cannot reach clearness on this point through our present knowledge
of the germ-plasm, because we possess no insight into its structure;
we can only draw conclusions as to the processes in the germ-plasm
from the observed phenomena of variation and inheritance. But two
facts stand in direct antithesis to one another, first, the high
power of adaptation possessed by all species, and the undoubted
occurrence of unrestricted persistence in a given direction of
variation, as seen in artificial selection, and in the disappearance
of parts which have ceased to function; and, secondly, the great
constancy of old-established species which do indeed always exhibit a
certain degree of individual variability, but without showing marked
deviations as a frequent occurrence or in all possible directions,
as they certainly would if every determinant favoured by a chance
increase in the nutritive stream necessarily and irresistibly went on
varying further in the same direction. Or can the constancy of such
species be maintained solely by means of personal selection, which
is continually setting aside all the determinants which rise above
the selection-value by eliminating their possessors? I was for long
satisfied that this was the true solution of the difficulty, and even
now I do not doubt that personal selection does, in point of fact,
maintain the constancy of the species at a certain level, but I do not
believe that this is sufficient, but rather that it is necessary to
recognize an equalizing influence due to germinal selection, and to
attribute to this a share in maintaining the constancy of a species
which has long been well adapted. I am led to this assumption chiefly
by the phenomena of variation in Man, for we find in him a thousand
kinds of minute hereditary individual variations, of which not one is
likely to attain to selection value. Of course the constant recurrence
of reducing divisions prevents any particular id which contains a
varying determinant from being inherited through many generations; for
so many ids are being continually removed from the genealogical tree
by the constant rejection of the half of all ids of every germ-plasm,
that only a small part of the ancestral id remains in the grandchild,
great-grandchild, and so on. Certainly some of the ids of the ancestors
compose the germ-plasm of the descendants, and if all the determinants
of one of these ids had begun to vary persistently upwards or downwards
in an ancestor, then all the determinants of the relative id in the
descendants would possess the variation in an intensified degree; and
however slowly the variation advanced it would attain selection-value
in some one or other of the descendants, and would thus break
the previously stable type of the most perfectly adapted species.
The descendant in question would then succumb in the struggle for
existence. But as the number of the determinants in the germ-plasm is
probably much greater than that of the descendants of one generation,
every descendant would in the course of time deviate unfavourably in
some one character from the type of the species, and then either all
the descendants would be eliminated or the type would become unstable.
But neither of these things happens, and there are undoubtedly
species which remain constant for long periods of time, therefore the
assumption must be false and every variation of a determinant does not
of necessity go on in the same direction without limit.
I therefore suppose that although slight variations are ceaselessly
taking place upwards or downwards in all determinants, even in constant
species, the majority of these turn again in the other direction before
they have attained to any important degree of increase, at least in
the germ-plasm of all species which have had a definitely established
equilibrium for thousands of generations. In such a germ-plasm, or
to speak more precisely, in the id of such a germ-plasm, marked
fluctuations in the nutritive stream will not be likely to occur as
long as the external conditions are unchanged, but slight fluctuations,
which will not be wanting even here, may often alternate and turn in
an opposite direction, and thus the upward movement of a determinant
may be transformed into a downward one. Every determinant is surrounded
by several others, and we can imagine that the regular nutritive
stream which we have assumed may be partially dammed up by a slight
enlargement of the determinant, and that this will drive the surplus
back again. But however we may picture these conditions, which are for
all time outside of the sphere of observation, the assumption of a
self-regulation of the germ-plasm, up to a certain degree, cannot be
regarded as inconceivable or unphysiological.
But there are limits to this self-regulation; as soon as the increase
or decrease of a determinant attains a certain degree, as soon as it
has got beyond the first slight deviation, it overcomes all obstacles,
and goes on increasing in the direction in which it has started.
This must happen even in the case of old and constant species, and
frequently enough to admit of an apparent capacity for adaptation in
all directions. Every part of a species can vary beyond the usual
individual fluctuations, and as this is possible only by means of
intra-germinal processes, we must assume that even in the case of
germ-plasms which have long remained in a state of stable equilibrium
there may occasionally be marked fluctuations in the nutritive stream,
and thus more than usually pronounced variations of the determinants
affected by it will occur. These yield the material for new adaptations
if they are in the direction of fitness, or they are eliminated either
by the chances of reducing division or by personal selection if new
adaptations are not required.
The old-established hereditary equilibrium of the germ-plasm must be
most easily disturbed when the species is in some way brought into new
conditions of existence, as, for instance, when plants or animals are
domesticated, and when in consequence, as we have already assumed, the
nutritive currents within the id gradually alter, quantitatively and
qualitatively; and on this account alone certain kinds of determinants
are favoured, while others are at a disadvantage. In this way there
arises the intensified general variability of domesticated animals
and cultivated plants which has been known since the time of Darwin.
Something analogous to this must occur in natural conditions, though
more slowly, when a species is subjected to a change of climatic
conditions, but we shall discuss this later on in more detail.
We have thus arrived at the idea that the slight variations of the
determinants may be counteracted whether they be directed upwards or
downwards, and that in the case of so-called constant species they
do frequently equalize themselves; but that more marked variations,
produced by more pronounced nutritive fluctuations, may in a sense go
on without limit, and then can only be restricted and controlled by
personal selection, that is, by the removal of the ids concerned from
the genealogical lineage of the species.
In one direction variation can be proved to go on without limit, and
that is downwards, as is proved by the fact of the disappearance of
_disused organs_, for here we have a variation-direction, which has
been followed to its utmost limit, and which is completely independent
of personal selection; it proceeds quite _uninterfered with_ by
personal selection, and is left entirely to itself. It is a significant
fact that the disappearance of the individual parts of a larger organ,
according to all the data that are as yet available, proceeds at a
very _unequal rate_, so that it evidently depends to a great extent on
chance whether a disused part begins to degenerate sooner or later.
Thus in one of the Crustaceans living in the darkness of the caves of
North America the optic lobes and optic nerves have disappeared, while
the retina of the eye, the lens, and the pigment have been retained,
and in others the reverse has taken place, and the nerve-centres
have persisted while the parts of the eye have been lost (Packard).
Variations of the relevant determinants towards the minus direction
may thus occur, sometimes sooner, sometimes later; but when once they
have started they proceed irresistibly, though with exceeding slowness.
But variation in an upward direction also, when it has once been set
a-going, may in many cases go on unchecked until limits are set to
it by personal selection, when the excess of the organ would disturb
the harmony of the parts, or in any other way lessen the individual's
chances of survival in the struggle for existence. This is proved
especially by the phenomena of artificial selection, for almost all
the parts of fowls and pigeons have been caused to vary to excess by
breeding, and must thus have been, so to speak, capable of unlimited
increase; and yet, as we have seen, personal selection cannot directly
cause progress in any direction of variation; it can only secure a
free course by excluding from breeding the bearers of variations
with an opposite tendency. The beards of hens, the tail-feathers of
the long-tailed domestic cocks, the long and short, straight and
curved bills of pigeons, the enormously long ruffled feathers of the
Jacobin, the multiplication of the tail-feathers in the fan-tail, and
innumerable other breed-characters of these playthings of the breeder,
prove that when variation-tendencies of any part are once present, that
is, when they have arisen through germinal selection, they apparently
go on unchecked until their further development would permanently and
irretrievably destroy the harmony of the parts. As soon as this is
threatened the breed loses its power of survival, and Darwin in his
time cited the case of many extremely short-billed breeds of pigeon,
which require the aid of the breeder before they can emerge from the
hard-shelled egg, because their short and soft bills no longer allow
them to break their way out. Here the correlation between the hardness
of the egg-shell and that of the pigeon's bill has been disturbed, and
the breed can now only be kept in existence by artificial aid.
There must be a possibility of something similar occurring in natural
conditions, and when it does the species concerned must die out. But in
the majority of cases the self-regulation which is afforded by personal
selection will be enough to force back an organ which is in the act
of increasing out of due proportion to within its proper limits. The
bearers of such excessively increased determinants succumb in the
struggle for existence, and the determinants are thus removed from the
genealogical lineage of the species.
Having now established the fact that determinants can continue their
direction of variation without limit because of internal, that is
intra-germinal, reasons, we have come nearer an understanding of
many secondary sexual characters, whose resemblance to the excessive
developments artificially produced in our domestic poultry is so
very striking. Here, too, we shall have to regard germinal selection
as the root of the variations of plumage and other distinguishing
characters, which have evolved by intra-germinal augmentation into
the magnificently coloured crests, tufts, and collars, into the long
or graduated, multiplied or erectile tail-feathers of the birds of
Paradise, pheasants, and humming-birds. The conception of sexual
selection formulated by Darwin will be so far modified, that we are no
longer compelled to regard every minute step in this cumulative process
as due to the selection of the males by the females. A preference of
the finest males may still take place, and is probably general, since
only thus could the distinguishing male characters become common
property, that is, be transmitted to all or the majority of the ids
of the germ-plasm, but the increase of the individual determinants
which are in the act of varying goes on in each individual id, quite
independently of this personal selection.
As it is not a single id with its determinant _a_ in ascending
variation that controls the organ _A_, but as it always requires
a majority of the ids _a_, this must be secured here by personal
selection just as it is in ordinary natural selection. If the
handsomest males are the successful competitors, then a majority of
the transformed ids _a´_ will be transmitted to a number of their
descendants, and the oftener this happens the larger will the majority
be, and the less becomes the danger that it will be dispersed again
by reducing division and amphimixis. Personal selection is thus in
no way rendered superfluous by germinal selection, only it does not
produce the augmentation of the distinguishing characters, but is
chiefly instrumental in fixing them in the germ-plasm; it collects,
so to speak, only the favourably varying ids, and, where complex
variations depending on the proper variation of many ids are concerned,
it combines these. How very great the influence of personal selection
may be in this case of secondary sexual characters we see clearly from
the soberly coloured mates of the brilliant males, for here natural
selection has been operative in conserving the coloration inherited
from remote ancestry.
But if the question be asked, how _the first majority_ of determinants
varying in the same direction is brought about, there are two
possibilities: first, by chance, and secondly, by influences which
cause particular determinants of all the ids to vary in almost exactly
the same manner. We shall find illustrations of the latter among
climatic varieties; but the cases of the first kind are the more
important, for they form the foundation and the starting-point for
processes of selection of a higher order, for personal selection. It
might seem perplexing that processes of such importance should depend
ultimately upon chance; but when we remember that there are only two
directions of variation, namely a plus direction or a minus direction,
we recognize that the chance of a majority in one direction or another
is much greater than that of absolute equilibrium between the two, and
there is therefore a very strong probability that in many individuals
of the species either the upward or the downward movement of a
determinant _A_ will preponderate.
Now as such variation movements, when they are of a certain strength,
increase automatically, we can easily see that they must gradually
attain to a level at which they acquire selection value, and how then,
by personal selection, the ids with favourably varying determinants may
be collected together.
Of course it is not possible to state positively the time at which
in individual cases a variation acquires a biological significance,
that is, selection value. We can only say in a general way that, as
soon as it attains this, personal selection either in a positive or a
negative sense _must_ intervene; an injurious variation tends to the
elimination of its possessor, a useful one increases the probability of
its survival.
There must, however, be for every variation a stage of development in
which it has as yet no decisive biological importance, and this stage
need not by any means be so insignificant that we cannot see it, or
can hardly do so: in other words, there are characters which have
arisen through germinal selection, which are of purely 'morphological
importance.'
It has often been disputed whether there can be any such thing
as 'purely morphological characters,' which are indifferent as
far as the existence of the species is concerned. This question
used to be an important one, because the sphere of operation, and
therefore the importance of the Darwin-Wallace selection--personal
selection--depends on the answer, since this mode of selection only
begins when a character has some biological importance. But as soon as
we take germinal selection into consideration the question loses its
importance, because we now know that every variation is indifferent
to begin with, but every one can, under favourable circumstances,
be increased to such a pitch that it attains biological importance,
and that personal selection then takes over the task of carrying it
on, either in a positive or a negative sense. We may therefore leave
this disputed point alone just now, for while germinal selection
seems still far from being generally recognized, we have to remember
that we are not at all in a position to judge with any certainty as
to the biological value of a character. What labour and painstaking
investigation it has cost to give a verdict as to this even in a
few instances! Innumerable characters appear indifferent, and are
nevertheless adaptations. Darwin in his day pointed out the need for
caution in this matter, referring to the case of animal coloration as
an example; very little attention had been directed to it for a long
time because it had been believed to be without significance. And how
many diverse kinds of characters among animals and plants, which had
likewise been regarded as 'purely morphological,' have on more careful
investigation shown themselves of very great biological importance.
I need only refer to the shape, position, hair-arrangement, colour,
and lustre of flowers, and their relation to cross-fertilization by
means of insects, or to the thickness and shape of the leaves of
tropical trees with their coating of wax and their gutter-like outlets
for carrying off the tropical rain which falls in terrible downpour
(Haberlandt, Schimper), or to the limp, perpendicular drooping of the
tufts of the young and tender leaves of the same trees, which also
secures protection from being battered and torn by the rain.
[Illustration: FIG. 107, _C._ Leptocephalus stage of an American Eel,
with seven pigment spots, of which three are on the left (_l_) and four
on the right (_r_) side. After Eigenmann.]
There are even characters the biological use of which is unknown to
us, but in regard to which we can affirm that they have a use. Thus
Eigenmann described the larva of an American eel, which differs from
other so-called 'Leptocephali' in that a row of seven black spots runs
along its side. Apparently all these lie upon the side turned towards
us, but in reality they are distributed on both sides, three lying on
the left and four on the right, and so arranged that they look like a
single row of spots at regular intervals, for the flat little fish is
absolutely transparent. The habits of this larva are not yet known, but
we may conclude that this appearance of a simple row of spots must have
some value for the animal, for such a significant asymmetry could not
have arisen for purely internal reasons (Fig. 107, _C_). It is possible
that the fish is thus made to resemble parts of some marine alga, and
that it is thereby protected from many enemies; that there is not a
complete row upon each side may depend upon the fact that the two rows
would be visible at the same time, and that they would blur each other
in the eyes of the swimming enemy, and so destroy the resemblance of
the picture to its unknown model.
But it cannot be denied that there are characters which have no special
biological significance. There are doubtless many such characters,
which stand beyond the threshold of good or bad, and which are
therefore not affected by personal selection; it is difficult and often
impossible to point these out with certainty. The shape of the human
nose and of the human ear, the colour of the hair and of the iris, may
be such indifferent characters whose peculiarities are to be referred
solely to germinal selection. On the other hand, I would not venture to
assert that the gay colouring and the complex markings on the wings of
our modern Lepidoptera are always and in all cases unimportant, even
when we cannot interpret their details either as protective, or as a
sign of nauseousness, or as mimetic. The usually very exact similarity
of the colour pattern in the individuals of each species seems to point
to the intervention of personal selection in some form or other, for in
what other way could such a large majority of variations in the same
direction have developed in the germ-plasm as this constancy of the
character indicates.
We know, of course, that the colours of butterflies and moths can be
caused to vary through external and especially climatic influences,
but this would only account for simple modifications of colour, and
not for the origin of the complex colour patterns that actually
occur. I therefore believe with Darwin that sexual selection has had
much to do with this by giving a slight preference to the variations
produced by spontaneous germinal selection, and thus preventing the
majority of varied ids once acquired from being scattered again, but
always collecting more of them, and so securing free play for the
increase of the new character through intra-germinal processes. In
this way have arisen not only the brilliance of our Lycænidæ and of
the large Morphidæ of South America, but also many of the coloured
spots, streaks, bands, eyes, and other components which have gradually
in the course of time evolved into the complex colour pattern of
many of our modern butterflies. I should like to remind any one who
doubts this of a fact which corroborates the view that personal
selection has co-operated in the production of these colours--I
refer to the inconspicuous colouring of the females of many of these
brilliant males--while in contradistinction to these cases there are
other species in which both sexes are alike brilliant, so that it is
impossible that mere spontaneous germinal selection can have determined
that the females, because of their femaleness, should vary in a
different manner from the males.
But while I believe that sexual selection in particular has had much to
do with producing the colours of Lepidoptera, the basis of all these
colour variations must still be looked for in germinal selection, and
we shall see later on how it is possible to think of the diversified
and often relatively abrupt transformations of marking as the resultant
of the co-operation of climatic influences with germinal selection.
Of course there must also be unimportant changes in butterfly-markings
which depend solely on the internal play of forces in the determinant
system, and to this must be referred the markings of many of the
'variable' species whose variations are mere fluctuations in the
details of marking, which have therefore caused much trouble to the
systematists. Truly unimportant variations will rarely or never
combine into a 'constant' form, and the fact that there are species
which are 'variable' in such a high degree is enough to make us refer
their variations to their lack of importance, for if they possessed
any biological value the less valuable among them would gradually be
removed by selection. Perhaps the variable species of certain moths
like _Arctia caja_, and especially _Arctia plantaginis_, the little
'bear' of the Alps and Apennines, must be reckoned among these.
But from the fact that there are such fluctuations in the markings
of Lepidoptera, it seems to me that we must conclude that species
which show a high degree of constancy in their markings have been
influenced by selection, or by climatic influences which turned the
play of forces within the determinant system in the same direction in
all individuals. All these considerations and conclusions are quite
sound and serviceable theoretically, but they are difficult to apply
to individual cases, and where this is attempted it must be with the
greatest caution, and, if possible, on a basis of investigations
specially undertaken for the purpose; for how should we know whether
a species which to-day is highly variable may not a geological
epoch later become a very constant one? We must in any case assume
that marked fluctuations of characters are associated with many
transformations.
LECTURE XXVI
GERMINAL SELECTION (_continued_)
Germinal selection, spontaneous and induced--Climatic
forms of _Polyommatus phlæas_--Deformities--Excessive
augmentation of variations--Can it lead to the elimination
of a species?--Saltatory variations, copper-beech, weeping
trees--Origin of sexual distinguishing characters--Formation
of breeds among domesticated animals--Degenerate jaws--Human
teeth--Short-sightedness--Milk-glands--Small hands and feet--Ascending
variation--Talents, intellect--Combination of mental endowments--The
ultimate roots of heritable variation--There are only plus- and
minus-variations--Relations of the determinants to their
determinates--The play of forces in the determinant system of the
id--Germinal selection inhibited by personal selection--Objection on
the score of the minuteness of the substance of the germ-plasm.