the germ-plasm in the case of purely asexual multiplication, and what
conclusions may be drawn from this, and then let us compare these
with the known observational data, and it will be apparent that in
individuals which have arisen by budding the complete germ-plasm
of the species must be contained; the number of ids will not only
remain the same in the bud as it was in the mother plant, but the
number of _different ids_ will not be diminished. The case is
analogous to that of pure parthenogenesis, in which the absence of
the second maturation-division of the ovum allows the germ-plasm to
retain the full complement of ids. Charles Darwin held that purely
asexual multiplication was 'closely analogous to long-continued
self-fertilization,' yet, as we have seen, according to our theory
there must be a not inconsiderable difference between the two
processes, depending on the fact that in exclusive self-fertilization
the number of different ids is continually decreasing, while in purely
asexual reproduction the germ-plasm loses nothing of the diversity of
its ids. If, therefore, the germ-plasm in purely asexual reproduction
no longer receives fresh ids through amphimixis, it at least loses none
of those it formerly possessed. Although we cannot consider it adapted
for entering upon new adaptations in many directions, yet we may expect
that the species will continue to reproduce unchanged for longer
than in the case of exclusive self-fertilization, the more so since
all unfavourable variational tendencies which crop up are eliminated
as soon as they attain to selection-value, and, as in the case of
parthenogenesis, they are eliminated without being mingled with other
lines of descent.
Let us take, for instance, the purely asexual reproduction which
obtains in Algæ of the genus _Laminaria_, in regard to which it is
stated that it multiplies only through asexual swarm-spores. There
are quite a number of species of this large tangle, and if it should
be established that in all these the spore-cells really do not
conjugate, then the case would prove that the species of a genus can
maintain a well-defined existence for a long time after amphimixis has
been given up. But this would not be a proof of the possibility of
_species-formation_, for that the ancestral forms of the Laminarians
must have possessed amphigony may be assumed, since their nearest
relatives exhibit it still. It cannot be proved, but there seems
nothing against the assumption that these tangles have existed for a
long time under uniform conditions, and have become adapted to these
with a high degree of constancy.
The conditions are similar in the marine Algæ of the genus _Caulerpa_,
the nearest relatives of which reproduce sexually, though they
themselves, as far as is known, reproduce only by spores.
In the Lichens, which represent, as we have already seen, a
life-partnership between Fungi and Algæ, amphimixis appears not to
occur at all; the unicellular Alga reproduces by cell-division, the
Fungus by producing a great number of swarm-spores, which do not
conjugate with one another. As far as the Alga is concerned we might
perhaps suppose that the simplicity of its structure makes it possible
for it to dispense with a constant recombination of its few characters
to bring about the most favourable composition in its idioplasm; in
support of this we may note that even the life-long combination with
the Fungus has caused no visible variation in the Alga, as we must
conclude from the fact that these Algæ can also live independently, and
that the same species of Alga may combine with several different Fungi
to form different species of lichen, just as the same Fungus may also
form part of several species of lichen. We might also imagine that we
have here no more than a direct influence of the Alga and Fungus upon
one another, and that there is no adaptation to the new conditions of
life at all, yet that can hardly be seriously maintained in regard to
species which live under such definite and diverse conditions. It now
seems to be established--contrary to the older statements--that the
lichen-fungus only reproduces asexually, and in face of this it seems
to me that nothing remains except to make the assumption that lichens
formerly possessed sexual reproduction, but that they have lost it,
though whether all have done so is, perhaps, not yet quite certain.
The same assumption must be made in regard to the Basidiomycetes among
the Fungi, and for most of the Ascomycetes, for in these groups of
Fungi sexual reproduction has only been demonstrated 'with certainty in
a few genera.' That in these cases also there has been a degeneration
of amphigony, until it has completely disappeared, seems probable from
the two other groups of Fungi, the Zygomycetes and Oomycetes, since in
these 'a reduction of sexuality amounting in some cases to complete
disappearance' can be demonstrated even in existing forms. But whether
it may be assumed that the Fungi which are now asexual are no longer
capable of new adaptations, and whether their parasitic habit may be
regarded as making up in some way for the lack of the remingling of
the germ-plasm, as the botanist Möbius supposes, I am not able to
decide. It is obvious that data in regard to amphimixis among the Fungi
are still incomplete, and recent investigations lead us to suspect
that sexual mingling may not be absent, but only disguised. Dangeard,
Harold Wager, and others have observed that a fusion of nuclei precedes
the formation of spores, and this may be regarded as amphimixis,
although the conjugating nuclei belong to cells of the same plant and
sometimes even to the same cell. But although we are here dealing
with a set of facts which cannot yet be satisfactorily formulated in
terms of our theory, it is nevertheless not contradictory to it that
amphimixis should be wholly absent in the higher Fungi. But the fact
would be contradictory to the unadulterated rejuvenescence-theory, for
if amphimixis were really a condition of the continuance of life, no
species--as we have already said--could continue to exist without it
for countless generations.
[Illustration: FIG. 38 (repeated). A fragment of a Lichen (_Ephebe
kerneri_), magnified 450 times. _a_, the green alga-cells. _P_, the
fungoid filaments. After Kerner.]
The same argument holds true for the higher plants, which have become
purely asexual under the influence of cultivation. I refer to many
of the well-marked varieties of our cultivated plants which multiply
exclusively, or almost exclusively, by means of tubers and slips,
as is the case with the potato, the manioc, the sugar-cane, the
arrowroot-plant (_Maranta arundinacea_), and others. All these facts
can easily be reconciled with our interpretation of the meaning of
amphimixis, although the attempt to range them as evidence against
our theory has more than once been made. We have thus arrived at the
conclusion that while many-sided adaptations, that is, variations
which transform the plant in accordance with the indirect influences
of new conditions of life, cannot be brought about without a
persistent mingling of germ-plasms, simple modifications may readily
appear although amphimixis is altogether absent. If a wild plant be
permanently transferred to a well-manured culture-bed, it is probable
that certain changes will occur in it, either gradually or at once.
But these are not adaptations; they are, so to speak, direct reactions
of the organism which do not even require selection to make them
increase, but depend upon the influencing of certain determinants of
the germ-plasm, and which, like all germinal variations, will follow
their course steadily until a halt is called either by germinal or by
personal selection. When a given plant is exposed to these new and
artificial conditions, the changes in question make their appearance
sooner or later, and follow their course, and go on increasing as long
as that is compatible with the harmony of the structure and functioning
of the plant, this depending, as in all individual development, on the
struggle between the parts, that is to say, on histonal selection.
Only in this respect is the utility or injuriousness of the change of
importance, for personal selection, the struggle between individuals,
does not affect plants which are under cultivation.
That such modifications may increase and may persist through many
generations, even with asexual multiplication, depends upon the fact
that the budding cells contain germ-plasm, as well as the germ-cells,
and if particular determinants of the germ-plasm in general are caused
to vary by these new influences, the variation may be transmitted
from bud to bud, from shoot to shoot, and so go on increasing as long
as the new conditions persist, as well as in amphigonic (bisexual)
reproduction, where they are transmitted from germ-cell to germ-cell.
It is not inconceivable that an individual adaptation, that is to
say a useful adjustment, might be effected in the course of asexual
reproduction, although it is improbable that direct influences
would give rise to just those changes which would be useful under
the new conditions. But there are a number of cases which have been
interpreted in this way. In several of the cultivated plants named,
the reproductive organs have themselves degenerated, either only the
male, or only the female, or both at the same time; and some observers,
accepting the hypothesis of an inheritance of functional modifications,
have regarded this as the direct result of disuse during the long
period of asexual reproduction.
Leaving out of account this erroneous presupposition, we may ask how
asexual reproduction, such as that of the potato by tubers instead of
by seed, which has gone on exclusively for several centuries, could
exercise any influence upon the flowers and seed-forming of this
species? In point of fact it has exercised none in most potatoes, for
the flowers and seeds are just as fertile now as they were when the
potato was first discovered.
Whether the pollen of a flower is utilized in one or other of its
thousands of pollen-grains by reaching the stigma of another plant
of the same species, or whether all the pollen-grains are uselessly
scattered abroad, cannot possibly affect the flower so as to cause
degeneration; the theory of disuse cannot be applied in this case.
What is true of the potato holds good also of the manioc (_Manihot
utilissima_), but, on the other hand, many of the best varieties of
common fruits--pears, figs, grapes, pine-apples, and bananas--are
seedless. In _Maranta arundinacea_ 'the whole wonderful structure
of the flower has persisted, but the pollen-grains, that is the
germ-cells, are wanting.' Whether this implies a permanent degeneration
of the sexual organs, that is to say, one that is embodied in the
primary constituents of the species, or whether it is only the result
of over-abundant nourishment, or of other causes in the circumstances
affecting the particular plant, can only be decided by experiment.
Probably both occur. The common ivy, for instance, does not now blossom
in the northern parts of Sweden and Russia, but it does so still in
the southern provinces. If plants were brought to us from the most
northerly zone of distribution, they would in all probability flower
and bear fruit with us, and in that case the absence of bloom in these
plants must have been a direct effect of the cold climate. But it is
quite conceivable that cultivated plants have in many cases become
hereditarily infertile, when they are constantly propagated only by
means of buds, layering, and so on, not however because of any direct
effect of this mode of propagation, but through chance germinal
variations. For in regard to many of them man has lost all interest in
the flowers and fruit, as, for instance, in the case of the potato; in
other cases he is even interested in procuring seedless fruits.
In the first case he will quite readily make use of plants with
imperfect flowers for propagating, if they are otherwise fit and
exhibit what he wants in other respects; in the second case, he will
give a preference to individuals with seedless fruits, and thus
increase and strengthen the tendency to degeneration of the seeds in
the race concerned.
All these cases are quite in harmony with our conception of amphimixis,
which, now that we have investigated the facts throughout the animate
kingdom, we may sum up in the following propositions. In the whole
organic world, from unicellular organisms up to the highest plants
and animals, amphimixis now means an augmentation of the organism's
power of adaptation to the conditions of its life, since it is only
through amphimixis that simultaneous harmonious adaptation of many
parts becomes possible. It effects this by the mingling and constant
recombination of the germ-plasm ids of different individuals, and thus
gives the selection-processes the chance of favouring advantageous
variational tendencies and eliminating those which are unfavourable,
as well as of collecting and combining all the variations which are
necessary for the further evolution of the species. This indirect
influence of amphimixis on the capacity of organisms for surviving
and being transformed is the fundamental reason for its general
introduction and for its persistence through the whole known realm of
organisms from unicellulars upwards.
The reason for its _first_ introduction among the lower forms of life
must have been a direct effect which had a favourable influence on the
metabolism, and this is so far coincident with the subsequent import of
amphimixis, inasmuch as it may be regarded not only as a heightening of
the power of adaptation, but as an immediate and direct increase and
extension of the power of assimilation. In any case, amphimixis is not
necessary to the actual preservation of life itself, but it does bring
about a wealth and diversity of organic architecture which without it
would have been unattainable.
If amphimixis has been abandoned in the course of phylogeny by isolated
groups of organisms, this has happened because other advantages accrued
to them in consequence, which gave them greater security in the
struggle for existence; but it must be admitted that they thereby lost
their perfect power of adaptation, and that they have thus bartered
their future for the temporary securing of their existence.
In addition to this variational influence, amphimixis has also played a
part in the evolution of sharply defined organic types, especially of
specific types; but of this we shall have more to say later on.
LECTURE XXXI
THE INFLUENCES OF ENVIRONMENT
Different modes and grades of selection--Changes due to the influences
of environment--Superfluity and lack of food--The horses and cattle
of the Falkland Islands--Angora animals--Protection against cold
in Arctic and marine mammals--Plant-galls--Nägeli's _Hieracium_
experiments--Experiments with _Polyommatus phlæas_--Artificially
produced _Vanessa_-aberrations--Vöchting's experiments on the
influence of light in the production of flower-forms--Heliotropism
and other tropisms--Primary and secondary reactions of
organisms--Herbst's 'lithium larvæ'--Schmankewitsch's experiments
with _Artemia_--Poulton's caterpillars with facultative colour
adaptation--Colour-change in fishes, chamæleon, &c.--Actual scope of
those influences which directly produce organic changes.
Through a long series of lectures we have devoted our attention to
those phenomena which bear some relation to the processes of selection;
we have attempted to gain clearness in regard to the modes and stages
of these, and we reached the result that all variations which have
taken place in organisms since the first appearance of living matter
are directed by processes of selection, that is, their direction and
duration are determined by these processes, although they may have
their roots in external influences. But it is not to be supposed that
this guidance is due solely to that one kind of selection which,
with Darwin and Wallace, we designate 'natural selection'; on the
contrary, we must regard this as only one of the different modes of
the processes of selection, necessarily occurring between all living
units which are equivalent to one another, and which, therefore, must
maintain a continual struggle with one another for space and food. If
the expression 'natural selection' were not already so firmly fixed
in its meaning, I should propose that it should be employed in the
most general sense for all the processes of selection collectively,
but we must keep to its original meaning and use it only for personal
selection.
We have seen that processes of selection take place even between the
elements of the germ-plasm in all organisms which possess a germ-plasm
as distinguished from the mass of the body, and that through these
processes there arise those hereditary individual variations which,
under some circumstances, form the basis of transformations in the
species.
Obviously this may come about in a twofold manner: firstly, a
variation movement originating in the germ-plasm may go on increasing
till it attains to selection-value, and then 'personal selection' steps
in, and seeks to make it the common property of the species. But it
is obviously also conceivable that variational tendencies arising in
the germ-plasm may never attain to selection-value at all, and then
in most cases they will only continue to exist through a longer or
shorter series of generations as individual distinguishing characters,
without being transmitted to a larger number of individuals or becoming
a constant character of the species. Their persistence will depend
essentially on the chance of mingling with other individuals, and on
the halving of the germ-plasm which precedes sexual reproduction.
Sooner or later these individual peculiarities disappear again, as may
often be observed in the case of abnormalities or morbid tendencies in
man, in as far as these do not weaken vitality. In the latter case they
attain selection-value, though only negatively.
But even quite indifferent germinal variations, which neither raise
nor lower the individual's power of survival, may, under some
circumstances, increase and lead to permanent variations of all the
individuals of a species, and this happens when they are conditioned by
external influences which affect all the individuals of a species, or
of the particular colony concerned, and it is this kind of organismal
change which we shall now study for a little in detail.
The ordinary never-ceasing, always active germinal selection depends,
we must assume, upon intra-germinal fluctuations of nutrition, or
inequalities in the nutritive stream which circulates within the
germ-plasm. The variations which it produces may, therefore, be
different in each individual, since these fluctuations are a matter
of chance and may affect the determinants _A_ in one individual and
the determinants _B_, _C_, or _X_ in another, or alternating groups
of these. Or it may be that the homologous determinants _A_ may vary
in a plus direction in one individual, and in a minus direction in
another, while in a third they may remain unchanged, and although the
same direction of variation of a determinant _N_ may occur in many
individuals, it will certainly not do so in all, and still less will
it occur in all along with the same combination of fluctuations in the
rest of the determinants. It is only if this occurs that the variation
can become a specific character.
We might expect on _a priori_ grounds that not only the chance
fluctuations of nutrition within the germ-plasm would cause its
elements to vary in this or that direction, but that there would also
be influences of a more general kind, especially those of nutrition and
climate, which would in the first place affect the body as a whole, but
with it also the germ-plasm, and which would therefore bring about
variations, either in all or only in certain determinants. In this case
all the individuals would vary in the same way, because all would be
similarly affected by the same causes of change.
This is actually the case; it is indubitable that external influences,
such as those emanating from the environment or media in which species
live, are able to cause direct variation of the germ-plasm, that is,
permanent, because hereditary variations. We have already referred to
this process and called it 'induced germinal selection.'
That such influences of environment may bring about changes in
_individual_ organisms is obvious enough; that, for instance, good
nutrition makes the body strong and vigorous, that too abundant food
makes it fat and causes degeneration, that insufficient food lessens
its stamina and vigour, are well-known facts. We have to inquire, on
the one hand, to what extent such influences are able to cause changes
in the individual body in the course of a lifetime, and, on the other
hand, more particularly, how far such changes or modifications of the
soma can call forth corresponding variations in the determinant system
of the germ-cells, and whether and under what circumstances they may be
transmitted; for where this is not the case there can be no permanent
hereditary variation of the whole species, and the variation will only
persist as long as the conditions which gave rise to it endure, and
will disappear again with these.
The influence of nutrition as a cause of variation has often been
over-estimated. The old statement which has gone the round of the
textbooks since the time of John Hunter, that the stomach of carnivores
may be transformed by vegetable diet into a herbivore stomach, is
absolutely unproved. Brandes at least, who not only subjected all the
statements in the literature on this point to a critical investigation,
but also instituted experiments of his own, regards the statement as
altogether unfounded. All the 'cases' cited, in which the stomach of a
gull or of an owl fed on grain became transformed into an organ with
stronger muscles and covered with horny plates, depend, according to
Brandes, upon inexact observation. There can therefore be no question
of any inheritance of this fictitious stomach-transformation, and the
idea that such a fundamental histological adaptation as the alleged
transformation of the stomach of the grain-eating bird should arise as
a direct effect of the food is wholly without foundation.
But it is quite otherwise with purely quantitative differences in
nutrition. That meagre diet influences individuals unfavourably is
indubitable, and we are certainly justified in considering whether
this may not have an effect on the germ-cells, and one which will
correspond to the changes induced on the body, so that if the
poor nutrition should last through many generations an hereditary
degeneration of the species would occur, which would not at once
disappear though the animals were transferred to more favourable
conditions.