xix. p. 165. 1877.
Footnote 241:
F. M. Balfour, ‘Comparative Embryology.’
Footnote 242:
Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre,’
München und Leipzig, 1884.
Footnote 243:
See the second and fourth Essays in the present volume.
Footnote 244:
Hensen, ‘Die Grundlagen der Vererbung,’ Zeitschr. f. wiss.
Landwirthschaft. Berlin, 1885, p. 749.
Footnote 245:
O. Hertwig, ‘Lehrbuch der Entwicklungsgeschichte des Menschen und der
Wirbelthiere.’ Jena, 1886.
Footnote 246:
Bütschli, ‘Gedanken über die morphologische Bedeutung der sog.
Richtungskörperchen,’ Biol. Centralblatt, Bd. VI. p. 5. 1884.
Footnote 247:
This observation was first published as a note at the end of the
fourth Essay in the present volume. See p. 249.
Footnote 248:
Weismann, ‘Richtungskörper bei parthenogenetischen Eieren,’ Zool.
Anzeiger, 1886, p. 570.
Footnote 249:
Blochmann, ‘Ueber die Richtungskörper bei den Insekteneiern,’ Biolog.
Centralblatt., April 15, 1887.
Footnote 250:
F. Stuhlmann, ‘Die Reifung des Arthropodeneies nach Beobachtungen an
Insekten, Spinnen, Myriapoden und Peripatus,’ Berichte der
naturforschenden Gesellschaft zu Freiburg i. Br., Bd. I. p. 101.
Footnote 251:
In the summer-eggs of Rotifera I have, together with Mr. Ischikawa,
observed one polar body, and we were able to establish for certain
that a second is not formed. The nuclear spindle had already been
observed by Tessin, and Billet had noticed polar bodies in
_Philodina_, but without attaching any importance to their number.
These latter observations were not conclusive proofs of the formation
of polar bodies in parthenogenetic eggs, so long as it was not known
whether the summer-eggs of Rotifera may develope parthenogenetically,
or whether they can only develope in this way. Knowing now that
parthenogenetic eggs expel only one polar body, we may perhaps be
permitted to draw the conclusion that the summer-egg of a Rotifer
(_Lacinularia_) which expelled only one polar body must have been a
parthenogenetic egg. But I may add that we have also succeeded in
directly proving the occurrence of parthenogenesis in Rotifera, as
will be described in detail in another paper.
Footnote 252:
See Essay IV, Part III. p. 225.
Footnote 253:
E. Bessels, ‘Die Landois’sche Theorie, widerlegt durch das
Experiment.’ Zeitschr. f. wiss. Zool. Bd. XVIII. p. 124. 1868.
Footnote 254:
l. c., p. 110.
Footnote 255:
Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang bei
den Phanerogamen als Grundlage einer Theorie der Zeugung.’ Jena, 1884.
Footnote 256:
Wilhelm Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’
Leipzig, 1884.
Footnote 257:
E. van Beneden, ‘Recherches sur la maturation de l’œuf, la
fécondation et la division cellulaire.’ Gand et Leipzig, Paris, 1883.
Footnote 258:
J. B. Carnoy, ‘La Cytodiérèse de l’œuf, la vésicule germinative et
les globules polaires de l’Ascaris megalocephala.’ Louvain, Gand,
Lierre, 1886.
Footnote 259:
See p. 364.
Footnote 260:
Wilhelm Roux, ‘Beiträge zur Entwicklungsmechanik des Embryo,’ No. 3,
Breslauer ärztliche Zeitschrift, 1885, p. 45.
Footnote 261:
Carnoy, ‘La Cytodiérèse chez les Arthropodes.’ Louvain, Gand, Lierre,
1885.
Footnote 262:
Flemming, ‘Neue Beiträge zur Kenntniss der Zelle.’ Arch. f. mikr.
Anat. Bd. XXIX, 1887.
Footnote 263:
Carnoy, ‘La Cytodiérèse de l’œuf; la vésicule germinative et les
globules polaires chez quelques Nématodes.’ Louvain, Gand, Lierre.
1886.
Footnote 264:
Hensen, ‘Die Grundlagen der Vererbung nach dem gegenwärtigen
Wissenskreis,’ Zeitschr. f. wissenschaftl. Landwirthschaft, Berlin,
1885, p. 731.
Footnote 265:
See the preceding Essay on ‘The Significance of Sexual Reproduction
in the theory of Natural Selection.’
Footnote 266:
E. van Beneden and Julin, ‘La Spermatogénèse chez l’Ascaride
mégalocéphale.’ Brussels, 1884.
Footnote 267:
Carnoy, ‘La Cytodiérèse chez les Arthropodes.’
Footnote 268:
Gustav Platner, ‘Die Karyokinese bei den Lepidopteren als Grundlage
für eine Theorie der Zelltheilung.’ Internation. Monatsschrift f.
Anatomie und Histologie, Bd. III. Heft 10. Leipzig, 1886.
Footnote 269:
La Valette St. George, ‘Ueber die Genese der Samenkörper.’ Fünfte
Mittheilung. Die Spermatogenese bei den Säugethieren und dem
Menschen,’ Archiv f. mikrosk. Anat. Bd. XV. 1878.
Footnote 270:
Weismann, ‘Studien zur Descendenztheorie,’ ii. p. 306, Leipzig, 1876,
translated by Meldola; see ‘Studies in the Theory of Descent,’ p. 680.
Footnote 271:
l. c., p. 92.
Footnote 272:
[The similar conclusion that identical ova lead to the appearance of
identical individuals was drawn from the same data by Francis Galton
in 1875. See ‘The history of the Twins, as a criterion of the
relative powers of Nature and Nurture,’ by Francis Galton, F.R.S.,
Journal of the Anthropological Institute, 1875, p. 391; also by the
same author, ‘Short Notes on Heredity, etc. in Twins,’ in the same
Journal, 1875, p. 325.
The author investigated about eighty cases of close similarity
between twins, and was able to obtain instructive details in
thirty-five of these. Of the latter there were no less than seven
cases ‘in which both twins suffered from some special ailment or had
some exceptional peculiarity;’ in nine cases it appeared that ‘both
twins are apt to sicken at the same time;’ in eleven cases there was
evidence for a remarkable association of ideas; in sixteen cases the
tastes and dispositions were described as closely similar. These
points of identity are given in addition to the more superficial
indications presented by the failure of strangers or even parents to
distinguish between the twins. A very interesting part of the
investigation was concerned with the after-lives of the thirty-five
twins. ‘In some cases the resemblance of body and mind had continued
unaltered up to old age, notwithstanding very different conditions of
life,’ in the other cases ‘the parents ascribed such dissimilarity as
there was, wholly, or almost wholly, to some form of illness.’
The conclusions of the author are as follows: ‘Twins who closely
resembled each other in childhood and early youth, and were reared
under not very dissimilar conditions, either grow unlike through the
development of natural characteristics which had lain dormant at
first, or else they continue their lives, keeping time like two
watches, hardly to be thrown out of accord except by some physical
jar. Nature is far stronger than nurture within the limited range
that I have been careful to assign to the latter.’ And again, ‘where
the maladies of twins are continually alike, the clocks of their two
lives move regularly on, and at the same rate, governed by their
internal mechanism. Necessitarians may derive new arguments from the
life histories of twins.’
The above facts and conclusions held for twins of the same sex, of
which at any rate the majority are shown by Kleinwächter’s
observations to have been enclosed in the same embryonic membranes,
and therefore presumably to have been derived from a single ovum; but
in rarer cases the twins, although also invariably of the same sex,
were marked by remarkable differences, greater than those which
usually distinguish children of the same family. Mr. Galton met with
twenty of these cases. In such twins the conditions of training, etc.
had been as similar as possible, so that the evidence of the power of
nature over nurture is strongly confirmed. Mr. Galton writes, ‘I have
not a single case in which my correspondents speak of originally
dissimilar characters having become assimilated through identity of
nurture. The impression that all this evidence leaves on the mind is
one of wonder whether nurture can do anything at all, beyond giving
instruction and professional training.’
The fact that twins produced from a single ovum seem to be invariably
of the same sex is in itself extremely interesting, for it proves
that the sex of the individual is predetermined in the fertilized
ovum.—E. B. P.]
Footnote 273:
Fol, Recherches sur la fécondation et le commencement de l’hénogénie:
Genève, Bâle, Lyon. 1879.
Footnote 274:
Born, ‘Ueber Doppelbildungen beim Frosch und deren Entstehung.’
Breslauer ärztl. Zeitschrift, 1882.
------------------------------------------------------------------------
VII.
ON THE SUPPOSED BOTANICAL PROOFS
OF THE
TRANSMISSION OF ACQUIRED CHARACTERS.
1888.
From ‘Biologisches Centralblatt,’ Bd. VIII. Nr. 3 and 4, pages 65
and 97: April 1888.
------------------------------------------------------------------------
VII.
ON THE SUPPOSED BOTANICAL PROOFS
OF THE
TRANSMISSION OF ACQUIRED CHARACTERS.
In a lecture on heredity, delivered in 1883[275], I first brought
forward the opinion that acquired characters cannot be transmitted; and
I then stated that there are no proofs of such transmission, that its
occurrence is theoretically improbable, and that we must attempt to
explain the transformation of species without its aid. Since that time
many biologists have expressed their opinions upon the subject, some of
them agreeing with me, while others have taken the opposite side. It is
unnecessary to allude to those who have attacked my opinions without
first understanding the real point in dispute, which turns upon the
true meaning of the phrase ‘acquired character.’ I think it is now
generally admitted that a very important problem is involved in this
question, the solution of which will contribute in a decisive manner
towards the formation of ideas as to the causes which have produced the
transformation of species. For if acquired characters cannot be
transmitted, the Lamarckian theory completely collapses, and we must
entirely abandon the principle by which alone Lamarck sought to explain
the transformation of species,—a principle of which the application has
been greatly restricted by Darwin in the discovery of natural
selection, but which was still to a large extent retained by him. Even
the apparently powerful factors in transformation—the use and disuse of
organs, the results of practice or neglect—cannot now be regarded as
possessing any direct transforming influence upon a species. And the
same is true of all the other direct influences, such as nutrition,
light, moisture, and that combination of different influences which we
call climate. All these, with use and disuse, may perhaps produce great
effects upon the body (_soma_) of the individual, but cannot produce
any effect in the transformation of the species, simply because they
can never reach the germ-cells from which the succeeding generation
arises. But if—as it seems to me—the facts of the case compel us to
reject the assumption of the transmission of acquired characters, there
only remains one principle by which we can explain the transformation
of species—the direct alteration of the germ-plasm, however we may
imagine that such alterations have been produced and combined to form
useful modifications of the body.
The difficulty of understanding these processes of transformation is by
no means lightened by abandoning the Lamarckian theory. The difficulty
in fact becomes much greater, for we are now compelled to seek a
different explanation of many phenomena which were previously believed
to be understood. But this can hardly be regarded as a reason for not
accepting the view: for we are in want of a correct explanation rather
than one which is easy and convenient. We seek truth, and when we
recognize that our path is leading in a wrong direction, we must leave
it and take another road even if it presents more difficulties.
My theory rests, on the one hand, upon certain theoretical
considerations which will be mentioned below, and which I have
attempted to develope in previous papers[276]. On the other hand, it
rests upon the want of any actual proof of the transmission of acquired
characters. My theory might be disproved in two ways,—either by
actually proving that acquired characters are transmitted, or by
showing that certain classes of phenomena admit of absolutely no
explanation unless such characters can be transmitted. It will be
admitted, however, that we must be very cautious in accepting proofs of
this latter kind, for the impossibility of explaining a given
phenomenon may be merely temporary, and may disappear with the progress
of science. No one could have explained the useful adaptations so
common in animals and plants, before the light of the theory of natural
selection had fallen on these phenomena; at that time we should have
been far from right if we had assumed that organisms possess a power
which causes them to respond to external influences by useful
modifications, a power unknown elsewhere, entirely unproved and only
supported by the fact that at that time it did not seem possible to
explain the phenomena in any other way.
Although my theory has not been disproved, I will nevertheless attempt
to bring into further accordance with it certain phenomena which seem
at first sight to oppose it. I first began to take this course in my
paper ‘On Heredity[277].’ In that paper I attempted to show how the
fact that disused organs become rudimentary may be readily explained
without assuming the transmission of acquired characters; and also that
the origin of instincts may in all cases be referred to the process of
natural selection[278], although many observers had followed Darwin in
explaining them as inherited habits,—a view which becomes untenable if
the habits adopted and practised in a single life cannot be transmitted.
Other phenomena which appeared to present difficulties were also
considered and brought into accordance with the theory, and I think
that I have been successful in showing that adequate and simple
explanations may be given.
There certainly remain many phenomena which seem to be opposed to my
theory and for which a new explanation must be found. Thus
Romanes[279], following Herbert Spencer[280], has recently pointed to
the phenomena of correlation as a proof of the transmission of acquired
characters; but, at no distant time, I hope to be able to consider this
objection, and to show that the apparent support given to the old idea
is in reality insecure and breaks down as soon as it is critically
examined. I believe that I shall be able to prove that correlation
cannot be used as the indirect proof of an hypothesis, of which all
direct evidence is still completely wanting. It must not be forgotten
that the _onus probandi_ rests with my opponents: they defend the
assertion that acquired characters can be transmitted, and they ought
therefore to bring forward actual proofs; for the mere fact that the
assertion has been hitherto accepted as a matter of course by almost
everyone, and has only been doubted by a very few (such as His, du
Bois-Reymond, and Pflüger), cannot be taken as any proof of its
validity. Not a single fact hitherto brought forward can be accepted as
a proof of the assumption. Such proofs ought to be found: facts ought
to be discovered which can only be understood with the aid of this
hypothesis. If, for instance, it could be shown that artificial
mutilation spontaneously re-appears in the offspring with sufficient
frequency to exclude all possibilities of chance, then such proof would
be forthcoming. The transmission of mutilations has been frequently
asserted, and has been even recently again brought forward, but all the
supposed instances have broken down when carefully examined. I think I
may here safely omit all further reference to the proofs dependent upon
transmitted mutilations, especially as Döderlein[281] has already, in
the most convincing manner, disposed of the argument derived from the
tailless cats which were so triumphantly exhibited at the last meeting
of the Association of German Naturalists[282].
I now come to the real subject of this paper—the supposed botanical
proofs of the transmission of acquired changes. The botanist Detmer has
recently brought forward certain phenomena in vegetable
physiology[283], as a support for the transmission of such changes, and
although I do not believe that they will bear this interpretation, the
discussion of them may perhaps be useful. I am even inclined to think
that these and a few other phenomena in vegetable physiology, upon
which I shall also touch, are very likely to throw new light upon the
whole question which has been so frequently misunderstood. I should
have preferred to leave this discussion to a botanist, but I do not
know whether my views will meet with any support from the followers of
this subject, and I must therefore attempt the discussion myself. And
perhaps it is of some assistance in clearing up the question, for one
who is not accustomed to the usual botanical views, and is more
conversant with other classes of biological knowledge, to consider the
facts brought to light by modern botany, from a general point of view.
Of course I shall not attempt to question the validity of the
observations, nor even the accuracy with which the facts have been
interpreted. I shall only deal with the conclusions which may be drawn
from the facts, and I do not think that it is absolutely necessary that
such criticism should be made by a botanist. Questions of general
biological significance such as that of heredity cannot be entirely
solved within the single domain of either zoological or botanical
facts. Both botanists and zoologists must give due weight to the facts
of the province which is not their own, and must see whether the views
which they have chiefly gained in the one province can be applied to
the other, or whether phenomena occur in the latter which are in
opposition to their previously formed views and which cause them to be
abandoned or modified.
Detmer begins by bringing forward certain facts which prove, as he
believes, that rather important changes in the organism can be directly
produced by external influences. He is of opinion that I under-estimate
the weight of these influences, and that I make light of the changes
which may thus arise in a single individual life. But obviously, it is
of no importance for the question of the transmission of acquired
characters, whether the changes directly produced by external
influences upon the _soma_ of an individual are greater or smaller: the
only question is whether they can be transmitted. If they can be
transmitted, the smallest changes might be increased by summation in
the course of generations, into characters of the highest degree of
importance. It is in this way that Lamarck and Darwin have supposed
that an organism is transformed by external influences. It is therefore
interesting to see what Detmer considers to be a change which has been
directly effected. We can in this way gain a very distinct appreciation
of the difference in views which is caused by the different spheres of
experience which belong to botany and zoology. It will be useful to
gain a clear idea of the differences which are thus caused.
Detmer first alludes to the dorso-ventral structure of the shoots of
_Thuja occidentalis_, chiefly shown in the fact that the upper sides of
these shoots contain the green palisade cells, while the under sides
which are turned away from the light possess green spheroidal
(isodiametric) cells. If the branches of _Thuja_ are turned upside down
and fixed in this position before the production of new shoots, it is
found that the anatomical structure of the latter, when developed, is
reversed. The side of the shoot which was destined to become the under
side, but which was artificially compelled to become the upper side,
assumes the structure of the upper side and developes the
characteristic palisade parenchyma; and on the other hand, the under
side which was intended to become the upper side developes the spongy
parenchyma which is characteristic of the under side. From these facts
Detmer concludes that the dorso-ventral structure of the shoots of
_Thuja_ has resulted from the continual operation of an external force,
and that the light must be considered as the cause of the structural
change.
But such a conclusion obviously depends upon a confusion of ideas. No
one will doubt that the light was the stimulus which led to the
reversal of the structures in the shoot, but this is a very different
thing from maintaining that it was the cause which conferred upon the
_Thuja_-shoot the power of producing palisade and spongy parenchyma.
When a phenomenon only occurs under certain conditions, it does not
follow that these conditions are the cause of the phenomenon. A certain
temperature is necessary for the development of a bird in the egg, but
surely no one will maintain that the temperature is the cause of the
capacity for such development. It is obvious that the egg has acquired
the power of producing a bird chiefly as the result of a long phyletic
course of development which has led to such a chemical and physical
structure in the egg and the fertilizing sperm-cell, that after their
union and development, a bird, and only a bird of a particular species,
must be produced. But of course certain conditions must be fulfilled in
order that such development may take place; and a definite temperature
is one of these conditions of development. Thus we may briefly say that
the physical nature of the egg is the cause of its development into a
bird, and we may similarly maintain that the physical nature of a
_Thuja_-shoot, and not the influence of light, is the cause of the
development of tissues which are characteristic of the species. In the
development of such a shoot the light plays precisely the same part
which is played by temperature in the development of a bird: it is one
of the conditions of development.