only the value of nuclei; l. c., p. 394.
Footnote 148:
I purposely abstain from using a more precise term, for the
complicated terminology employed in spermatogenesis hardly
contributes anything to the elucidation of the phenomena themselves.
Why do we not simply speak of sperm-cells and spermatoblasts, and
distinguish the latter by numbers when they occur in successive
generations of different form? Moreover, all the names which have
been suggested for successive stages of development, can only be
applied to the special group of animals upon which the observations
have been made. Hence great confusion results from the use of such
terms as spermatoblasts, spermatogonia, spermatomeres, spermatocysts,
spermatocytes, spermatogemmae, etc.
Footnote 149:
Fol, ‘Sur l’origine des cellules du follicule et de l’ovule chez les
Ascidies.’ Compt. rend., 28 mai, 1883.
Footnote 150:
Roule, ‘La structure de l’ovaire et la formation des œufs chez les
Phallusiadées.’ Ibid., 9 avril, 1883.
Footnote 151:
Balbiani, ‘Sur l’origine des cellules du follicule et du noyau
vitellin de l’œuf chez les Géophiles.’ Zool. Anzeiger, 1883, Nos.
155, 156.
Footnote 152:
Will, ‘Ueber die Entstehung des Dotters und der Epithelzellen bei den
Amphibien und Insecten.’ Ibid., 1884, Nos. 167, 168.
Footnote 153:
[It is almost certain that this vesicle is not derived from the
nucleus, but from the cytoplasm of the sperm-mother-cell. See Douglas
H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden’ in
Berichte der deutschen botanischen Gesellschaft, vol. v, 1887, p.
122.—S. S.]
Footnote 154:
Bütschli, ‘Gedanken über die morphologische Bedeutung der sogenannten
Richtungskörperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884.
Footnote 155:
F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63.
Footnote 156:
The formation of a polar body in parthenogenetic eggs has now been
proved: see note at the end of this Essay; see also Essay VI.—A. W.,
1888.
Footnote 157:
R. Leuckart,—article ‘Zeugung,’ in R. Wagner’s ‘Handwörterbuch der
Physiologie,’ 1853, Bd. IV. p. 958. Similar observations were made by
Max Schultze. These observations appear however to be erroneous, for
Pflüger has since shown that the eggs of frogs never develope if the
necessary precautions are taken to prevent the access of any
spermatozoa to the water.—A. W., 1888.
Footnote 158:
Oellacher, ‘Die Veränderungen des unbefruchteten Keims des
Hühncheneies. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXII.
p. 181. 1872.
Footnote 159:
Hensen, ‘Centralblatt,’ 1869, No. 26.
Footnote 160:
Weismann, ‘Beiträge zur Naturgeschichte der Daphnoiden,’ Leipzig,
1876-79, Abhandlung VII, and ‘Zeitschrift für wissenschaftliche
Zoologie,’ Bd. XXXIII.
Footnote 161:
Weismann, ‘Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im
Insectenei,’ Bonn, 1882, p. 106.
Footnote 162:
W. Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig,
1883.
Footnote 163:
We now know that the number of loops varies considerably in different
species, even when they belong to the same group of animals (e.g.
Nematodes).—A.W., 1888.
Footnote 164:
This expression is used by bee-keepers, for instance by the
well-known Baron Berlepsch. Of course, it would be more accurate to
say that the queen, seeing the cell of a drone, is stimulated to lay
an unfertilized egg, and that, on the other hand, she is stimulated
to lay a fertilized egg when she sees the cell of a worker, or that
of a queen.
Footnote 165:
E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das
Experiment.’ Zeitschrift für wissenschaftliche Zoologie, Bd. XVIII.
p. 124. 1868.
Footnote 166:
‘Daphniden,’ Abhandlung, vi. p. 324.
Footnote 167:
l. c., p. 150.
Footnote 168:
Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena.
1884.
Footnote 169:
I intend to publish these experiments elsewhere in connexion with
other observations.
Footnote 170:
Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The
Principles of Biology,’ 1864, vol. i. pp. 229, 230.
Footnote 171:
The same fact has since been ascertained in species belonging to
several groups of animal.
Footnote 172:
Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73.
Footnote 173:
‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107.
1873.
Footnote 174:
Valaoritis, l. c., p. 6.
Footnote 175:
I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. II.
p. 219. He further states that these are the only instances hitherto
known in which undoubted male cells have proved to be capable of
further development when they have been unable to exercise their
powers of fertilization. It must be added that the two kinds of
germ-cells do not differ in appearance, but only in behaviour; the
female germ-cells becoming fixed, and withdrawing one of their two
flagella, while the male cells continue to swarm. But even this
slight degree of differentiation requires the supposition of internal
molecular differentiation.
------------------------------------------------------------------------
V.
THE SIGNIFICANCE OF SEXUAL REPRODUCTION
IN THE THEORY OF NATURAL SELECTION.
1886.
------------------------------------------------------------------------
SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC.
PREFACE.
The greater part of the present essay was delivered at the first
general meeting of the Association of German Naturalists, at
Strassburg, on September 18th, 1885, and is printed in the Proceedings
of the fifty-eighth meeting of that Society.
The form of a lecture has been retained in the present publication, but
its contents have been extended in many ways. Besides many small and a
few large additions to the text, I have added six appendices in order
to treat of certain subjects more fully than was possible in the
lecture itself, in which I was often obliged to be content with mere
hints and suggestions. This appears to be all the more necessary
because it is impossible to suppose that many views and ideas upon
which the lecture was based would be well known to all readers,
although they have been described in my former papers. It was above all
necessary to deal with the class of acquired characters, which, as it
seems to me, is easily confounded, especially by the medical
profession, with the much broader class of new characters generally.
Only those new characters can be called ‘acquired’ which owe their
origin to external influences, and the term ‘acquired’ must be denied
to those which depend upon the mysterious relationship between the
different hereditary tendencies which meet in the fertilized ovum.
These latter are not ‘acquired’ but inherited, although the ancestors
did not possess them as such, but only as it were the elements of which
they are composed. Such new characters as these do not at present admit
of an exact analysis: we have to be satisfied with the undoubted fact
of their occurrence. The transmission or non-transmission of acquired
characters must be of the highest importance for a theory of heredity,
and therefore for the true appreciation of the causes which lead to the
transformation of species. Any one who believes, as I do, that acquired
characters are not transmitted, will be compelled to assume that the
process of natural selection has had a far larger share in the
transformation of species than has been as yet accorded to it; for if
such characters are not transmitted, the modifying influence of
external circumstances in many cases remains restricted to the
individual, and cannot have any part in producing transformation. We
shall also be compelled to abandon the ideas as to the origin of
individual variability which have been hitherto accepted, and shall be
obliged to look for a new source of this phenomenon, upon which the
processes of selection entirely depend.
In the following pages I have attempted to suggest such a source.
A. W.
Freiburg I. Br.,
_November 22, 1885._
------------------------------------------------------------------------
SIGNIFICANCE OF SEXUAL REPRODUCTION, etc.
CONTENTS.
PAGE
1. Can we dispense with the principle of natural 255
selection?
2. Nägeli’s theory of transformation from internal 256
causes
3. A definite course of development is possible 258
without a self-changing idioplasm
4. Conclusive importance of ‘adaptations’ 260
5. The structure of whales as an example of 261
adaptation
6. Transformation takes place by the smallest steps 264
7. The foundation of such minute changes depends upon 266
individual variability
8. Difficulty in accounting for variability on the 266
supposition of a continuity of the germ-plasm
9. Previous theories by which variability has been 267
accounted for
10. Non-transmission of acquired characters 267
11. Nägeli’s and Alexis Jordan’s experiments 269
12. Germ-plasm is only altered with great difficulty 271
13. The source of individual variation lies in sexual 272
reproduction
14. The process of natural selection does not operate 274
when asexual reproduction takes place
15. Origin of variability in unicellular organisms 278
16. Sexual reproduction effects combination 279
17. E. van Beneden’s and V. Hensen’s theory of sexual 282
reproduction as a process of rejuvenescence
18. Theoretical objections to such a view 283
19. Original significance of conjugation 286
20. Preservation of sexual reproduction by means of 287
heredity
21. It is lost in parthenogenesis for reasons of 289
utility
22. Parthenogenesis prevents further transformations 290
23. It excludes Panmixia and thus prevents disused 291
organs from becoming rudimentary
24. Final considerations 294
APPENDICES.
I. Further considerations which oppose Nägeli’s 298
explanation of Transformation as due to internal
causes
II. Nägeli’s Explanation of Adaptation 300
III. Adaptations in Plants 308
IV. On the Supposed Transmission of Acquired 310
Characters
1. Brown-Séquard’s experiments on Guinea-pigs 310
2. A case which at first sight appears to prove 320
the transmission of acquired characters
V. On the Origin of Parthenogenesis 323
VI. W. K. Brooks’ Theory of Heredity 326
------------------------------------------------------------------------
V.
THE SIGNIFICANCE OF SEXUAL REPRODUCTION
IN THE THEORY OF NATURAL SELECTION.
During the quarter of a century which has elapsed since Biology began
to occupy itself again with general problems, at least one main fact
has been made clear by the united labours of numerous men of science,
viz. the fact that the Theory of Descent, the idea of development in
the organic world, is the only conception as to the origin of the
latter, which is scientifically tenable. It is not only that, in the
light of this theory, numerous facts receive for the first time a
meaning and significance; it is not only that, under its influence, all
the ascertained facts can be harmoniously grouped together; but in some
departments it has already yielded the highest results which can be
expected from any theory, it has rendered possible the prediction of
facts, not indeed with the absolute certainty of calculation, but still
with a high degree of probability. It has been predicted that man, who,
in the adult state, only possesses twelve pairs of ribs, would be found
to have thirteen or fourteen in the embryonic state: it has been
predicted that, at this early period in his existence, he would possess
the insignificant remnant of a very small bone in the wrist, the
so-called _os centrale_, which must have existed in the adult condition
of his extremely remote ancestors. Both predictions have been
fulfilled, just as the planet Neptune was discovered after its
existence had been predicted from the disturbances induced in the orbit
of Uranus.
That existing species have not arisen independently, but have been
derived from other and mostly extinct species, and that on the whole
this development has taken place in the direction of greater
complexity, may be maintained with the same degree of certainty as that
with which astronomy asserts that the earth moves round the sun; for a
conclusion may be arrived at as safely by other methods as by
mathematical calculation.
If I make this assertion so unhesitatingly, I do not make it in the
belief that I am bringing forward anything new nor because I think that
any opposition will be encountered, but simply because I wish to begin
by pointing out the firm ground on which we stand, before considering
the numerous problems which still remain unsolved. Such problems appear
as soon as we pass from the facts of the case to their explanation; as
soon as we pass from the statement ‘The organic world has arisen by
development,’ to the question ‘But how has this been effected, by the
action of what forces, by what means, and under what circumstances?’
In attempting to answer these questions we are very far from dealing
with certainties; and opinions are still conflicting. But the answer
lies in the domain of future investigation, that unknown country which
we have to explore.
It is true that this country is not entirely unknown, and if I am not
mistaken, Charles Darwin, who in our time has been the first to revive
the long-dormant theory of descent, has already given a sketch, which
may well serve as a basis for the complete map of the domain; although
perhaps many details will be added, and many others taken away. In the
principle of natural selection, Darwin has indicated the route by which
we must enter this unknown land.
But this opinion is not universal, and only recently Carl Nägeli[176],
the famous botanist, has expressed decided doubts as to the general
applicability of the principle of natural selection. According to
Nägeli, the co-operation of the external conditions of life with the
known forces of the organism, viz. heredity and variability, are
insufficient to explain the regular course of development pursued by
the organic world. He considers that natural selection is at best an
auxiliary principle, which accepts or rejects existing characters, but
which is unable to create anything new: he believes that the causes of
transformation reside within the organism alone. Nägeli further assumes
that organisms contain forces which cause periodical transformation of
the species, and he imagines that the organic world, as a whole, has
arisen in a manner similar to that in which a single individual arises.
Just as a seed produces a certain plant because it possesses a certain
constitution, and just as, in this process, certain conditions must be
favourable (light, warmth, moisture, &c.) in order that development may
take place, although they do not determine the kind or the manner of
development; so, in precisely the same way, the tree of the whole
organic world has grown up from the first and lowest forms of life on
our planet, under a necessity arising from within, and on the whole
independently of external influences. According to Nägeli, the cause
which compels every form of living substance to change, from time to
time, in the course of its secular growth, and which moulds it afresh
into new species, must lie within the organic substance itself, and
must depend upon its molecular structure.
It is with sincere admiration and real pleasure that we read the
exposition in which Nägeli gives, as it were, the result of all his
researches which bear upon the great question of the development of the
organic world. But although we derive true enjoyment from the
contemplation of the elaborate and ingeniously wrought-out theoretical
conception,—which like a beautiful building or a work of art is
complete in itself,—and although we must be convinced that its rise has
depended upon the progress of knowledge, and that by its means we shall
eventually reach a fuller knowledge; it is nevertheless true that we
cannot accept the author’s fundamental hypothesis. I at least believe
that I am not alone in this respect, and that but few zoologists will
be found who can adopt the hypothesis which forms the foundation of
Nägeli’s theory.
It is not my intention at present to justify my own widely different
views, but the subject of this lecture compels me to briefly explain my
position in relation to Nägeli, and to give some of the reasons why I
cannot accept his theory of an active force of transformation arising
and working within the organism; and I must also explain the reasons
which induce me to adhere to the theory of natural selection.
The supposition of such a phyletic force of transformation (see
Appendix I, p.298) possesses, in my opinion, the greatest defect that
any theory can have,—it does not explain the phenomena. I do not mean
to imply that it is incapable of rendering certain subordinate
phenomena intelligible, but that it leaves a larger number of facts
entirely unexplained. It does not afford any explanation of the
purposefulness seen in organisms: and this is just the main problem
which the organic world offers for our solution. That species are, from
time to time, transformed into new ones might perhaps be understood by
means of an internal transforming force, but that they are so changed
as to become better adapted to the new conditions under which they have
to live, is left entirely unintelligible by this theory. For we
certainly cannot accept as an explanation Nägeli’s statement that
organisms possess the power of being transformed in an adaptive manner
simply by the action of an external stimulus (see Appendix II, p. 300).
In addition to this fundamental defect, we must also note that there
are absolutely no proofs in support of the foundation of this theory,
viz. of the existence of an internal transforming force.
Nägeli has very ingeniously worked out his conception of idioplasm, and
this conception is certainly an important acquisition and one that will
last, although without the special meaning given to it by its author.
But is this special meaning anything more than pure hypothesis? Can we
say more than this of the ingenious description of the minute molecular
structure of the hypothetical basis of life? Could not idioplasm be
built up in a manner entirely different from that which Nägeli
supposes? And can conclusions drawn from its supposed structure be
brought forward to prove anything? The only proof that idioplasm must
necessarily change, in the course of time, as the result of its own
structure, is to be found in the fact that Nägeli has so constructed
it; and no one will doubt that the structure of idioplasm might have
been so conceived as to render any transformation from within itself
entirely impossible.
But even if it is theoretically possible to imagine that idioplasm
possesses such a structure that it changes in a certain manner, as the
result of mere growth, we should not be justified in thus assuming the
existence of a new and totally unknown principle until it had been
proved that known forces are insufficient for the explanation of the
observed phenomena.
Can any one assert that this proof has been forthcoming? It has been
again and again pointed out that the phyletic development of the
vegetable kingdom proceeds with regularity and according to law, as we
see in the preponderance and constancy of so-called purely
‘morphological’ characters in plants. The formation of natural groups
in the animal and vegetable kingdoms compels us to admit that organic
evolution has frequently proceeded for longer or shorter periods along
certain developmental lines. But we are not on this account compelled
to adopt the supposition of unknown internal forces which have
determined such lines of development.
Many years ago I attempted to prove[177] that the constitution or
physical nature of an organism must exercise a restricting influence
upon its capacity for variation. A given species cannot change into any
other species, which may be thought of. A beetle could not be
transformed into a vertebrate animal: it could not even become a
grasshopper or a butterfly; but it could change into a new species of
beetle, although only at first into a species of the same genus. Every
new species must have been directly continuous with the old one from
which it arose, and this fact alone implies that phyletic development
must necessarily follow certain lines.
I can fully understand how it is that a botanist has more inclination
than a zoologist to take refuge in internal developmental forces. The
relation of form to function, the adaptation of the organism to the
internal and external conditions of life, is less prominent in plants
than in animals; and it is even true that a large amount of observation
and ingenuity is often necessary in order to make out any adaptation at
all. The temptation to accept the view that everything depends upon
internal directing causes is therefore all the greater. Nägeli indeed
looks at the subject from the opposite point of view, and considers
that the true underlying cause of transformation is in animals obscured
by adaptation, but is more apparent in plants[178]. Sufficient
justification for this opinion cannot, however, be furnished by the
fact that in plants many characters have not been as yet explained by
adaptation. We should do well to remember the extent to which the
number of so-called ‘morphological’ characters in plants has been
lessened during the last twenty years. What a flood of light was thrown
upon the forms and colours of flowers, so often curious and apparently
arbitrary, when Sprengel’s long-neglected discovery was extended and
duly appreciated as the result of Darwin’s investigations, and when the
subject was further advanced by Hermann Müller’s admirable researches!
Even the venation of leaves, which was formerly considered to be
entirely without significance, has been shown to possess a high
biological value by the ingenious investigations of J. Sachs (see
Appendix III, p. 308). We have not yet reached the limits of
investigation, and no reason can be assigned for the belief that we
shall not some day receive an explanation of characters which are now
unintelligible[179].
It is obvious that the zoologist cannot lay too much stress upon the
intimate connexion between form and function, a connexion which extends
to the minutest details: it is almost impossible to insist too much
upon the perfect manner in which adaptation to certain conditions of
life is carried out in the animal body. In the animal body we find
nothing without a meaning, nothing which might be otherwise; each
organ, even each cell or part of a cell is, as it were, tuned for the
special part it has to perform in relation to the surroundings.
It is true that we are as yet unable to explain the adaptive character
of every structure in any single species, but whenever we succeed in
making out the significance of a structure, it always proves to be a
fresh example of adaptation. Any one who has attempted to study the
structure of a species in detail, and to account for the relation of
its parts to the functions of the whole, will be altogether inclined to
believe with me that everything depends upon adaptation. There is no
part of the body of an individual or of any of its ancestors, not even
the minutest and most insignificant part, which has arisen in any other
way than under the influence of the conditions of life; and the parts
of the body conform to these conditions, as the channel of a river is
shaped by the stream which flows over it.