recognise mother and daughter in the two resulting organisms. But in
_Euglypha_ and allied forms the existence of a shell introduces a
distinguishing mark by which it is possible to discriminate between the
products of fission; so that the offspring can be differentiated from
the parent. The parent organism, before division, builds the parts of
the shell for the daughter form. These parts are arranged on the
surface of that part of the protoplasm, external to the old shell,
which will be subsequently separated as the daughter-cell. On this part
the spicules are arranged and unite to form the new shell. The division
of the nucleus takes place after that of the protoplasm, so that the
daughter-cell is for some time without a nucleus. Although we can in
this species recognise the daughter-cell for some time after separation
from the parent by the greater transparency of its younger shell, it is
nevertheless impossible to admit that the characteristics of the two
animals are in any way different, for just before the separation of the
two individuals a circulation of the protoplasm through both shells
takes place after the manner described in the text, and there is
therefore a complete intermingling of the substance of the two bodies.
The difference between the products is even greater after transverse
fission of the _Infusoria_, for a new anus must be formed at the
anterior part and a new mouth posteriorly. It is not known whether any
circulation of the protoplasm takes place, as in _Euglypha_. But even
if this does not occur, there is no reason for believing that the two
products of division possess a different duration of life.
The process of fission in the _Diatomaceae_ seems to me to be
theoretically important, because here, as in the previously-mentioned
_Monothalamia_ (_Euglypha_, etc.), the new silicious skeleton is built
up within the primary organism, but not, as in _Euglypha_, for the new
individual only, but for both parent and daughter-cell alike[31]. If we
compare the diatom shell to a box, then the two halves of the old shell
would form two lids, one for each of the products of fission, while a
new box is built up afresh for each of them. In this case there is an
absolute equality between the products of fission, so far as the shell
is concerned.
Note 11. Regeneration.
A number of experiments have been recently undertaken, in connection
with a prize thesis at Würzburg, in order to test the powers of
regeneration possessed by various animals. In all essential respects
the results confirm the statements of the older observers, such as
Spallanzani. Carrière has also proved that snails can regenerate not
only their horns and eyes, but also part of the head when it has been
cut off, although he has shown that Spallanzani's old statement that
they can regenerate the whole head, including the nervous system, is
erroneous[32].
Note 12. The Duration of Life in Plants.
The title of the work on this subject mentioned in the Text is ‘Die
Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache und ihre
Entwicklung,’ F. Hildebrand, Engler’s botanische Jahrbücher, Bd. II. 1.
und 2. Heft, Leipzig, 1881.
Note 13.
[Many interesting facts and conclusions upon the subject of this essay
will be found in a volume by Professor E. Ray Lankester, ‘On
comparative Longevity in Man and the lower Animals,’ Macmillan and Co.,
1870.—E. B. P.]
------------------------------------------------------------------------
Footnotes for the Appendix to Essay I.
Footnote 1:
Humboldt’s ‘Ausichten der Natur.’
Footnote 2:
This estimate is derived from observation of the time during which
these insects are to be seen upon the wing. Direct observations upon
the duration of life in this species are unknown to me.
Footnote 3:
[Sir John Lubbock has now kept a queen ant alive for nearly 15 years.
See note 2 {note 18 below} on p. 51.—E. B. P.]
Footnote 4:
[After reading these proofs Dr. A. R. Wallace kindly sent me an
unpublished note upon the production of death by means of natural
selection, written by him some time between 1865 and 1870. The note
contains some ideas on the subject, which were jotted down for
further elaboration, and were then forgotten until recalled by the
argument of this Essay. The note is of great interest in relation to
Dr. Weismann’s suggestions, and with Dr. Wallace’s permission I print
it in full below.
‘The Action of Natural Selection in Producing Old Age, Decay, and
Death.
‘Supposing organisms ever existed that had not the power of natural
reproduction, then since the absorptive surface would only increase
as the square of the dimensions while the bulk to be nourished and
renewed would increase as the cube, there must soon arrive a limit
of growth. Now if such an organism did not produce its like,
accidental destruction would put an end to the species. Any
organism therefore that, by accidental or spontaneous fission,
could become two organisms, and thus multiply itself indefinitely
without increasing in size beyond the limits most favourable for
nourishment and existence, could not be thus exterminated: since
the individual only could be accidentally destroyed,—the race would
survive. But if individuals did not die they would soon multiply
inordinately and would interfere with each other’s healthy
existence. Food would become scarce, and hence the larger
individuals would probably decompose or diminish in size. The
deficiency of nourishment would lead to parts of the organism not
being renewed; they would become fixed, and liable to more or less
slow decomposition as dead parts within a living body. The smaller
organisms would have a better chance of finding food, the larger
ones less chance. That one which gave off several small portions to
form each a new organism would have a better chance of leaving
descendants like itself than one which divided equally or gave off
a large part of itself. Hence it would happen that those which gave
off very small portions would probably soon after cease to maintain
their own existence while they would leave a numerous offspring.
This state of things would be in any case for the advantage of the
race, and would therefore, by natural selection, soon become
established as the regular course of things, and thus we have the
origin of _old age_, _decay_, and _death_; for it is evident that
when one or more individuals have provided a sufficient number of
successors they themselves, as consumers of nourishment in a
constantly increasing degree, are an injury to those successors.
Natural selection therefore weeds them out, and in many cases
favours such races as die almost immediately after they have left
successors. Many moths and other insects are in this condition,
living only to propagate their kind and then immediately dying,
some not even taking any food in the perfect and reproductive
state.’—E. B. P.]
Footnote 5:
Johannes Müller, ‘Physiologie,’ Bd. I. p. 31, Berlin, 1840.
Footnote 6:
Oken, ‘Naturgeschichte,’ Stuttgart, 1837, Bd. IV. Abth. 1.
Footnote 7:
Brehm, ‘Leben der Vögel,’ p. 278.
Footnote 8:
‘Naturwissenschaftliche Thatsachen und Probleme,’ Populäre Vorträge,
Berlin, 1880; _vide_ Appendix.
Footnote 9:
‘Entomolog. Mag.,’ vol. i. p. 527, 1833.
Footnote 10:
Imhof, ‘Beiträge zur Anatomie der _Perla maxima_,’ Inaug. Diss.,
Aarau, 1881.
Footnote 11:
Mr. Edwards has meanwhile published these communications in full; cf.
‘On the length of life of Butterflies,’ Canadian Entomologist, 1881,
p. 205.
Footnote 12:
When no authority is given, the observations are my own.
Footnote 13:
In the paper quoted above, Edwards, after weighing all the evidence,
reduces the length of life from three to four weeks.
Footnote 14:
‘Entomolog. Mag.,’ vol. i. p. 527, 1823.
Footnote 15:
Ibid.
Footnote 16:
Ibid.
Footnote 17:
‘Recherches sur les mœurs des Fourmis indigènes,’ Genève, 1810.
Footnote 18:
These two female ants were still alive on the 25th of September
following Sir John Lubbock’s letter, so that they live at least seven
years. Cf. ‘Observations on Ants, Bees, and Wasps,’ Part VIII. p.
385; Linn. Soc. Journ. Zool., vol. xv. 1881.
[Sir John Lubbock has kindly given me further information upon the
duration of life of these two queen ants. Since the receipt of his
letter, the facts have been published in the Journal of the Linnean
Society (Zoology), vol. xx. p. 133. I quote in full the passage which
refers to these ants:—
‘Longevity.—It may be remembered that my nests have enabled me to
keep ants under observation for long periods, and that I have
identified workers of _Lasius niger_ and _Formica fusca_ which were
at least seven years old, and two queens of _Formica fusca_ which
have lived with me ever since December 1874. One of these queens,
after ailing for some days, died on the 30th July, 1887. She must
then have been more than thirteen years old. I was at first afraid
that the other one might be affected by the death of her companion.
She lived, however, until the 8th August, 1888, when she must have
been nearly fifteen years old, and is therefore by far the oldest
insect on record.
‘Moreover, what is very extraordinary, she continued to lay fertile
eggs. This remarkable fact is most interesting from a physiological
point of view. Fertilization took place in 1874 at the latest. There
has been no male in the nest since then, and, moreover, it is, I
believe, well established that queen ants and queen bees are
fertilized once for all. Hence the spermatozoa of 1874 must have
retained their life and energy for thirteen years, a fact, I believe,
unparalleled in physiology.’
* * * * *
‘I had another queen of _Formica fusca_ which lived to be thirteen
years old, and I have now a queen of _Lasius niger_ which is more
than nine years old, and still lays fertile eggs, which produce
female ants.’
Both the above-mentioned queens may have been considerably older, for
it is impossible to estimate their age at the time of capture. It is
only certain (as Sir John Lubbock informs me in his letter) that they
must have been at least nine months old (when captured), as the eggs
of _F. fusca_ are laid in March or early in April.’ The queens became
gradually ‘somewhat lethargic and stiff in their movements (before
their death), but there was no loss of any limb nor any abrasion.’
This last observation seems to indicate that queen ants may live for
a much longer period in the wild state, for it is stated above that
the chitin is often greatly worn, and some of the limbs lost (see pp.
48, 51, and 52).—E. B. P.]
Footnote 19:
A. von Berlepsch, ‘Die Biene und ihre Zucht,’ etc., 3rd ed.;
Mannheim, 1872.
Footnote 20:
E. Bevan, ‘Ueber die Honigbiene und die Länge ihres Lebens;’ abstract
in Oken’s ‘Isis,’ 1844, p. 506.
Footnote 21:
Dalyell, ‘Rare and Remarkable Animals of Scotland,’ vol. ii. p. 203;
London, 1848.
Footnote 22:
[Mr. J. S. Haldane has kindly obtained details of the death of the
sea anemone referred to by the author. It died, by a natural death,
on August 4, 1887, after having appeared to become gradually weaker
for some months previous to this date. It had lived ever since 1828
in the same small glass jar in which it was placed by Sir John
Dalyell. It must have been at least 66 years old when it died.—E.B.P.]
Footnote 23:
Bronn, ‘Klassen und Ordnungen des Thierreichs,’ Bd. III. p. 466;
Leipzig.
Footnote 24:
Bronn, l. c.
Footnote 25:
Cf. the article ‘Mort’ in the ‘Encyclop. Scienc. Méd.’ vol. M. p. 520.
Footnote 26:
Roux, in his work ‘Der Kampf der Theile im Organismus,’ Jena 1881,
has attempted to explain the manner in which division of labour has
arisen among the cells of the higher organisms, and to render
intelligible the mechanical processes by which the purposeful
adaptations of the organism have arisen.
Footnote 27:
von Berlepsch, ‘Die Biene und ihre Zucht,’ etc.
Footnote 28:
Oken, ‘Isis,’ 1844, p. 506.
Footnote 29:
von Berlepsch, l. c., p. 165.
Footnote 30:
Cf. August Gruber, ‘Der Theilungsvorgang bei Euglypha alveolata,’
and ‘Die Theilung der monothalamen Rhizopoden,’ Z. f. W. Z., Bd.
XXXV. and XXXVI., p. 104, 1881.
Footnote 31:
Cf. Victor Hensen, ‘Physiologie d. Zeugung,’ p. 152.
Footnote 32:
Cf. J. Carrière, ‘Ueber Regeneration bei Landpulmonaten,’ Tagebl. der
52. Versammlg. deutsch. Naturf. pp. 225-226.
------------------------------------------------------------------------
II.
ON HEREDITY.
1883.
------------------------------------------------------------------------
ON HEREDITY.
PREFACE.
The following essay was my inaugural lecture as Pro-Rector of the
University of Freiburg, and was delivered publicly in the hall of the
University, on June 21, 1883; it first appeared in print in the
following August. Only a few copies of the first edition were available
for the public, and it is therefore now reprinted as a second edition,
which only differs from the first in a few not unimportant improvements
and additions.
The title which I have chosen requires some explanation. I do not
propose to treat of the whole problem of heredity, but only of a
certain aspect of it—the transmission of acquired characters which has
been hitherto assumed to occur. In taking this course I may say that it
was impossible to avoid going back to the foundation of all the
phenomena of heredity, and to determine the substance with which they
must be connected. In my opinion this can only be the substance of the
germ-cells; and this substance transfers its hereditary tendencies from
generation to generation, at first unchanged, and always uninfluenced
in any corresponding manner, by that which happens during the life of
the individual which bears it. If these views, which are indicated
rather than elaborated in this paper, be correct, all our ideas upon
the transformation of species require thorough modification, for the
whole principle of evolution by means of exercise (use and disuse), as
proposed by Lamarck, and accepted in some cases by Darwin, entirely
collapses.
The nature of the present paper—which is a lecture and not an elaborate
treatise—necessitates that only suggestions and not an exhaustive
treatment of the subject could be given. I have also abstained from
giving further details in the form of an appendix, chiefly because I
could hardly have attempted to complete a treatment of the whole range
of the subject, and I hope to refer again to these questions in the
future, when new experiments and observations have been made.
I am very glad to see that such an important authority as Pflüger[33]
has in the meantime come to the same opinion, from an entirely
different direction—an opinion which forms the foundation of the views
here brought forward, namely, that heredity depends upon the continuity
of the molecular substance of the germ from generation to generation.
A. W.
II.
ON HEREDITY.
With your permission I wish to bring before you to-day my views on a
problem of general biological interest—the problem of heredity.
Heredity is the process which renders possible that persistence of
organic beings throughout successive generations, which is generally
thought to be so well understood and to need no special explanation.
Nevertheless our minds cannot fail to be much perplexed by the
multiplicity of its manifestations, and to be greatly puzzled as to its
real nature. A celebrated German physiologist says[34], ‘Although many
hands have at all times endeavoured to break the seal which hides the
theory of heredity from our view, the results achieved have been but
small; and we are in a certain degree justified in looking with little
hope upon new efforts undertaken in this direction. We must
nevertheless endeavour from time to time to ascertain how far we have
advanced towards a complete explanation.’
Such a course is in every way advisable, for we are not dealing with
phenomena which from their very nature are incomprehensible by man. The
great complexity of the subject has alone rendered it hitherto
insuperable, but in the province of heredity we certainly have not
reached the limits of attainable knowledge.
From this point of view heredity bears some resemblance to certain
anatomical and physiological problems, e. g. the structure and function
of the human brain. Its structure—with so many millions of nerve-fibres
and nerve-cells—is of such extraordinary complexity that we might well
despair of ever completely understanding it. Each fibre is nevertheless
distinct in itself, while its connection with the nearest nerve-cell
can be frequently traced, and the function of many groups of cell
elements is already known. But it would seem to be impossible to
unravel the excessively complex network into which the cells and fibres
are knit together; and hence to arrive at the function of each single
element appears to be also beyond our reach. We have not however
commenced to untie this Gordian knot without some hope of success, for
who can say how far human perseverance may be able to penetrate into
the mechanism of the brain, and to reveal a connected structure and a
common principle in its countless elements? But surely this work will
be most materially assisted by the simultaneous investigation of the
structure and function of the nervous system in the lower forms of
life—in the polypes and jelly-fish, worms and Crustacea. In the same
way we should not abandon the hope of arriving at a satisfactory
knowledge of the processes of heredity, if we consider the simplest
processes of the lower animals as well as the more complex processes
met with in the higher forms.
The word heredity in its common acceptation, means that property of an
organism by which its peculiar nature is transmitted to its
descendants. From an eagle’s egg an eagle of the same species
developes; and not only are the characteristics of the species
transmitted to the following generation, but even the individual
peculiarities. The offspring resemble their parents among animals as
well as among men.
On what does this common property of all organisms depend?
Häckel was probably the first to describe reproduction as ‘an
overgrowth of the individual,’ and he attempted to explain heredity as
a simple continuity of growth. This definition might be considered as a
play upon words, but it is more than this; and such an interpretation
rightly applied, points to the only path which, in my opinion, can lead
to the comprehension of heredity.
Unicellular organisms, such as Rhizopoda and Infusoria, increase by
means of fission. Each individual grows to a certain size, and then
divides into two parts, which are exactly alike in size and structure,
so that it is impossible to decide whether one of them is younger or
older than the other. Hence in a certain sense these organisms possess
immortality: they can, it is true, be destroyed, but, if protected from
a violent death, they would live on indefinitely, and would only from
time to time reduce the size of their overgrown bodies by division.
Each individual of any such unicellular species living on the earth
to-day is far older than mankind, and is almost as old as life itself.
From these unicellular organisms we can to a certain extent understand
why the offspring, being in fact a part of its parents, must therefore
resemble the latter. The question as to why the part should resemble
the whole leads us to a new problem, that of assimilation, which also
awaits solution. It is, at any rate, an undoubted fact that the
organism possesses the power of taking up certain foreign substances,
viz. food, and of converting them into the substance of its own body.
Among these unicellular organisms, heredity depends upon the continuity
of the individual during the continual increase of its body by means of
assimilation.
But how is it with the multicellular organisms which do not reproduce
by means of simple division, and in which the whole body of the parent
does not pass over into the offspring?
In such animals sexual reproduction is the chief means of
multiplication. In no case has it always been completely wanting, and
in the majority of cases it is the only kind of reproduction.
In these animals the power of reproduction is connected with certain
cells which, as germ-cells, may be contrasted with those which form the
rest of the body; for the former have a totally different rôle to play;
they are without significance for the life of the individual[35], and
yet they alone possess the power of preserving the species. Each of
them can, under certain conditions, develope into a complete organism
of the same species as the parent, with every individual peculiarity of
the latter reproduced more or less completely. How can such hereditary
transmission of the characters of the parent take place? how can a
single reproductive cell reproduce the whole body in all its details?
Such a question could be easily answered if we were only concerned with
the continuity of the substance of the reproductive cells from one
generation to another; for this can be demonstrated in some cases, and
is very probable in all. In certain insects the development of the egg
into the embryo, that is the segmentation of the egg, begins with the
separation of a few small cells from the main body of the egg. These
are the reproductive cells, and at a later period they are taken into
the interior of the animal and form its reproductive organs. Again, in
certain small freshwater Crustacea (_Daphnidae_) the future
reproductive cells become distinct at a very early period, although not
quite at the beginning of segmentation, i. e. when the egg has divided
into not more than thirty segments. Here also the cells which are
separated early form the reproductive organs of the animal. The
separation of the reproductive cells from those of the body takes place
at a still later period, viz. at the close of segmentation, in
_Sagitta_—a pelagic free-swimming form. In Vertebrata they do not
become distinct from the other cells of the body until the embryo is
completely formed. Thus, as their development shows, a marked
antithesis exists between the substance of the undying reproductive
cells and that of the perishable body-cells. We cannot explain this
fact except by the supposition that each reproductive cell potentially
contains two kinds of substance, which at a variable time after the
commencement of embryonic development, separate from one another, and
finally produce two sharply contrasted groups of cells.
It is evidently unimportant, as regards the question of heredity,
whether this separation takes place early or late, inasmuch as the
molecular constitution of the reproductive substance is determined
before the beginning of development. In order to understand the growth
and multiplication of cells, it must be conceded that all protoplasmic
molecules possess the power of growing, that is of assimilating food,
and of increasing by means of division. In the same manner the
molecules of the reproductive protoplasm, when well nourished, grow and
increase without altering their peculiar nature, and without modifying
the hereditary tendencies derived from the parents. It is therefore
quite conceivable that the reproductive cells might separate from the
somatic cells much later than in the examples mentioned above, without
changing the hereditary tendencies of which they are the bearers. There
may be in fact cases in which such separation does not take place until
after the animal is completely formed, and others, as I believe that I
have shown[36], in which it first arises one or more generations later,
viz. in the buds produced by the parent. Here also there is no ground
for the belief that the hereditary tendencies of the reproductive
molecules are in any way changed by the length of time which elapses
before their separation from the somatic molecules. And this
theoretical deduction is confirmed by observation, for from the egg of
a Medusa, produced by the budding of a Polype, a Polype, in the first
instance, and not a Medusa arises. Here the molecules of the
reproductive substance first formed part of the Polype, and later, part
of the Medusa bud, and, although they separated from the somatic cells
in the bud, they nevertheless always retain the tendency to develope
into a Polype.