place periodically under natural conditions, the leaves closing at dusk
as a result of changes in the stimulus provided by the light. In this
case also, when the plants are transferred to constant darkness, the
periodicity in the movements of the leaves continues for several days.
All this is certainly very interesting, and it proves that periodical
stimuli produce periodical processes in the plant, which are not
immediately arrested when the stimulus is withdrawn, and only become
uniform gradually and after the lapse of a considerable time. But I
certainly claim the right to ask what connexion there is between these
facts and the transmission of acquired characters. All these
peculiarities produced by external influences remain restricted to the
individual in which they arose; most of them disappear comparatively
soon, and long before the death of the individual. No example of the
transmission of such a peculiarity is known. Although successive
generations of sunflowers have been exposed for thousands of years to
the daily alternation of light and darkness, the periodicity in the
flow of sap has not become hereditary, and does not take place at all
in plants which have always been kept in darkness. Detmer specially
tells us that we can even reverse the periods of opening and closing
the leaves in _Mimosa pudica_ by keeping them in darkness during the
day, but exposed to light at night; an experiment which was performed
by Pfeffer. Here again we see the proof that influences which have
acted upon countless generations have left no impression whatever upon
the germ-plasm.
Detmer himself admits this when he says that the after-effects are only
witnessed during the life of the individual, but he nevertheless adds
that he has been for many years convinced that the phenomena of
heredity and after-effect differ in degree and not in kind. He even
goes so far as to assert that, in spite of the obvious non-transmission
of after-effect, the similarity between the natures of these two
classes of phenomena cannot escape the intelligent observer.
It seems to me that this question does not demand the attention of the
observer (for the observations have already been made) so much as that
of the thinker. It is not a correct train of reasoning to conclude that
after-effect and heredity are identical in nature, from the fact that
certain periodical influences, acting upon a single individual, set up
periodical physiological processes which continue for a time after the
influences have ceased to act. We might almost as well argue that the
oscillations of a pendulum, which continue as after-effects when the
pendulum has been set going, are of an identical nature with the
process of heredity. All these phenomena have indeed this much in
common:—a cause which acted at some time in the past, but which is no
longer visible at the time when the phenomenon appears. But the
likeness ends here, and the supposed identity in nature merely depends
upon wild speculation. One difference is very obvious, for the
phenomena of after-effect gradually cease after the withdrawal of the
stimulus, just like the oscillations of the pendulum, while the
phenomena of heredity continue without any interruption. As far as
heredity is concerned the physiological processes of after-effect are
not distinguishable from any of the other well-known acquired
characters which are recognizable as morphological changes.
After-effects are not transmitted, and compared with this fact but
little importance can be attached to the use of vague analogies by
Detmer, who would wish to conclude that heredity is only the
after-effect of processes which had been set going in the parent
organism.
At the end of his paper Detmer applies the ideas which he has gained
from the consideration of after-effect to certain phenomena in the
normal life of plants. He suggests that the periodical change of leaf
in trees and shrubs may have been produced by the direct effect of
climate. If branches bearing winter buds are cut off in the autumn and
are placed in a hot-house, with their cut ends in water, the buds do
not at once develope, and months may often elapse before they begin to
break. He argues that this experiment proves that the annual
periodicity of the plant no longer depends directly upon external
influences; these latter produced the periodicity at some earlier time,
but it has been gradually fixed in the organism by after-effect and
heredity(!), so that its disappearance does not now take place when the
stimulus is withdrawn, and changes would only happen very gradually
under the influence of changed climatic conditions. He considers that
this is proved from the fact that our cherry has become an evergreen in
Ceylon.
Such are Detmer’s opinions, and every one will agree with him in
believing that the periodical change of leaf in temperate climates has
been produced in relation to the recurring alternation of summer and
winter. This is certainly the case, and it cannot be doubted that the
character has become fixed by heredity. Where, however, is the proof
that this hereditary character has been produced by the direct
influence of climate? What right have we to look upon the hereditary
appearance of the character as an after-effect of the direct influence
exerted by changes of temperature upon previous generations? Such an
opinion derives but little support from the previously described
experiments upon after-effect, which showed that these phenomena were
never hereditary.
It appears to me that there are certain points in this change of leaf
and its accompanying phenomena, which distinctly indicate that natural
selection has been at work. Can Detmer imagine that the brown scales
which form the characteristic protective covering of winter buds have
been produced by the direct action of the cold? If, however, the
peculiar structure of these buds is to be referred to the specific
constitution of the individual rather than to the direct effects of
climate, would it be so very improbable for their physiological
peculiarity of lying dormant for several months to have been developed
simultaneously with the structure, by the operation of natural
selection? And if this explanation be correct, we can at once see why
the character has become hereditary, for natural selection works upon
variations of the germ-plasm, and these are transferred from one
generation to another with the germ-plasm itself.
But Detmer attempts to establish the converse conclusion, and he argues
that the hereditary change of leaf has been abandoned under the
long-continued effect of changed climatic conditions; but this opinion
is based upon the single instance of the alteration in the habit of the
European cherry in Ceylon. If it were proved that our cherry, grown
from seed in Ceylon and propagated by seed for several generations,
became evergreen gradually and not suddenly in the first generation:
if, under such circumstances, it came to retain its leaves in the
autumn and ceased to produce the dormant winter buds:—then indeed the
transmission of acquired characters could hardly be doubted. I am not a
botanist, but I believe I am right in supposing that the wild cherry
reproduces itself by seeds, while the edible domesticated cherry is
propagated by grafting. Grafts are, however, parts of the _soma_ of a
previously existent tree, and we are not therefore concerned, in this
method of propagation, with a succession of generations, but with the
successive distribution of one and the same individual over many wild
stocks. But no one will doubt that one and the same individual can be
gradually changed during the course of its life, by the direct action
of external influences. The really doubtful point is whether such
changes can be transmitted by means of the germ-cells. If, as I
presume, the English in Ceylon do not care to eat wild cherries but
prefer the cultivated kinds, it follows that the branches which bear
fruit in that island have not been developed from germ-cells, at any
time since their introduction, and there is nothing to prevent them
from gradually changing their anatomical and physiological characters
in consequence of the direct influence of climate.
Hence the instance which Detmer looks upon as plainly conclusive, can
hardly be accepted in support of such a far-reaching assumption as the
transmission of acquired characters.
It is therefore clear that none of the facts brought forward by Detmer
really afford the proofs which he believes that they offer. But another
botanist, Professor Hoffman of Marburg, well known for his
long-continued experiments on variation, has recently called attention
to certain other botanical facts in support of the transmission of
acquired characters. These facts are indeed conclusive, if we accept
the author’s use of the term ‘acquired,’ but it will be found that they
lead to hardly any modification in the state of existing opinion upon
the subject.
In a short note, dated Jan. 1, 1888, the author communicated to this
journal (‘Biologisches Centralblatt’) the statement that changes in the
structure of flowers caused by poor nutrition can be proved to be
hereditary to a greater or less extent[288].
A more elaborate account of the experiments will be found in several
numbers of the ‘Botanische Zeitung,’ and the author expresses his final
results in the following words (see Bot. Zeit. 1887, p. 773):—‘These
experiments prove with certainty (1) that insufficient nutrition may
cause considerable morphological changes (viz. qualitative variations)
which are in the first place acquired by the sexual apparatus of the
flower, (2) that the “transient” (Weismann) characters acquired by the
individual can be transmitted[289].’
The data upon which Hoffman bases these opinions are certain
experiments conducted upon various plants, in order to determine the
conditions of life under which abnormal flowers or any other variations
occur most frequently: to decide, in short, how far variations are
caused by the change of conditions.
It is obvious that the attention of the author was not at first
directed to the question of the transmission of acquired characters.
His experiments are of a much older date than the present condition and
significance of the question before us. Hoffmann has, in fact,
re-examined his former results from the new point of view, and this
explains why his proofs are not always sufficiently convincing when
applied to the present issue. But this is of no great importance,
inasmuch as there is no necessity for me to question the correctness of
his assumptions.
The essential details of the experiments to which he directs attention
are as follows.
Different plants with normal flowers were subjected to greatly changed
conditions of life for a series of generations. They were, for example,
crowded together in small pots. Under these circumstances the plants
were of course poorly nourished, and in the course of generations,
several species produced a variable proportion of abnormal—viz.
double-flowers. This, however, was not always the case, for such
flowers did not appear in _Matthiola annua_ and _Helianthemum
polifolium_. In other species, such as _Nigella damascena_, _Papaver
alpinum_ and _Tagetes patula_, they appeared and often increased in
numbers in the course of generations, although this was not a constant
result. For instance, four successive generations of _Nigella
damascena_, when closely sown, produced the following results:—
1883. No double flowers.
1884. No double flowers.
1885. 23 typical flowers: 6 double flowers.
1886. 10 typical flowers: 1 double flower.
But it was not always the case that the double flowers continued to
appear after they had been once produced. In _Papaver alpinum_, which
Hoffman has cultivated in successive generations since 1862, other
changes in addition to the doubling of the flowers first appeared in
1882, viz. a slight variability in the form of the leaf, and a greater
variability in the colours of the flowers. The production of double
flowers appeared to be favoured by poor nutrition caused by crowding
the plants. The results as regards the number of double flowers
produced in this species by close sowing, from 1882-1886, have been as
follows:—
Experiment XI. 1881. 40 per cent. of double flowers.
1882. 4 per cent. of double flowers.
1883. 5·3 per cent. of double flowers.
Experiment XVII. 1884. 13· per cent. of double flowers.
1885. 0·0 per cent. of double flowers.
1886. 0·0 per cent. of double flowers.
Although in these and some other series of generations the double
flowers again disappeared in the later generations, yet there can be
hardly any doubt that their first appearance was due to the abnormal
conditions of nutrition. This conclusion is also unaffected by the fact
that double flowers appeared in nearly the same proportions in
consequence of cultivation in ordinary garden soil. The plants which
were crowded in pots produced 2879 normal flowers, and 256 (=8·8 per
cent.) abnormal and mostly double ones, while 867 normal and 62 (=7·0
per cent.) abnormal ones were produced on garden beds. Hoffman will not
indeed admit that such a comparison can be fairly made, for the plants
in the garden beds were raised from seed which was in part taken from
the double flowers, and was therefore, he believed, under a strong
hereditary influence. But this latter assumption is not supported by
the results of his own experiments.
Thus experiment XVIII., conducted upon _Papaver alpinum_, is described
in these words,—‘Seeds yielded by double flowers from experiment XI.
(1883), were sown in pots, and the resulting plants produced from
1884-1886, fifty-three single flowers and no double ones.’
In the converse experiment XIX. ‘The seeds of single flowers from
different stocks were sown in pots, and the resulting plants produced
in 1885 and 1886 forty-three flowers, of which all were typical except
one;’ while plants produced in the garden by seed from the same
sources, yielded 166 single and five double flowers. Hoffman also
describes other experiments in which the seeds from double flowers
produced plants which also yielded many double flowers. Thus, for
example, in experiment XXI. seeds yielded by the double flowers of
_Papaver alpinum_ were sown in the garden and produced numerous plants,
which in 1885 and 1886 bore 284 single and twenty-one double flowers,
that is 7 per cent. of the latter.
It will therefore be seen that the transmission of the abnormality is
by no means proved beyond the possibility of doubt, for who can decide
between the effects due to heredity and changed conditions in the last
experiment? I have no doubt however that the results are at any rate in
part due to the operation of heredity, for I do not see how the
phenomena can be otherwise understood. Nevertheless I cannot admit the
transmission of acquired characters on this evidence, for the changes
which have appeared are not ‘acquired’ in the sense in which I use the
term and in the sense required by the general theory of evolution. It
is true that they may be described by the use of this word: inasmuch as
they are characters which the plant has come to possess; we are not
however engaged in a mere dispute about terms, but in the discussion of
a weighty scientific question. Our object is to decide whether changes
in the _soma_ (the body, as opposed to the germ-cells) which have been
produced by the direct action of external influences, including use and
disuse, can be transmitted; whether they can influence the germ-cells
in such a manner that the latter will cause the spontaneous appearance
of corresponding changes in the next generation. This is the question
which demands an answer; and, as has been shown above, such an answer
would decide whether the Lamarckian principle of transformation must be
retained or abandoned.
I have never doubted about the transmission of changes which depend
upon an alteration in the germ-plasm of the reproductive cells, for I
have always asserted that these changes, and these alone, must be
transmitted. If any one makes the contrary assertion, he merely proves
that he does not understand what I have said upon the subject. In what
other way could the transformation of species be produced, if changes
in the germ-plasm cannot be transmitted? And how could the germ-plasm
be changed except by the operation of external influences, using the
words in their widest sense; unless indeed we assume with Nägeli, that
changes occur from internal causes, and imagine that the phyletic
development of the organic world was planned in the molecular structure
of the first and simplest organism, so that all forms of life were
compelled to arise from it, in the course of time, and would have
arisen under any conditions of life. This is the outcome of Nägeli’s
view, against which I have contended for years.
If we now use the term ‘acquired characters’ for changes in the soma
which, like spontaneous abnormalities, depend upon previous changes in
the germ-plasm—it is of course easy to prove that acquired characters
are transmitted; but this is hardly the way to advance science, for
nothing but confusion would be produced by such a use of terms[290]. I
am not aware that any one has ever doubted that spontaneous characters,
such as extra fingers or toes, patches of grey hair, moles, etc., can
be transmitted. It is true that such characters are sometimes called
‘acquired’ in pathological works, but His has rightly insisted that
such an obviously inaccurate use of the term ought to be avoided, in
order to prevent misunderstanding. If every new character is said to be
‘acquired’ the term at once loses its scientific value, which lies in
the restricted use. If generally used, it would mean no more than the
word ‘new’; but new characters may arise in various ways,—by artificial
or natural selection, by the spontaneous variations of the germ, or by
the direct effect of external influences upon the body, including the
use and disuse of parts. If we assume that these latter characters are
transmitted, the further ‘assumption of complicated relations between
the organs and the essential substance of the germ becomes necessary’
(His), while the transmission of the other kinds of characters do not
involve any theoretical difficulties. There is therefore obviously a
wide difference between these two groups of characters as far as
heredity is concerned, quite apart from the question as to whether
acquired characters are really transmitted. It is at all events
necessary to have distinct terms which cannot be misunderstood.
His[291] has proposed to call those characters which are due to
selection ‘changes produced by breeding’ (‘erzüchtete Abänderungen’),
those which appear spontaneously—‘spontaneous changes’ (‘eingesprengte
Abänderungen’), and these two groups of characters would then be
opposed to those which he calls ‘acquired changes’ (‘erworbene
Abänderungen’), of course using the term in the restricted sense.
Science has always claimed the right of taking certain expressions and
applying them in a special sense, and I see no reason why it should not
exercise this right in the case of the term ‘acquired.’ It appears
moreover that this word has not always been used in this vague sense by
pathological anatomists, such as Virchow and Orth; for Weigert and
Ernst Ziegler have employed it in precisely the same sense as that in
which it has been used by Darwin, du Bois-Reymond, Pflüger, His and
many others, including myself.
It is certainly necessary to have two terms which distinguish sharply
between the two chief groups of characters—the primary characters which
first appear in the body itself, and the secondary ones which owe their
appearance to variations in the germ, however such variations may have
arisen. We have hitherto been accustomed to call the former ‘acquired
characters,’ but we might also call them ‘_somatogenic_,’ because they
follow from the reaction of the _soma_ under external influences; while
all other characters might be contrasted as ‘_blastogenic_,’ because
they include all those characters in the body which have arisen from
changes in the germ. In this way we might perhaps prevent the
possibility of misunderstanding. We maintain that the ‘_somatogenic_’
characters cannot be transmitted, or rather, that those who assert that
they can be transmitted, must furnish the requisite proofs. The
_somatogenic_ characters not only include the effects of mutilation,
but the changes which follow from increased or diminished performance
of function, and those which are directly due to nutrition and any of
the other external influences which act upon the body. Among the
_blastogenic_ characters, we include not only all the changes produced
by natural selection operating upon variations in the germ, but all
other characters which result from this latter cause.
If we now wish to place Hoffmann’s results in their right position, we
must regard all of them as ‘_blastogenic_’ characters, for no one of
them can be considered as belonging to the group which has been
hitherto spoken of as ‘acquired,’ in the literature of evolution: they
are not due to _somatogenic_ but to _blastogenic_ changes. The body of
the plant—the _soma_—has not been directly affected by external
influences, in Hoffman’s experiments, but changes have been wrought in
the germ-plasm of the germ-cells and, only after this, in the _soma_ of
succeeding generations.
There is no difficulty in finding facts in support of this statement,
among Hoffmann’s experiments. The proof chiefly lies in the fact that
in no one of his numerous experiments did any change appear in the
first generation. The seeds of different species of wild plants, with
normal flowers, were cultivated in the garden and in pots (thickly sown
in the latter case), but no one of the plants produced by these wild
seeds possessed a single double flower. It was only after a greater or
less number of generations had elapsed that a variable proportion of
double flowers appeared, sometimes accompanied by changes in the leaves
and in the colours of the flowers. This fact admits of only one
interpretation;—the changed conditions at first produced slight and
ineffectual changes in the idioplasm of the individual, which was
transmitted to the following generation: in this again the same causes
operated and increased the changes in the idioplasm which was again
handed down. Thus the idioplasm was changed more and more, in the
course of generations, until at last the change became great enough to
produce a visible character in the _soma_ developed from it, such as,
for example, the appearance of a double flower. Now the idioplasm of
the first ontogenetic stage (viz. germ-plasm) alone passes from one
generation to another, and hence it is clear that the germ-plasm itself
must have been gradually changed by the conditions of life until the
alteration became sufficient to produce changes in the _soma_, which
appeared as visible characters in either the flower or leaf[292].
In addition to the above-mentioned cases Hoffmann also quotes some
facts of a somewhat different kind. He succeeded in inducing
considerable changes in the structure of the root of the wild carrot
(_Daucus carota_) by means of the changes in nutrition implied by
garden cultivation. These changes also proved to be hereditary.
Unfortunately, I have not the literature of the subject at hand, and
hence I am unable to read the accounts of these older experiments _in
extenso_; but it is sufficiently obvious that in this case we are also
concerned with a change which did not become visible until after some
generations had elapsed, and which was therefore a change in the
germ-plasm.