathematically demonstrable
that this is the case, but as the whole process of transformation which
makes an old adaptation into a new one begins with minimal fluctuations
of the determinants, which must first be brought by germinal selection
to the level of selection-value, and must then be subject to personal
selection, so the whole process goes on so gradually and by such
small steps that the harmony of the parts is maintained by functional
adaptation during the individual life in a great number of individuals.
But these are just the individuals which survive in the struggle for
existence, and at the same time possess at every stage of the process
_the best combination of favourably varying determinants_. As these
favourable variations are, in consequence of germinal selection, not
mere isolated variations of fluctuating importance, but variations in
a _definite direction_, the whole process of variation must persist
in every single part in the direction imposed upon it by personal
selection. But since at every reducing division the ids of the
germ-cells are brought down to half their number, a possibility is
offered for gradually removing the unfavourable ids from the germ-plasm
of the species, since the descendants resulting from the most
unfavourable id-combinations always perish, and so from generation to
generation the germ-plasm gets rid of its unfavourably varying ids, and
the most propitious combinations afforded by amphimixis are preserved,
till ultimately there remain only those combinations which are varying
appropriately, or at least only those in which the appropriately
varying determinants are in the majority, and so have controlling
influence.
Logically this deduction is undoubtedly indisputable, from the
standpoint of the germ-plasm theory; but whether it may be regarded as
a sufficient reason for the introduction of amphimixis, and for its
extremely tenacious persistence throughout the course of the long and
intricate phylogeny, cannot be maintained without special investigation.
Against my position the objection has often been urged that an
arrangement cannot arise or be maintained through natural selection
unless it is _of direct use_ to the individual in which it occurs.
Sexual reproduction cannot therefore have been established simply
because it advances, or even because it makes possible the adaptations
of species, for these adaptations only came about occasionally, perhaps
once in a thousand generations or even less frequently; thus the
intervening generations could derive no advantage of any kind from the
arrangement in question, and therefore, according to the law of the
degeneration of unused characters, it must have long since been lost. I
mentioned this objection before, but was obliged to postpone a detailed
consideration of it until we had discussed germinal selection.
We admit, of course, that characters are only preserved intact as
long as they are of advantage sufficient to turn the scale in favour
of their possessors, and that they begin to fall from their height of
perfection when that is no longer the case; we admit also that new
adaptations are not continually necessary, but are so only at intervals
of long series of generations, and yet the objection cited seems to me
baseless.
Leaving out of account, for the moment, the first introduction of
amphimixis, let us deal with it as an existing occurrence, for the
tenacious persistence of which we wish to find reasons.
Is it really the case that amphimixis is only of importance in
connexion with the new adaptation of a species, and that it has nothing
to do with the persistence of the species in the state of adaptation
already attained? According to the conception of the processes within
the germ-plasm which we have already stated, it is impossible that this
should be the case, for continual slight fluctuations are occurring in
the determinants in consequence of the fluctuations of the nutritive
stream, and these slight variations, plus or minus, do not in many
cases equalize one another or counteract one another by turning again
in a contrary direction; they go on increasing in the direction in
which they have begun. It is only when personal selection opposes them
that they come to a standstill, and this can only happen when they
attain to selection-value, that is to say, when they reach a level at
which they become disadvantageous in the struggle of persons. But as
germinal variations of this kind are continually occurring, personal
selection must keep continual watch over them, and eradicate them as
soon as they have attained selection-value.
Therefore, when a species is most perfectly adapted to its conditions,
it would of necessity begin to degenerate if personal selection were
not continually guarding it, and setting aside everything that is in
excess or deficient as soon as it begins to be prejudicial. But the
adaptation of a species does not depend upon _one_ character persisting
at its normal level, but on the persistence of very many, and many of
these vary simultaneously upwards or downwards, and reach the limit of
selection-value at one time or another. If there were no amphimixis,
then either all individuals with any excessive variant would be at
once eliminated, or the species would go on deteriorating until this
excessive variant was so numerously and strongly represented in all
its individuals that it would perish through degeneration. But even
in the first of these cases the species would drift towards the fate
of extinction, because excessive variations do occur even in every
asexual generation, and would appear in an increasingly large number
of determinants if there were no possibility of rejecting them and
eliminating them from the lineage of the species.
This is made possible through the periodic intervention of amphimixis;
it is actually effected thereby; and in this way alone the species
is kept at its high-water mark of adaptation. It is not necessary
to assume that every single determinant which is varying in an
unfavourable direction is at once eliminated as soon as it becomes
prejudicial, that is, reaches negative selection-value, or--to make
use of an expression introduced by Ammon--as soon as it oversteps the
boundaries of the 'playground of variations,' the limits within which
variations are neither favourable nor unfavourable. But _in the course
of generations_ they are unfailingly eliminated, especially when a
large number of unfavourably varying determinants are coincident in
the germ-plasm. Then the individuals which arise from a germ-plasm
thus composed must perish in the struggle for existence, and thus the
id-combinations with excessive determinants are eliminated from the
germinal constitution of the species. As this is repeated as often as
excesses of the ids occur, the species is kept pure.
It might be objected that, through such a continual weeding-out of
rebellious determinants, the germ-plasm would become so constant in
its constitution that it would ultimately be secure from all such
aberrations of it on the part of its determinants, and therefore would
in time become quite incapable of diverging from its proper path at
all, and would thus no longer require this continual correction through
amphimixis.
I do not wish to contradict this conclusion; indeed, I believe that the
constitution of the species becomes more and more constant in the way
I have indicated, and that an ever more perfect and stable equilibrium
of the whole determinant system is thus brought about. It follows that
in the course of generations the diverse determinants of the germ-plasm
will vary within a progressively shortened radius, and will thus more
and more rarely overstep the limits of the 'variation-playground'--and
yet I still believe that this justifiable conclusion tells in favour of
my interpretation of the utility of the persistence of amphigony once
introduced.
Let it be remarked, in the first place, that it is by no means
essential to the preservation of a useful institution that it should
practically justify its utility in _every_ generation. Although, for
instance, the warm winter coat of a species of mammal may be necessary
to its survival, it does not disappear at once when a winter happens
to occur which is so warm that even individuals with poor pellage can
survive. Indeed, several such mild winters might occur in succession,
in which there was no weeding-out of the individuals with poor fur, and
yet the thickness of the winter fur of the species would not become
less fixed, just because this character no longer varies perceptibly
in an old-established species which has long been perfectly adapted,
and it could only be brought into a state of marked fluctuation very
slowly through direct influence on the germ-plasm, or through panmixia.
But exactly the same thing is true in regard to the determinants of the
reproductive cells, in respect of their adaptation to amphimixis, only
very much more emphatically.
Before going further, I should like to show that the conclusion
we have just deduced from the theory, namely, that the equilibrium
of the determinant system of a species increases in stability with
the duration of its persistence, holds good not only for the whole
system, but for its individual parts, that is, for the individual
characters and adaptations. Experience teaches that characters are the
more exactly and constantly transmitted the older they are; generic
characters are more constant than species-characters, order-characters
more persistent than family-characters--this is implied even in their
name. But we are able to show even in relation to the characters of
a species that those which have been fixed for a long time are most
precisely and purely transmitted; that is, that their determinants are
least inclined to overstep the limits of the 'variation-playground'
either in an upward or downward direction.
Two groups of facts prove this: first the observed fact that the very
different degree of variability which the different species exhibit is
by no means common to all the characters of the species in the same
measure; for individual characters may be variable or constant in very
different degrees.
Many years ago[21] I drew attention to the fact that the different
stages in the life-history of insects, especially of Lepidoptera,
might be variable in quite different degrees. Thus, for instance,
the caterpillar might be very variable, and yet the butterfly which
arises from it might be extremely constant. I concluded from this--what
probably no one now will dispute--that the various stages may vary
phyletically independently of one another, that, for instance, the
caterpillar may adapt itself to a new manner of life, a new food-plant,
a new means of defence, while the butterfly, unaffected by this, goes
on quietly as it was before. Every new adaptation necessarily implies
variability, and so the stage which is in process of transformation
must have its period of variability, which gradually returns again to
greater constancy, and this the more completely the longer the series
of generations through which the weeding out of the less well-adapted
has endured.
[21] _Studien zur Descendenztheorie_, Leipzig, 1876.
But it is not only the individual stages of development that may be
unequally variable; the same is true of the characters of a species
which occur simultaneously. The most striking example of this known to
me is the leaf-butterfly, which I have already mentioned many times
in the course of these lectures--the Indian _Kallima paralecta_.
In this species the brown and red upper surface is almost alike in
colour and marking in all individuals, but the under surface, the
colour and marking of which is so deceptively mimetic of a leaf, is
variable to such a degree that it is difficult, among a large number
of specimens, to find even a few which are as like one another as are
the members of species in which the under side is constant. It need
not be urged that this is due to the complexity of the marking on the
under side. In many of our indigenous butterflies the under side is
just as complex in coloration and marking, and nevertheless it is very
constant, being almost identical in all individuals, as for instance
in _Vanessa cardui_. In _Kallima_ the great variability of the under
surface certainly depends not merely on the fact that the mimetic
character has been only recently acquired (phyletically speaking), but
chiefly on the fact that the dead leaves to which they approximate are
themselves very diverse in appearance, for many are dry, others moist
and covered with mould, and that the adaptations have therefore gone in
different directions, and as yet, at least, have neither combined to
form a single constant type, nor diverged to form two or three distinct
types. The various 'leaf-pictures' seem equally effective in concealing
the insects from their enemies, and thus there is still a continual
crossing and mingling of the different essays at leaf-picturing.
A second group of facts, which indicates that old-established
characters have less tendency to overstep the limits of the neutral
'variation-playground,' is to be found in the experience of breeders,
and especially that of gardeners who have brought wild plants under
cultivation in order to procure varieties.
It has been proved that the wild plants often exhibit no hereditary
variations for a long series of generations, notwithstanding the
greatly altered conditions of life, but that then a moment comes in
which isolated variations crop up, which may then be intensified by
the manipulations of the breeder to form sport-species with large
conspicuously coloured flowers, or with some other distinctive
character. Darwin called this a shattering of the constitution of the
plant; but the stable and slowly varying 'constitution' simply means
that in old-established and well-adapted species the determinants
possess only a very restricted 'variation-playground,' and because of
their firmly based harmonious correlation are not easily and never very
quickly induced to overstep its limits in any marked degree.
Let us now apply all this to the institution of amphimixis and
amphigony, and it is immediately obvious that these determinants of the
germ-plasm which control the characters relating to sexual reproduction
must be _more stable and less variable than all others which a species
possesses, for they are infinitely older_. They are older than all
species-characters, older than the characters of the genus, of the
family, of the class, and indeed of the whole series or phylum to
which a higher animal, a vertebrate, for instance, belongs. We cannot
wonder, therefore, that amphigony has persisted through hundreds and
thousands of generations, even if it had not been reinforced in the
germ-plasm during this period by selection. We should rather wonder
that an institution so primaeval, and so firmly engrained in the
germ-plasm, can ever be departed from, even when its abandonment is to
the advantage of the species, as has happened in parthenogenesis.
I have entered upon this long discussion because I believe that we
require to appreciate this power of persistence on the part of the
sexual determinants before we can explain the general occurrence of
amphigony. The occurrence of pure parthenogenesis, unaccompanied by
any degeneration of the species, can hardly be understood except on
the assumption that the constancy of the species, when it has once
been attained, may be preserved without the continual intervention of
amphimixis. How long it can be preserved is another question, which it
is difficult or impossible to answer, since species exhibiting _pure_
parthenogenesis are rare, and since we cannot tell with certainty
how long it is since amphimixis ceased to occur in them. Generally
speaking, the answer in regard to the few species which have to be
taken into account in this connexion would be 'not long,' but whether
this 'not long' signifies hundreds of generations or thousands of
generations we must leave undecided. So much only we can say, that
in all species of animals in which the male sex has quite died out
or has dwindled to a minimal remnant, there are as yet no traces of
degeneration to be found, and that even organs which have fallen into
disuse and become functionless because amphigony has disappeared, are
nevertheless in several cases retained in perfect completeness. I shall
return to this subject later on, but in the meantime I wish to work
out our conception of the actual efficacy of amphigony or ordinary
sexual reproduction, and thereby increase our understanding of its
significance and power of persistence.
We have seen that amphigony not only renders possible the novel
'harmonious adaptations' which are continually required, but that it
also leads, by a continual crossing of individuals, simultaneously with
the elimination of the less fit, to a gradually increasing constancy of
the species. This has been regarded by some writers as its sole effect;
thus recently by Hatschek, whose view has already been refuted.