_Thuja_-shoot possesses the possibility of development in two different
ways instead of only one. The upper side of the shoot can assume the
structure of the under side and _vice versa_, and this structural
reversal depends upon the way in which the light is thrown upon the
shoot. But even if the light causes the structural reversal, does this
justify us in assuming that the structure itself is also the direct
consequence of the influence of light? I see no reason for rejecting
the supposition that the physical nature of part of a plant may be of
such a kind that this or that structure may be produced according as
this or that condition of development prevails. Thus with stronger
light the structure of the upper side of the shoot developes; with
weaker light, the structure of the under side. But this physical nature
of the _Thuja_-bud depends, like that of a bird’s egg, upon its
phyletic history, as we must assume to be the case with the germs
producing all individual developments. It is therefore quite impossible
to interpret the reversal of the structure in the _Thuja_-shoot as the
result of modification produced by the direct influence of external
conditions. It is an instance of double adaptation—one of those cases
in which the specific nature of a germ, an organism, or a part of an
organism, possesses such a constitution that it reacts differently
under the incidence of different stimuli. An entirely analogous example
of reversal occurs in the climbing shoots of the Ivy, and is described
in Sachs’ lectures on the physiology of plants. Such shoots produce
leaves only on the side directed towards the light, and roots (which
are made use of in climbing) only upon the opposite side. If however
the position of the plant be altered so that the root-bearing side is
turned towards the light, while the leafy side is shaded, a reversal
occurs, so that from that time the former only produces leaves, and the
latter nothing but roots. In other words, the Ivy-shoot reacts under
strong light with the production of leaves and under weak light with
the production of roots, just as litmus-paper becomes red with an acid
and blue with an alkali. The physical nature of the Ivy-shoot was
present before the production of either structure, and was no more due
to the action of light itself, than the physical nature of litmus-paper
is due to an acid or an alkali. But this is quite consistent with the
possession of a physical nature which reacts differently under the two
different conditions afforded by light and shade.
No one would think of bringing forward the changes in the colour of the
green frog (_Hyla_) as a proof of the power of direct influences in
causing structural modifications in the animal body. The frog is light
green when it is resting upon green leaves, but it becomes dark brown
or nearly black when transferred to dark surroundings. This is an
obvious instance of adaptation, for the changes in the colour of the
frog depend upon a complex reflex mechanism. The changes in the shape
of the chromatophores of the skin are not produced by the direct
influence of the different rays of light upon the body-surface, but in
consequence of the action of these rays upon the retina. Blind frogs do
not react under the changes of light. Hence it is impossible that any
one can maintain that the skin of the frog has gained its green colour
as the direct result of the green light reflected from its usual
surroundings. It must be admitted that in this and in all similar
cases, there is only one possible explanation, viz. an appeal to the
operation of natural selection. It may be objected that we are not here
dealing, as in the _Thuja_ and Ivy, with changes in the course of
ontogenetic development following upon the occurrence of this or that
external condition, but only with the different reactions of a mature
organism. But nevertheless, cases of the former kind appear to be also
present in the animal kingdom.
Thus the very careful and extensive investigations of Poulton[284] upon
the colours of certain caterpillars have distinctly shown that some
species possess the possibility of development in two directions, and
that the actual direction taken by the individual is decided by the
influence of external conditions. Poulton surrounded certain larvae of
Geometrae with an abundance of dark branches, in addition to the leaves
upon which they fed. When such conditions prevailed from the beginning
of larval life, the caterpillars as they developed, gradually assumed
the dark colour of the twigs and branches upon which they rested. When
other larvae of the same species (and in many experiments hatched from
the same batch of eggs) were similarly exposed to the green leaves of
the same food-plant, they did not indeed become bright green like the
leaves, but were invariably of a much lighter colour than the other
larvae, while many of them gained a brownish-green tint. The larvae of
_Smerinthus ocellatus_[285] also possess the power of assuming
different shades of green and of thus approaching, to some extent, the
green of the plant upon which they happen to live. It is quite
impossible to explain the phyletic development of the green colour of
these and other caterpillars as due to the direct action upon the skin
of the green light reflected from the leaves upon which they sit. The
impossibility of such an effect was pointed out long ago by Darwin, and
also followed from my own investigations. Here, as in the other cases,
the only possible solution is afforded by natural selection. The colour
of the caterpillars has become gradually more and more perfectly
adapted to the colour of the leaves,—and often to the particular side
of the leaves upon which these animals rest,—not by the direct effect
of reflected light, but by the selection of those individuals which
were best protected. Poulton’s experiments quoted above prove that
certain species which occur upon different plants with different
colours (or even in some cases upon the differently coloured parts of
the same plant), present us with a further complication in the process
of adaptation, inasmuch as each individual has acquired the power of
assuming a lighter or darker colour[286]. The light which falls upon a
single individual caterpillar during the course of its growth
determines whether the lighter or darker colour shall be developed.
Here therefore we have a case exactly parallel to that of the
_Thuja_-shoot in which the palisade or spongy parenchyma is developed
according to the position in which the shoot is fixed.
As far as it is possible in the present condition of our knowledge to
offer any opinion upon the origin of sex in bisexual animals, it may be
suggested that this problem is also capable of an essentially similar
solution. Each germ-cell may possess the possibility of developing in
either of two directions, the one resulting in a male individual, and
the other resulting in a female, while the decision as to which of the
two possible alternatives is actually taken may rest with the external
conditions. We must, however, include among the external circumstances
everything which is not germ-plasm. Moreover, this explanation is by no
means certain, and I only mention it as an instance which, if we assume
it to be correct, further illustrates my views upon the phenomena
presented by the _Thuja_-shoot.
The two other facts brought forward by Detmer as proofs of the
transforming power of external influences can be explained in precisely
the same manner. These instances are—the fact that _Tropaeolum_ when
grown in moist air produces leaves with anatomical characters different
from those produced when the plant is grown in dry air; and the
differences in the structure of the leaves of many plants, according as
they have been grown in the sun or shade respectively. Such differences
do not by any means afford proof of the direct production of structural
changes by means of external influences. How would such an explanation
be consistent with the fact that the leaves are, in all these cases,
changed in a highly purposeful manner? Or is it assumed that these
organs were so constituted from the beginning, that they are compelled
to respond to external conditions by the production of useful changes?
Any one who made such an assertion nowadays, or who even thought of
such a thing as a possibility, would prove that he is entirely ignorant
of the facts of organic nature, and that he has no claim to be heard
upon the question of the transformation of species. The very first
necessity in any scientific question is to gain acquaintance with that
which has been thought and said upon the subject. And it has been
frequently shown that whole groups of useful characters cannot by any
possibility have been produced by the direct action of external
influences. If a caterpillar, which hides itself by day in the crevices
of the bark, possesses the same colour as the latter, while other
caterpillars which rest on leaves are of a green colour, these facts
cannot be explained as the results of the direct influence of the bark
and leaves. And it would be even less possible to explain upon the same
principle all the details of marking and colour by which these animals
gain still further protection. If the upper side of the upper wings of
certain moths is grey like the stone on which they rest by day, while
in butterflies the under side of both wings which are exposed during
rest, exhibits analogous protective colours, these facts cannot be due
to the direct influence of the surroundings which are resembled, but,
if they have arisen in any natural manner, they must have been
indirectly produced by the surroundings. One may reasonably complain
when compelled to repeat again and again these elements of knowledge
and of thought upon the causes of transformation!
Any one who remembers these things, and is aware of the countless
number of purposeful characters which cannot possibly depend upon such
direct influences, will be very cautious in yielding to any single
instance which at first sight appears to be the direct consequence of
external conditions. If Detmer had been thus cautious he would hardly
have written the following sentence as a _résumé_ of the physiological
experiments on plants which have been already alluded to: ‘In certain
cases it is possible, as we have seen, to artificially modify the
anatomical structure of certain parts of plants. In such cases the
relation between the structure and the external influences is
undoubtedly clear: the latter act as the cause; the anatomical
structure of the members of the plant is the consequence of this
cause.’ A little more logic would have prevented the author from
expressing such an opinion, for, as has been already shown, it is
founded on a confusion between the true cause of a phenomenon and one
of the conditions which are necessary for its production. We might as
well consider the phenomena of _geotropism_, _hydrotropism_, and
_heliotropism_—which have been established, and investigated in such a
brilliant way by modern vegetable physiologists—as the direct results
of the attraction of the earth, of water, and of light; and it is not
improbable that some botanists are even inclined to make this
assumption. And yet it is perfectly easy to show that this cannot be
the case. By geotropism we mean the power possessed by the parts of a
plant of growing along lines which make certain angles with the
direction of the earth’s attraction. For example, the chief root grows
parallel with the earth’s attraction, viz. towards the centre of the
earth, and it is described as positively geotropic: conversely the main
shoot grows along the same line but in an opposite direction, and it is
negatively geotropic. But geotropism is not a primitive attribute of
the plant, and it is even now absent from those plants which, like many
Algae, have no definite position. Geotropism cannot have arisen before
plants first became fixed in the earth. If any one were to assume that
the direct influence of gravity, continuous through countless
generations, had at length conferred upon the root the power of growing
in a geotropic direction, how would it be possible to explain the fact
that the shoot which has been under precisely the same influence has
acquired the power of growing in an exactly opposite direction? The
characteristic differences between root and shoot cannot have appeared
until the plant became fixed in the ground, and how can we imagine that
the same influence of gravity has since that time directly produced the
two antagonistic results of positive and negative geotropism, in two
structures, which were originally and essentially similar? It should
also be remembered that it is only the main root which exhibits true
positive geotropism. The lateral roots form angles with the main root,
and do not therefore grow towards the earth’s centre; and the same is
true of the lateral shoots which grow obliquely, and not
perpendicularly upwards, like the main shoot. Moreover the angles which
the lateral roots make with the main root, and the lateral shoots with
the main shoot, are quite different in different species. How is it
possible that all these different modes of reaction witnessed in the
different parts of plants can be the direct results of one and the same
external force? It is quite obvious that these are all cases of
adaptation. The main root has not acquired the power of growing
perpendicularly downwards under the stimulus of gravity, because this
force has acted upon it for numberless generations, but because such a
direction for such a part was the most useful to the plant. Hence
natural selection has conferred upon the root the power of reacting
under the stimulus of gravity by growing in a direction parallel to
this force. For the main shoot, the opposite reaction was the most
useful and has been established by natural selection, while still
another reaction has been similarly established for the lateral roots
and another for the lateral shoots.
Each part of a plant has received its special mode of reacting under
the stimulus of gravity because it was useful for the whole plant,
inasmuch as the position of its different parts relatively to one
another and to the soil became thus fixed and regulated. These modes of
reaction have become different in different species, because the
conditions of life peculiar to each require special arrangements.
The same argument also holds with regard to heliotropism. The power of
growing towards the light possessed by green shoots cannot be a
primitive character of the plant: it must have arisen secondarily. If
it were an essential and original character it could not be reversed in
certain parts of the plant; but, the roots are negatively heliotropic,
for they grow away from the light. There are also shoots, such as the
climbing shoots of Ivy, which are similarly negatively heliotropic.
Whenever the heliotropic power is thus reversed in shoots, the change
is of a useful kind. Thus the shoots of the Ivy gain the power of
clinging closely to a perpendicular wall or to some horizontal
plane[287]. In this case, however, it is only the shoot which is
negatively heliotropic, its leaves turn towards the light; and the same
is true of the flower-bearing shoots which do not climb. All these are
clearly adaptations and not the results of direct influence. The light
only provides the stimulus which calls forth the characteristic
reaction from each part of the plant, but the cause of each peculiar
reaction lies in the specific nature of the part itself which has not
been produced by light, but as we believe by processes of natural
selection. If this explanation does not account for the facts we may as
well abandon all attempts at understanding the useful arrangements in
organisms.
Sachs has used the term _anisotropism_ to express the fact that the
various organs of a plant assume the most diverse directions of growth
under the influence of the same forces. He also states that
anisotropism is one of the most general characteristics of vegetable
organization, and that it is quite impossible to form any idea as to
how plants would appear or how they could live if their different
organs were not anisotropic. Since anisotropism is nothing more than
the expression of different kinds of susceptibility to the action of
gravity, light, &c., it is obvious that the configuration of the plant
is to be traced to such specific susceptibilities.
Now these specific susceptibilities cannot have been produced by the
direct effect of the various external influences (as was shown above),
and the only other possible explanation is to recognise them as
adaptations, and to admit that they have arisen by the operation of
natural selection upon the general variability of plant organization.
Simple as these conclusions are, I have failed to meet with them in any
of the writings of botanists, and they may perhaps be of use in helping
to shake the vaguely-felt opinion that the characters of plants are to
be chiefly referred to the direct action of external influences.
At all events it cannot be maintained that the phenomena of
anisotropism support the opinion mentioned above; and the mere
assertion that it is highly probable that hereditary characters arise
as the result of external influences, is no more than the expression of
an unfounded individual opinion. It is remarkable that Detmer should
make such an assertion as the outcome of his discussion of the reversed
_Thuja_-shoot, &c., for even if we admit that the dorso-ventral
structure of the shoot is—as Detmer believes—the direct and primary
effect of the action of light, the experiment with the reversed shoot
would prove that no part of this effect has become hereditary. Although
the upper side of the shoot has produced the palisade parenchyma under
the influence of light for thousands of generations, there is
nevertheless no tendency towards the establishment of any hereditary
effect, for as soon as the upper side of the growing shoot is
artificially transformed into the under side, its normal structure is
at once abandoned. Hence so far from lending any support to the
assumption that acquired characters can be transmitted, Detmer’s
experiment rather tends to disprove this opinion.
I think I have sufficiently shown that Detmer’s reproach—that I have
under-estimated the effects of external influences upon an organism—may
be fairly directed against its author. If we can believe that every
structural arrangement in plants, which depends upon certain external
conditions, has been produced in a phyletic sense by these latter, it
becomes very easy to explain the transformation of species; but in
accepting such an explanation we are building without any foundation,
for the proof that acquired characters can be transmitted has yet to be
given.
As a further disproof of my views Detmer quotes the so-called phenomena
of correlation in plants, and he believes that these instances help us
to conceive how the acquired changes of the body (_soma_) of the plant
may also influence the sexual cells. If the apical shoot of a young
spruce fir be cut off, one of the lateral shoots of the whorl next
below the section rises and becomes an apical shoot: it not only
assumes the orthotropic growth of such a shoot, but also its mode of
branching. The phenomenon itself is well known, and I have often
observed it myself in my garden without making any botanical
experiments; for this experiment is not uncommonly made by Nature
herself, when the apical shoot is destroyed by insects (for example the
gall-making _Chermes_). The change of the lateral into an apical shoot
occurs here in consequence of the loss of the true apical shoot, and is
therefore really dependent upon it. The only difficulty is to
understand how these and many other kindred phenomena can be considered
to prove the transmission of acquired characters. That correlation
exists between the parts of an organism, that correlated changes are
not only common but nearly always accompany some primary change, has
been perfectly well known since Darwin’s time, and I am not aware that
it has been disputed by any one. I further believe that hardly any one
would maintain that it is impossible for the reproductive organs to be
influenced by correlation. But this is very far from the admission that
such changes would occur in the germ-cells as would be necessary for
the transmission of acquired characters. For such transmission to occur
it would be necessary for the germ-plasm (the bearer of hereditary
tendencies) to undergo a transformation corresponding to that produced
by the external influences;—such a transformation as would cause the
future organism to spontaneously develope changes similar to those
which its parent had acquired. But since the germ-plasm is not an
organism in the sense of being a microscopic facsimile which only has
to increase in size in order to become a mature organism, it is obvious
that the developmental tendencies must exist in the specific molecular
structure, and perhaps also in the chemical constitution of the
germ-plasm itself. It therefore follows that the changes in the
germ-plasm which would be required for the transmission of an acquired
character must be of an entirely different nature from the change
itself acquired by the body of the parent plant: and yet it is supposed
that the former is produced by the latter as a result of correlation. I
will illustrate this by an example. Let us suppose that the influence
of climate had caused a plant to change the form of its leaves from an
ovate into a lobate shape: now such a change could not be transferred
to the germ-plasm in the pollen and the ovules, as anything similar to
leaves or the form of leaves; for such specialized morphological
features have no existence in the germ-plasm. The only thing which
could happen would be changes in its molecular structure which bear no
resemblance to those changes which are implied by the direct alteration
of the form of the leaf in the parent plant. Any one who clearly
appreciates this difficulty will hesitate in admitting the possibility
of the transmission of acquired characters, because it is possible that
the sexual cells may be affected by correlated influences. If the
change in the form of a leaf exercises any influence at all upon the
germ-plasm, why should it produce a corresponding (in the
above-mentioned sense) change in its molecular structure? Why should it
not produce some other out of the immense number of possible changes?
There must be as many possible changes in the structure of germ-plasm
as there are possible variations in each part of a plant that arises
from it. Why then should the corresponding change always occur,—a
change which had never previously existed in the whole phyletic
development of the organic world; for the plant with the latest
modification can have never existed before? The occurrence of a
particular change out of the countless possible changes would be about
as likely as if one out of a hundred thousand pins thrown out of a
window were to balance on its point when it reached the ground. The
assumption scarcely deserves to be called a scientific hypothesis, and
yet it must be made by all who accept the transmission of acquired
characters,—that is unless they adopt the hypothesis of pangenesis,
which is quite as improbable, and which even Darwin did not look upon
as a real, but only as a formal explanation.
Detmer is also greatly mistaken when he says that I refuse to admit the
transmission of acquired characters, because I am prejudiced in favour
of my doctrine of the continuity of the germ-plasm. This doctrine is
either right or wrong, and there is no middle course: to this extent I
quite admit that I am prejudiced. But the question as to whether
acquired characters can be impressed upon the germ and thus transmitted
would not be by any means settled in this way; for even if we admit
that the germ-plasm is not continuous from one generation to another,
but that it must be produced afresh in each individual, this would by
no means necessarily imply that it would potentially receive and retain
every change produced in every part of the individual, and at any time
in its life. It seems to me that the problem of the transmission or
non-transmission of acquired characters remains, whether the theory of
the continuity of the germ-plasm be accepted or rejected.
I will now proceed to examine the last group of phenomena which Detmer
brings forward in favour of the transmission of acquired characters. He
charges me with not having taken into account, in discussing the
problem of heredity, the very important facts which are known about the
strange phenomena of ‘after-effect’ in plants. Among these
‘after-effects’ are the following.
If vigorous plants of the sun-flower, grown in the open air, be cut off
close to the ground and transferred to complete darkness, the
examination of a tube fixed to the cut surface of the stem will show
that the escape of sap does not take place uniformly, but undergoes
periodical fluctuation, being strongest in the afternoon and weakest in
the early morning. Now the cause of this daily periodicity in the flow
of sap depends upon the periodical changes due to the light to which
the plant was exposed when it was growing under normal conditions. When
plants which have been grown in darkness from the first are similarly
treated, the flow of sap does not exhibit any such periodicity.
Another instance is as follows:—it is well known that darkness
accelerates, while light retards the growth of plants, and therefore
plants usually grow more strongly by night than by day. If now plants
are transferred from the open air into constant darkness, the
periodicity in their growth does not immediately disappear, and often
persists for a long time as a phenomenon of after-effect.