one of them is to be found in the history of the garden pansy, which
Hoffmann has succeeded in producing from the wild form, _Viola
tricolor_, in the course of eighteen years. Darwin some time ago
pointed out in his work upon ‘The Variation of Animals and Plants under
Domestication,’ that, in the case of the pansy and all other ‘improved’
garden flowers, the wild form remained unchanged for many generations
after its transference to the garden, apparently uninfluenced by the
new conditions of life. At length single varieties began to appear, and
these were further developed by artificial selection and appropriate
crossing, into well-marked races distinguished by peculiar colours,
forms, etc.
In these cases also, changes in the germ-plasm are the first results of
the new conditions, and there is no evidence for the occurrence of
acquired characters, using the term in its restricted sense.
I now come to the last botanical fact brought forward by Hoffmann in
support of the transmission of acquired characters. He states that
specimens of _Solidago virgaurea_ brought from the Alps of the Valais,
commenced flowering in the botanical garden at Giessen, at a time which
differed by several weeks from that at which specimens from the
surrounding country, planted beside them, began to flower. In other
words, the time of flowering must have been fixed by heredity in the
alpine _Solidago_, for the external conditions would have favoured a
time which was simultaneous with that of the Giessen plants.
What conclusions can be drawn from these facts? Hoffmann of course sees
in them the proof of the transmission of acquired characters, but this
presupposes that the time of flowering was originally an acquired
character. Hoffmann indeed appears to entertain this opinion when he
somewhat vaguely states that the time at which flowering begins has
been acquired by accommodation—that is by the influence of
climate—during a long series of generations, and has become hereditary.
But what does Hoffmann mean by ‘accommodation’? He presumably means
that which, since the appearance of Darwin’s writings, has been
generally called adaptation:—that is a purposeful arrangement, suited
to certain conditions. The majority of biologists have followed Darwin
in believing that such adaptations have been produced by processes of
natural selection. Hoffmann seems to imagine that they have arisen in
some other way: perhaps he believes, with Nägeli, that they have been
directly produced by external influences.
The fixation of the time at which flowering begins, is an adaptation
which formerly could have been very well explained as the direct result
of external conditions. The question we have to decide is whether such
an explanation is the true one. We might imagine that the plant would
be forced into quicker development by an earlier appearance of the warm
season. Hence when transferred into a warmer climate the plant would at
first flower rather earlier, the habit would then be transmitted, and
would increase in successive generations from the continued influence
of climate, until it advanced as far as the organization of the plant
permitted. But in this explanation, as in so many others of the same
kind, it has unfortunately been forgotten that the transmission of
acquired characters which is presupposed in the explanation is a
totally unproved hypothesis. It is sufficiently obvious that by
interpreting a phenomenon in a manner which presupposes the
transmission of acquired characters, we cannot furnish a proof of the
existence of such transmission.
It always seemed to me that the fixation of the commencement of
flowering, together with similar physiological phenomena in the animal
kingdom (for example, the hatching of insects from winter eggs), could
be explained very satisfactorily by the operation of natural selection:
and even now this explanation appears to me to be the simplest and most
natural. In Freiburg, where the vine is largely grown, the harvest is
often injured by frosts in spring, which kill the young shoots, buds
and flowers. Accordingly, different kinds of vine, which do not push
their buds so early, have now been planted. Any one, who has seen all
the shoots of the former destroyed by the frosts at the end of April,
while the latter, not having opened their buds, were spared, would not
doubt that the former must have been long ago exterminated, if they had
been compelled to struggle for existence with the others, under natural
conditions. Now the time of flowering fluctuates slightly in the
individuals of every species of plant, and can therefore be modified by
natural selection. It is therefore difficult to see why the time at
which each plant flowers should not have been fixed in the most
favourable manner for each habitat, by natural selection alone.
Hoffmann is obviously unaware of the fundamental distinction between
the characters primarily acquired by the _soma_, and the secondary
characters which follow from changes in the germ-plasm.
If the author had appreciated this distinction he would not have
attempted to strengthen his opinions by following up the botanical
facts which exclusively belong to the second class of characters, with
the enumeration of certain instances selected from the animal kingdom
(viz., the supposed transmission of mutilations), all of which belong
to the first class. I will not discuss these latter instances, for most
of them are old friends, and they are all far too uncertain and
inaccurate to have any claim on scientific consideration.
I believe that I have shown that no botanical facts have been hitherto
brought forward which prove the transmission of acquired characters (in
the restricted sense), and that there are not even any facts which
render such transmission probable.
A. W.
Naples, Zoological Station,
_Jan. 11, 1888_.
------------------------------------------------------------------------
Footnotes for Essay VII.
Footnote 275:
See the second Essay.
Footnote 276:
Consult ‘Ueber die Vererbung,’ Jena, 1883; ‘Die Kontinuität des
Keimplasmas,’ Jena, 1885; ‘Ueber die Zahl der Richtungskörper und
über ihre Bedeutung für die Vererbung,’ Jena, 1887. These papers are
translated as the second, fourth and sixth Essays in the present
volume.
Footnote 277:
See the second Essay.
Footnote 278:
[See R. Meldola in Ann. and Mag. Nat. Hist., 1878, vol. i. pp.
158-161. The author discusses many cases among insects in which
instinct is related to protective structure or colouring: he also
considers that instinct is to be explained by the principle of
natural selection which accounts for the other protective
features.—E. B. P.]
Footnote 279:
[See ‘Nature,’ vol. 36, pp. 491-507.—E. B. P.]
Footnote 280:
[See ‘The Factors of organic Evolution’ in ‘The Nineteenth Century’
for April and May 1886.—E. B. P.]
Footnote 281:
See ‘Biol. Centralbl.’ Bd. VII. No. 23.
Footnote 282:
See the next Essay (VIII).
Footnote 283:
Detmer, ‘Zum Problem der Vererbung,’ Pflüger’s Archiv f. Physiologie,
Bd. 41, (1887), p. 203.
Footnote 284:
[Dr. Weismann is here alluding to experiments upon the larvae of
_Rumia Crataegata_. A short account of the results will be found in
the Report of the British Association at Manchester (1887), and in
‘Nature,’ vol. 36, p. 594. I have now obtained similar results with
many other species (see Trans. Ent. Soc., Lond. 1888, p. 553); but
many of the results are as yet unpublished.—E. B. P.]
Footnote 285:
[See the editorial notes by Raphael Meldola, in his translation of
Weismann’s ‘Studies in the Theory of Descent’ (the Essay on ‘The
Origin of the Markings of Caterpillars,’ pp. 241 and 306): also E. B.
Poulton, in ‘Proc. Roy. Soc.,’ vol. xxxviii. pp. 296-314; and in
‘Proc. Roy. Soc.,’ vol. xl. p. 135.—E. B. P.]
Footnote 286:
[Professor Meldola first called attention to the scattered instances
of the kind here alluded to by Professor Weismann, in 1873: see
‘Proc. Zool. Soc.,’ 1873, p. 153. The author explains the relation of
this ‘variable protective colouring’ to other protective appearances,
and he is strongly of the opinion that the former as well as the
latter is to be explained by the action of the ‘survival of the
fittest.’
The validity of Dr. Weismann’s interpretation of these effects as due
to adaptation, through the operation of natural selection, is
conclusively proved by the following facts. The light reflected from
green leaves becomes the stimulus for _the production of dark brown
pigment_ in those cases in which the leaves constitute the
surroundings for many months. Under these circumstances the leaves of
course become brown at a relatively early date, and protection is
thus afforded for the remainder of the period, although the dark
pigment is produced before the change in the colour of the leaf.
Instances of this kind are seen in the colours of cocoons spun among
leaves by certain lepidopterous larvae (see ‘Proc. Ent. Soc. Lond.,’
1887, pp. l, li, and 1888, p. xxviii), the cocoons of the same
species being of a creamy white colour when spun upon white paper.
Conversely, the light reflected from the same surfaces serves as the
stimulus for _withholding pigment_ in the cases alluded to by Dr.
Weismann (larvae of _R. Crataegata_, &c.), in all of which the
organism only remains in contact with the leaves while they are
green, viz. at a time when the dark colour would be disadvantageous.
Hence precisely opposite effects are produced by the operation of the
same force; the nature of the effect which actually follows in any
case being solely determined by the advantage afforded to the
organism.—E. B. P.]
Footnote 287:
Compare Sachs, ‘Lectures on the Physiology of Plants,’ translated by
H. Marshall Ward, p. 710.
Footnote 288:
Compare Biol. Centralbl. Bd. VII. No. 21.
Footnote 289:
I have used the expression ‘transient’ (‘passant’) in the same sense
as ‘acquired,’ in order to enforce the conclusion that they are
merely temporary, and disappear with the individual in which they
arise. Since the characters of which Hoffmann speaks are hereditary,
the term cannot be rightly applied to them, and I shall prove later
on that they cannot be regarded as acquired characters in the sense
required by the theory of descent.
Footnote 290:
Compare a paper by J. Orth, ‘Ueber die Entstehung und Vererbung
individueller Eigenschaften,’ Leipzig, 1887. This author considers my
theory of the non-transmission of acquired characters to be
incorrect, because he will insist upon using the term ‘acquired’ for
those characters which are due to spontaneous changes in the germ;
although he considers that they are only indirectly acquired. He also
reproaches me with not having discriminated with sufficient clearness
between the two modes in which new characters are acquired by the
body, and with having altogether failed to take into account the
class of characters which are due to variations in the germ. On the
very same page he quotes the following sentence from my
writings:—‘Every change of the germ-plasm itself, however it may have
arisen, must be transmitted to the following generation by the
continuity of the germ-plasm; and hence also any changes in the
_soma_ which arise from the germ-plasm must be transmitted to the
following generation.’ Not only does the transmission of Orth’s
‘indirectly acquired characters’ necessarily follow from this
sentence, but it is even distinctly asserted by it. I cannot
understand how any one who is aware of what happened at the meeting
of the Association of German naturalists at Strassburg in 1885, can
charge me with the confusion of ideas which has prevailed since
Virchow took part in the discussion of this question.
Footnote 291:
His, ‘Unsere Körperform,’ Leipzig, 1874, p. 58.
Footnote 292:
Compare on this point Nägeli in his ‘Theorie der Abstammungslehre.’
This writer also concludes from similar facts that external
influences have wrought in the idioplasm, changes which were at first
ineffectual, and which only increased during the course of
generations up to a point at which they could produce visible changes
in the plant. He does not, however, draw the further conclusion that
these changes only influence the germ-plasm, for he was not aware of
the distinction between germ-plasm and somatoplasm.
------------------------------------------------------------------------
VIII.
THE SUPPOSED TRANSMISSION OF
MUTILATIONS.
1888.
A lecture delivered at the Meeting of the Association of
German Naturalists at Cologne, September 1888.
------------------------------------------------------------------------
VIII.
THE SUPPOSED TRANSMISSION OF
MUTILATIONS.
We know well the manner in which Lamarck imagined that the gradual
transformation of species occurred, when he first made the attempt to
penetrate into the mechanism of the process of evolution, and to
ascertain the causes by which it is produced. In his opinion, a change
in the structure of any part of an organism was chiefly brought about
when the species in question met with new conditions of life and was
thus forced to assume new habits. Such habits caused an increased or
diminished activity, and therefore a stronger or weaker development, of
certain parts, and the modified parts were then transmitted to the
offspring. Inasmuch as the offspring continued to live under the same
changed conditions, and kept up the altered manner of using the part in
question, the inherited changes would be increased in the same
direction during the course of their life, and would be further
increased in each successive generation, until the greatest possible
change had been effected.
In this way Lamarck was able to give an apparently satisfactory
explanation of at any rate those changes which consist in the mere
enlargement or diminution of a part; such, for instance, as the great
length of neck in the swan and other swimming birds, which he believed
to have been produced by the habit of stretching after food at the
bottom of the water; or the webbed feet of the same animals, supposed
to be produced by the habit of striking the water with outspread toes,
etc. In this way he was also able to explain the disappearance of a
part after it had ceased to be of use; as, for instance, the
degeneration of the eyes of animals inhabiting caves or the sunless
depths of lakes or the sea.
But it is obvious that such an explanation tacitly assumes that changes
produced by use or disuse can be transmitted to the offspring; _it
assumes the transmission of acquired characters_.
Lamarck made this assumption as a matter of course, and when half a
century later Charles Darwin, his more fortunate successor, refounded
the theory of organic evolution, he also believed that we could not
entirely dispense with the Lamarckian principle of explanation,
although he added the new and extremely far-reaching principle of
natural selection. But he certainly attempted to decide whether the
Lamarckian principle of the effects of use and disuse is truly
efficient, by asking himself the question whether such changes, as for
example those produced by exercise during an individual life, can be
transmitted to the offspring. Many observations appeared to him, if not
to prove the transmission directly, yet to render it extremely
probable; and he thus came to the conclusion that there is no
sufficient reason for denying the transmission of acquired changes.
Hence, in Darwin’s works, use and disuse still play important parts as
direct factors of transformation, in addition to natural selection.
Darwin was not only an original genius, but also an extraordinarily
unbiassed and careful investigator. Whatever he expressed as his
opinion had been carefully tested and considered. This impression is
gained by every one who has studied Darwin’s writings, and perhaps it
in part explains the fact that doubts as to the correctness of the
Lamarckian principle adopted by him have only arisen during the last
few years. These doubts have, however, culminated in the decided denial
of the assumption that changes acquired by the body can be transmitted.
I for one frankly admit that I was in this respect under the influence
of Darwin for a long time, and that only by approaching the subject
from an entirely different direction was I led to doubt the
transmission of acquired characters. In the course of further
investigations I gradually gained a more decided conviction that such
transmission has no existence in fact.
Doubts on this point have been expressed not only by me but also by
others, such as du Bois-Reymond and Pflüger. Indeed, concerning a
certain class of acquired characters, viz. mutilations, the great
German philosopher, Kant, has distinctly denied that transmission can
take place[293]; and in more recent times Wilhelm His has expressed the
same opinion[294].
But if the transmission of acquired characters is truly impossible our
theory of evolution must undergo material changes. We must completely
abandon the Lamarckian principle, while the principle of Darwin and
Wallace, viz. natural selection, will gain an immensely increased
importance.
When I first expressed this opinion in my essay ‘On Heredity[295],’ I
was well aware of the consequences of such an idea. I knew well that
apparently insurmountable obstacles would be raised against any
explanation of evolution, from which the principle of the direct
transformation of the species by external influences had been excluded.
I therefore endeavoured to show that these difficulties are not in
reality insurmountable, and that it is quite possible to explain
certain phenomena, such as the degeneration of useless parts, without
the aid of the Lamarckian principle. Furthermore it can be shown that a
not inconsiderable number of instincts, viz. all those which are
exercised only once in a lifetime, cannot possibly have arisen by
transmitted practice. This fact renders it unnecessary to make use of
the Lamarckian principle for the explanation of other kinds of
instinct. I do not mean to deny the existence of phenomena for which
such an explanation has not yet been found, or at least has not been
brought forward; but on the other hand it appears to me that it has
never been proved that we cannot dispense with the Lamarckian principle
in the explanation of these phenomena. At any rate, I do not know of
any facts which could induce us to abandon from the first any hope of
finding an explanation without the aid of this hypothesis.
If we are able to prove that we may dispense with the assumption of the
transmission of acquired characters in explaining such phenomena, of
course it by no means follows that we _must_ dispense with it; or, in
other words, it does not follow that the transmission of acquired
changes cannot take place. It would be as unsafe to make this assertion
as to state of a ship seen at a great distance, that it is only moving
by sails and not by steam simply because the movement appears to be
explicable by sails alone. We ought first to attempt to show that the
ship does not possess a steam-engine, or at least that the existence of
such an engine cannot be proved.
I believe that I am able to show that the actual existence of the
transmission of acquired characters cannot be directly proved; that
there are no direct proofs supporting the Lamarckian principle.
If we ask for the facts which can be brought forward by the supporters
of the theory of the transmission of acquired characters, if we inquire
for the observations which induced Darwin, for instance, to adopt such
an hypothesis, or which at least prevented him from rejecting it,—a
very brief answer can be given. There are a small number of
observations made upon man and the higher animals which seem to prove
that injuries or mutilations of the body can, under certain
circumstances, be transmitted to the offspring.
A cow which had accidentally lost its horn, produced a calf with an
abnormal horn; a bull which had accidentally lost its tail, from that
time begat tailless calves: a woman whose thumb had been crushed and
malformed in youth, afterwards had a daughter with a malformed thumb,
and so on.
In a great number of such cases every guarantee for the trustworthiness
of the statements is entirely wanting, and, as His and still earlier
Kant have already said, they are of no greater value as evidence than
the merest tales. But in other cases this assertion cannot be made
without further examination; and a small number of such observations
can indeed claim a scientific investigation and value. I shall
presently discuss this point in greater detail, but I wish now to lay
stress upon the fact that, as far as direct evidence goes, we cannot
bring forward any proofs in favour of the transmission of acquired
characters, except these cases of mutilations. There are no
observations which prove the transmission of functional hypertrophy or
atrophy, and it is hardly to be expected that we shall obtain such
proofs in future, for the cases are not of a kind which lend themselves
to an experimental investigation. The hypothesis that acquired
characters can be transmitted is therefore only directly supported by
the above-mentioned instances of the transmission of mutilations. For
this reason, the defenders of the Lamarckian principle, who have come
forward in rather large numbers recently[296], have endeavoured to show
that these observations are conclusive, and therefore of the highest
importance. For the same reason I believe that it is my duty, as I take
the opposite view, to explain what I think of the value of these
apparent proofs of transmitted mutilations.
It can hardly be doubted that mutilations are acquired characters: they
do not arise from any tendency contained in the germ, but are merely
the reaction of the body under external influences. They are, as I have
recently expressed it, purely somatogenic characters[297], viz.
characters which emanate from the body (_soma_) only, as opposed to the
germ-cells; they are therefore characters which do not arise from the
germ itself.
If mutilations must necessarily be transmitted, or even if they might
occasionally be transmitted, a powerful support would be given to the
Lamarckian principle, and the transmission of functional hypertrophy or
atrophy would thus become highly probable. For this reason it is
absolutely necessary that we should try to come to a definite
conclusion as to whether mutilations can or cannot be transmitted.
We will now consider in greater detail the facts which have hitherto
been brought forward upon this point. It is not my purpose to discuss
every single case which has been mentioned anywhere or by anybody; such
a discussion would hardly lead to any result. I propose to select a
small number of such instances, in order to show why they cannot be
maintained as proofs. I shall chiefly deal with cases which have been
brought forward as especially strong proofs by my opponents, and which
have been carefully and completely examined. I shall attempt to show
that these are not conclusive and that they must be explained in an
entirely different manner. The insufficiency of the proof does not
always depend upon the same circumstances, and, according to the
latter, we may distinguish different classes of cases.
First of all we may briefly mention those instances in which the
necessary precautions have not been taken before drawing conclusions.