germ-plasm, upon which we based the process of germinal selection,
from _chance local_ fluctuations in nutrition, such as must occur in
an individual id, independently of the nutrition of the other ids of
the same germ-plasm. But there are doubtless also influences which set
up similar nutritive changes in _all ids_, and by which, therefore,
all homologous determinants, in as far as they are sensitive to the
nutritive change in question, are affected in the same manner. To
this category belong changes in the external conditions of life, and
particularly climatic changes. It is, then, germinal selection alone
which brings about the presence of a majority of ids with determinants
varying in the same direction, and personal selection has no part
in the transformation of the species. Many years ago I instituted
experiments with a small butterfly, _Pararga egeria_, and these
showed that a heightened temperature so influenced the pupæ of this
form that the butterflies emerged with a different and deeper yellow
ground-colour, similar to that of the long-known southern variety
_Meione_. More thoroughly decisive, however, were the experiments on
_Polyommatus phlæas_, the small 'fire-butterfly,' which were carried
on in the eighties by Merrifield in England and by myself almost at
the same time. I shall discuss these later in more detail, and will
only say here that this butterfly, whose range extends from Lapland
to Sicily, occurs in two forms, the southern distinguished by a
'dusting' of deep black from the northern, in which the wing-surfaces
are of a pure red-gold. The experiments showed that the southern
form can be artificially produced by warmth, and the interpretation
must be that the direct influence of higher temperature affects the
quality of the nutritive fluids in the germ-plasm, and thereby at
the same time the determinants of one or more kinds of wing-scales
are caused to vary in all the ids in the same direction, in such a
fashion that they give rise to black scales instead of the former
red-gold ones. It is thus certain that there are external influences
which cause particular determinants to vary in a particular manner.
I call this form of germinal variation 'induced' germinal selection,
and contrast it with 'spontaneous' selection, which is caused, not by
extra-germinal influences, but by the chances of the intra-germinal
nutritive conditions, and which will, therefore, not readily occur at
the same time in all the ids of a germ-plasm, and so will not give rise
to variation of the same kind in the homologous determinants of all the
ids.
The two processes must also be distinguished from each other in their
relation to personal selection, for induced germinal selection will
go on increasing until the maximum of variation corresponding to the
nature of the external influences and of the determinants concerned
is reached. Since _all_ the ids are equally affected and caused to
vary in the same way, personal selection has nothing to take hold of,
and the variation might go on intensifying even if it should become
biologically prejudicial. But it is quite otherwise with spontaneous
germinal selection, which has its roots not in all, but only in a
majority of the ids. Here the variation may go on increasing by
germinal selection alone, but only until it acquires a positive or
negative biological value, that is, until it becomes advantageous or
prejudicial to the life of the individual; then personal selection
intervenes and decides whether it is to go on increasing or not.
Spontaneous germinal selection can therefore only lead to the general
variation of a whole species when it is supplemented by some external
factor such as, especially, the utility of the variation.
This does not imply, however, that indifferent variations of large
amount could not arise through spontaneous germinal selection, but
they would remain confined to a small number of individuals, and
would sooner or later disappear again. The congenital deformities of
Man may in part fall under this category. If, for instance, certain
determinants are, by reason of specially favourable local nutritive
conditions, maintained for a long time in progressive variation, they
will become so strong that the part which they determine will turn
out excessive, perhaps double. Hereditary polydactylism in Man may
perhaps be explained on this principle, and I had already referred
it to the more rapid growth and duplication of certain determinants
of the germ even before formulating the idea of germinal selection.
In this I was at one with the pathologist Ernst Ziegler, who had
designated polydactylism as a germ-variation, and in contrast to
others had not interpreted it in an atavistic sense, as a reversion to
unknown six-fingered ancestors. All excessive or defective hereditary
malformations may be referred to germinal selection alone, that is, to
the long-continued progressive or regressive variation of particular
determinant-groups in a majority of ids.
The fact that, as far as our experience goes, superfluous fingers
are never inherited for more than five generations may be simply
explained, for there has been no reason for the intervention of
personal selection, either in the negative sense, for the six-fingered
state does not threaten life, nor in the positive, since it is not of
advantage. The deformity depends on spontaneous germinal variation,
which must have taken place in a majority of ids or it would not
have become manifest. But such a majority of 'polydactylous' ids is
liable to become scattered again in every new descendant, and to be
reduced again into a minority which can no longer make itself felt by
the chances of reducing division and the admixture of normal ids in
amphimixis. A polydactylous race of men could only arise through the
assistance of personal selection; in that case there would doubtless
be just as much chance of success in breeding a six-fingered race as
there was in breeding the crooked-legged Ancon sheep from a single ram
which was malformed in this manner. Without a gradual setting aside of
the germs with normal ids, that is, without personal selection, such
spontaneous deformities, and indeed all _spontaneous_ variations, must
fail of attaining to permanent mastery.
This must frequently be the case in free nature also, but we shall have
to investigate later on, in the section devoted to the formation of
species, whether external circumstances (inbreeding) may not also occur
which make it possible for spontaneous variations to become constant
breed-characters, even although they remain neither good nor bad, and
are thus not subject to the action of personal selection.
In general, however, amphigony with its reduction of the ids and its
constant mingling of strange ids will form the corrective to the
deviations which may arise through the processes of selection within
the id, and which lead to excessive or superfluous development of
certain structures, to a complete disturbance of the harmony of the
parts, and ultimately to the elimination of the species.
It must be admitted, however, that Emery was probably right when he
directed attention to the possibility of a 'conflict between germinal
and personal selection.' It is quite conceivable that in cases of
useful variations, that is, of adaptations, the processes of selection
within the germ-plasm may lead to excessive developments, which
personal selection cannot control, because, on account of their earlier
usefulness, they have in the course of a series of generations and
species become fixed not only in a majority of ids, but in almost all
the ids of the collective germ-plasm of the species. In this case a
reversal must be difficult and slow, for the gathering together of ids
with relatively weaker determinants can only take place slowly, and
it is questionable whether the species would survive long enough for
the slow process to take effect. But, apart from the question of time,
such a reduction of an excessive development would sometimes be quite
impossible, for the simple reason that there is nothing for personal
selection to take hold of.
Döderlein has pointed out that many characters go on increasing through
whole series of extinct species, and ultimately grow to such excess
that they bring about the destruction of the species, as, for instance,
the antlers of the giant stag or the sabre-like teeth of certain
carnivores in the diluvial period. I shall have to discuss this in more
detail in speaking of the extinction of species; it is enough to say
here that such long-continued augmentations in the same direction can
never be referred _solely_ to germinal selection, since it is hardly
conceivable that a species--much less a whole series of species--should
arise with injurious characters; they would have become extinct while
they were still in process of arising. Although we see that the
Irish stag, with his enormous antlers over ten feet across from tip
to tip, was heavily burdened, we are hardly justified in concluding
that the size and weight of the burden on his head tended to his
destruction from the first--for in that case the species would never
have developed at all--but it may well be that at some time or other
the life-conditions of the species altered in such a manner that the
heavy antlers became fatal to it. In this case the variation-direction
which had gained the mastery in all ids could no longer be sufficiently
held in check by personal selection, because the variations in the
contrary direction would be much too slight to attain to selective
value. Sudden, or at least rapidly occurring changes in the conditions
of life, such as the appearance of a powerful enemy, exclude all chance
of adaptation by the slow operation of personal selection.
If we look into the matter more carefully, we see that it is not
strictly true to say that germinal selection alone brings about the
extinction of a species by cumulative augmentation of structures which
are already excessive; _it is the incapacity of personal selection
to keep pace with the more rapid changes in the conditions of life
and to reduce excessive developments to any considerable extent in a
short time_. This would always be possible in a long time, for the
determinants of the excessive organ _E_ can never be equally strong in
all the ids; they always fluctuate about a mean, however high this mean
may be. Here again it must still be possible that reducing-divisions
and amphimixis may lead to the formation of majorities of ids with
weaker _E_-determinants, and if sufficient time be allowed, artificial
selection could, by consistently selecting the individuals with, let
us say, weaker antlers, give rise to a descending variation-movement.
There are no variation-movements which cannot be checked; every
direction can be reversed, but time and something to take hold of
must be granted. That was wanting in the case of the giant stag, for
it would not have been saved even if its antlers had at once become a
couple of feet shorter, and germinal selection can hardly make so much
difference as that.
Analogous to hereditary deformities, and of special interest in
connexion with the processes within the germ-plasm, are '_sports_'
variations of considerable magnitude which suddenly appear without our
being able to see any definite external reason for them. I have already
discussed these in detail in my _Germ-plasm_, and have shown how simply
these apparently capricious phenomena of heredity can be understood _in
principle_ from the standpoint of the germ-plasm theory.
The chances of the transmission of the saltatory variation will be
greater or less according to whether the variation of the relevant
determinants involves a bare majority of ids or a large majority, for
the more ids that have varied, the greater is the probability that the
majority will be maintained throughout the course of ensuing reducing
divisions and amphimixis, that is, that the seeds of the plant will
reproduce the variation, and will not revert to the ancestral form.
Although one of the most satisfactory results of the id-theory lies
precisely in the interpretation of these conditions, I do not wish
to enter into the matter here, but will refer to the details in my
_Germ-plasm_, published in 1894, which I consider valid still. At that
time I had not formulated the idea of germinal selection, but the
explanation of the occurrence of such sport-variations which I gave was
based upon the assumption of nutritive fluctuations in the germ-plasm,
which gave rise to variations in certain determinants. There was still
lacking the recognition that the direction of variation once taken must
be adhered to until resistance was met with, and that the determinants
stand in nutritive correlation with one another, so that changes in
one determinant must re-act upon the neighbouring ones, as I shall
explain more fully afterwards. I also showed from definite cases that
such sports, though they are sudden--'saltatory'--in their mode of
occurrence, are long being prepared for by intimate processes in the
germ-plasm. This 'invisible prelude' of variation depends on germinal
selection. When a wild plant is sown in garden-ground it does not
require to vary at once; several, even many, generations may succeed
each other which show no sports; suddenly, however, sports appear,
at first singly, then, perhaps, in considerable numbers. It is not,
however, by any means always the case that considerable numbers occur,
for some varieties of our garden flowers have arisen only once, and
then have been propagated by seed; and such saltatory sports in plants
which are raised from seed are usually constant in their seed, and if
they are fertilized with their own pollen they breed true--a proof that
the same variations must have taken place in the relevant determinants
in a large majority of ids.
In animals, it would appear, such saltatory variations occur much more
rarely than in plants; the case examined in detail by Darwin of the
'black-shouldered peacock' which suddenly appeared in a poultry-yard is
an example of this kind. Much more numerous, however, are the instances
among plants, and especially among plants which are under cultivation.
This indicates that we have here to do with the effect of external
conditions, of nutritive influences which cause the slow variation of
certain determinants, sometimes abetting and sometimes checking. As
soon as a majority of ids varied in this way comes to lie in a seed, a
sport springs up suddenly and apparently discontinuously--a plant with
differently coloured or shaped petals or leaves, with double flowers,
with degenerate stamens, or with some other distinguishing mark, and
these new characters persist if the variety is propagated without
inter-crossing.
But it happens sometimes, though more rarely, that not the whole
plant but individual shoots may exhibit the variation. To this class
belong the 'bud-variations' of our forest trees, the copper-beeches,
copper-oaks, and copper-hazels, the various fasciated varieties of oak,
beech, maple, and birch, and the 'weeping trees'; also the numerous
varieties of potato, plantain, and sugar-cane. It seems that only a
few of these breed true when reproduced from seed, or in other words,
they usually exhibit reversions to the ancestral form: on the other
hand, in the weeping oak for instance, nearly all the seedlings exhibit
the character of the new variety, though 'in varying degrees.' The
records as to the transmissibility of bud-variations through seed are
probably not all to be relied upon, and new investigations are much
to be desired, but the fact that in many cases they may be propagated
not only by means of layers and cuttings but by seed also, is most
important in our present discussion, for it proves that here too the
varied determinants must be contained in a majority of ids. As it
is only a single shoot that exhibits the saltatory variation, only
the germ-plasm which was contained in the cells of this one shoot
can have varied, and it must have done so in so many ids that the
variation prevailed and found expression. But that, in this case also,
the variation does not appear in all, but only in a small majority
of ids, is proved by the frequent reversion of bud-varieties to the
ancestral form. I have already reported a case of this kind shown to
me by Professor Strasburger in the Botanic Gardens in Bonn, where a
hornbeam with deeply indented 'oak-leaves' had one branch which bore
quite normal hornbeam leaves. In my own garden there is an oak shrub
of the 'fern-leaved' variety, whose branches bear some leaves of the
ordinary form; variegated maples with almost white leaves often exhibit
in individual branches a reversion to the fresh green leaves of the
ancestral form. We see from this that what is so energetically disputed
by many must in reality occur--namely, differential or non-equivalent
nuclear division--for otherwise it would be unintelligible how
the ids of the new variety, if they once attain a majority in the
tree, could give place in an individual branch to a majority of the
ancestral ids. Only differential nuclear division, in the manner of a
reducing division, can be the cause of this. Of course this implies
only a dissimilar or differential distribution of the ids between the
two daughter-nuclei, not a splitting up of the individual ids into
non-equivalents.
That in free Nature bud-variations left to themselves can ever become
permanent varieties is probably an unlikely assumption, because of the
inconstancy of their seeds which only breed true in rare cases; nor
is it likely that such variations as the copper-beech, the weeping
ash, and so on could hold their own in the struggle for existence
with the older species; but there is certainly nothing to prevent our
assuming that, in certain circumstances, saltatory variations, when
they have a germinal origin, may become persistent varieties and may
even lead to a splitting of the species. This may happen, for instance,
when the variations remain outside the limits of good and bad, and
thus are neither of advantage to the existence of the species nor a
drawback thereto. In the next chapter we shall discuss the influence of
isolation upon the formation of species, and it will be seen that in
certain conditions even indifferent variations may be preserved, and
that saltatory variations, as for instance in the evolution of species
of land-snails or butterflies, may have materially contributed to bring
this about.
I should like to emphasize still more the part played by saltatory
variations arising from germinal selection in the origin of secondary
sexual characters. As soon as personal selection, whether sexual
or ordinary, prefers as useful in any sense a saltatory variation,
it is not only preserved and becomes a character of a variety, but
it may increase, and we have to ask whether such sudden variations
are frequently of a useful kind, especially when not individual
characters alone, but whole combinations of them are implicated. If
we may judge from the sports of the flowers and the leaves of plants,
transformations useful to the species as a whole rarely occur suddenly,
that is, they occur only in a few out of very numerous sports; they
are much more frequently indifferent, although quite visible and often
conspicuous variations.
For this reason I am disposed to attribute to saltatory variations a
considerable share in the production of distinctive sexual characters.
From saltatory variations in flowers, fruits, and leaves we know that
these may be conspicuous enough even on their first appearance, and so
we are justified in finding in such variations the first beginnings of
many of the decorative distinguishing characters which occur in the
males of so many animals, especially butterflies and birds. As soon as
it is admitted that variations of considerable amount, which have been
slowly prepared in the germ-plasm by means of germinal selection, can
suddenly attain to expression, one of the objections against sexual
selection is disposed of, for conspicuous variations are necessary for
the operation of this kind of selection, since the changes in question
must attract the attention of the females if they are to be preferred.
Without such preference, even though it be not quite strict and
consistent, a long-continued augmentation of the decorative characters
is inconceivable.
But as intra-germinal disturbances of the position of equilibrium in
the determinant system is at the root of the saltatory variations of
our cultivated plants, it must also have played a large share in the
evolution of breeds among our domesticated animals, which is therefore
by no means wholly due to artificial selection operating upon the
variation of individual characters. In all breeds in the formation of
which the production of more than a single definite character was
concerned, as, for instance, in the broad-nosed breeds of dog--bull-dog
and pug-dog--we may refer the peculiar variation of many parts to
disturbances of the equilibrium of the determinant system, which bring
to light, not suddenly as in the case of saltatory variations, but
gradually and increasingly, the curious complex of characters. Darwin
referred such transformations of the whole animal facies, where a
single varying character is deliberately selected, to correlation, and
by this he understood the mutual influence of the parts of an animal
upon one another. Such correlation certainly exists, as we have already
seen in discussing histonal selection, but here we have rather to do
with the correlation of the parts of the germ-plasm, with the effects
of germinal selection, which, affected by the artificial selection of
particular characters, gradually brings about a more marked disturbance
in the whole determinant system.
In the evolution of our breeds of domesticated animals, germinal
selection in the negative sense must also have played a part--I mean
through the weakening and degeneration of individual determinants.
Only in this way, it seems to me, can we explain the tameness of our
domestic animals, dogs, cats, horses, &c., in which all the instincts
of wildness, fleeing from Man, the inclination to bite, and to attack,
have at least partly disappeared. It is, of course, very difficult to
estimate how much of this is to be ascribed to acquired habitude during
the individual lifetime. The case of the elephant might be cited in
evidence of tameness which arises in the individual lifetime, for all
tame elephants are caught wild, but it seems that captured young beasts
of prey, such as the fox, wolf, and wild cat, not to speak of lions and
tigers, never attain to the degree of tameness exhibited by many of our
domesticated dogs and cats. The very considerable differences in the
degree of tameness of dogs and cats go to show that the case is one of
instincts varying in different degree.
If this be so, then the instinct of wildness, if I may express myself
so for the sake of brevity, has degenerated in consequence of its
superfluity, and through the process of germinal selection, which
allowed the determinants of the brain-parts concerned to set out on a
path of downward variation upon which they met with no resistance on
the part of personal selection.
Herbert Spencer adduced against my position the case of the reduction
in the size of the jaws in many breeds of dog, especially in pugs and
other lap-dogs, which he regarded as evidence of the inheritance of
acquired characters. But this and analogous cases of the degeneration
of an organ during a long period in which the animal had been withdrawn
from the conditions of natural life is intelligible enough on the
assumption of persistent germinal selection aided by panmixia. The jaws
and teeth in these spoilt pets no longer require to be maintained at
the level of strength and sharpness essential to their ancestors which
depended on these characters, and so they fell below it, became smaller
and weaker, but could not disappear altogether, for the process of
degeneration was brought, or is being brought, to a standstill by the
intervention of personal selection.
Even the lower jaw in Man is declared by many authors to be degenerate.
Collins found that the lower jaw of the modern Englishman was one-ninth
smaller than that of the ancient Briton, and one-half smaller than that
of the Australians; Flower showed that we are a microdont race like
the Egyptians, while the Chinese, Indians, Malays, and Negroes are
mesodont, and the Andamanese, Melanese, Australians, and Tasmanians are
macrodont. This does not of itself imply that we exhibit a degeneration
of dentition, though this conclusion is hinted at by other facts,
such as the variability of the wisdom-teeth. It need not surprise us,
indeed, that a retrogressive variation tendency should have started
in this case, for, with higher culture and more refined methods of
eating, the claims which personal selection was obliged to make on the
dentition have been greatly diminished, and germinal selection would
thus intervene.
Every one knows how the quality of human teeth has deteriorated with
culture, and this not in the higher classes only, but even among the
peasantry, as Ammon has observed. The time is past when raw flesh
was a dainty, and when bad teeth meant poor nutrition, if not actual
starvation. Even nowadays famine plays a terrible and periodically
recurrent rôle as an eliminator among some negroid races.
Many other organs in man have been reduced from their former pitch of
perfection through culture, and some of them are still in process of
dwindling. When I formulated the idea of panmixia and applied it to
explain cases which had previously been referred to the inheritance of
the results of disuse, I regarded the short-sightedness of civilized
Man from this point of view. My opinion aroused lively opposition at
the time, especially on the part of oculists, who very emphatically
referred the phenomenon to the inheritance of acquired shortsight,
and indeed regarded it as a proof of the transmission of functional
modifications.