means of which a species might secure continued existence, for it is a
method for which it would have to pay very dearly at a later period. If
my theory as to the causes of hereditary individual variability be
correct, it follows that all species with purely parthenogenetic
reproduction are sure to die out; not, indeed, because of any failure
in meeting the existing conditions of life, but because they are
incapable of transforming themselves into new species, or, in fact, of
adapting themselves to any new conditions. Such species can no longer
be subject to the process of natural selection, because, with the
disappearance of sexual reproduction, they have also lost the power of
combining and increasing those hereditary individual characters which
they possess.
All the facts with which we are acquainted confirm this conclusion, for
whole groups of purely parthenogenetic species or genera are never met
with, as would certainly be the case if parthenogenesis had been the
only method of reproduction through a successional series of species.
We always find it in isolated instances, and under conditions which
compel the conclusion that it has become predominant in the species in
question, and has not been transmitted from any preceding species.
There still remains a very different class of facts which, so far as we
can judge, are in accordance with my theory as to the significance of
sexual reproduction, and which may be quoted in its support. I refer to
the condition of functionless organs in species with parthenogenetic
reproduction.
Under the supposition that acquired characters cannot be
transmitted—and this forms the foundation of the views here set
forth—organs which are of no further use cannot become rudimentary in
the direct and simple manner in which it has been hitherto imagined
that degeneration takes place. It is true that an organ which does not
perform any function exhibits a marked decrease of strength and
perfection in the individual which possesses it, but such acquired
degradation is not transmitted to its descendants, and we must
therefore look for some other explanation of the firmly established
fact that organs do become rudimentary through a series of generations.
In seeking this explanation, we shall have to start from the
supposition that new forms are not only created by natural selection,
but are also preserved by its means. In order that any part of the body
of an individual of any species may be kept at the maximum degree of
development, it is necessary that all individuals possessing it in a
less perfect form must be prevented from propagation—they must succumb
in the struggle for existence. I will illustrate this by a special
instance. In species which, like the birds of prey[193], depend for
food upon the acuteness of their vision, all individuals with
relatively weak eyesight must be exterminated, because they will fail
in the competition for food. Such birds will perish before they have
reproduced themselves, and their imperfect vision is not further
transmitted. In this way the keen eyesight of birds of prey is kept up
to its maximum.
But as soon as an organ becomes useless, the continued selection of
individuals in which it is best developed must cease, and a process
which I have termed _panmixia_ takes place. When this process is in
operation, not only those individuals with the best-developed organs
have the chance of reproducing themselves, but also those individuals
in which the organs are less well-developed. Hence follows a mixture of
all possible degrees of perfection, which must in the course of time
result in the deterioration of the average development of the organ.
Thus a species which has retired into dark caverns must necessarily
come to gradually possess less developed powers of vision; for defects
in the structure of the eyes, which occur in consequence of individual
variability, are not eliminated by natural selection, but may be
transmitted and fixed in the descendants[194]. This result is all the
more likely to happen, inasmuch as other organs which are of importance
for the life of the species will gain what the functionless organ loses
in size and nutrition. As at each stage of retrogressive transformation
individual fluctuations always occur, a continued decline from the
original degree of development will inevitably, although very slowly,
take place, until the last remnant finally disappears. How
inconceivably slowly this process goes on is shown by the numerous
cases of rudimentary organs: by the above-mentioned embryonic sixth
finger of man, or by the hind limbs of whales buried beneath the
surface of the body, or by their embryonic tooth-germs. I believe that
the very slowness with which functionless organs gradually disappear,
agrees much better with my theory than with the one which has been
hitherto held. The result of the disuse of an organ is considerable,
even in the course of a single individual life, and if only a small
fraction of such a result were transmitted to the descendants, the
organ would be necessarily reduced to a minimum, in a hundred or at any
rate in a thousand generations. But how many millions of generations
may have elapsed since e. g. the teeth of the whalebone whales became
useless, and were replaced by whalebone! We do not know the actual
number of years, but we know that the whole material of the tertiary
rocks has been derived from the older strata, deposited in the sea,
elevated, and has been itself largely removed by denudation, since that
time.
Now if this theory as to the causes of deterioration in disused organs
be correct, it follows that rudimentary organs can only occur in
species with sexual reproduction, and that they cannot be formed in
species which are exclusively reproduced by the parthenogenetic method:
for, according to my theory, variability depends upon sexual
reproduction, while the deterioration of an organ when disused, no less
than its improvement when in use, depends upon variability. There are
therefore two reasons which lead us to expect that organs which are no
longer used will remain unreduced in species with asexual reproduction:
first, because only a very slight degree of hereditary variability can
be present, viz. such a degree as was transmitted from the time when
sexual reproduction was first abandoned by the ancestors; and,
secondly, because even these slight degrees of variability are not
combined, or, in other words, because panmixia cannot occur.
And the facts seem to point in the direction required by the theory,
for superfluous organs do not become rudimentary in parthenogenetic
species. For example, as far as my experience goes, the _receptaculum
seminis_ does not deteriorate, although it is, of course, altogether
functionless when parthenogenesis has become established. I do not
attach much importance to the fact that the Psychids and
Solenobias—(genera of Lepidoptera which Siebold and Leuckart have shown
to include species with parthenogenetic reproduction)—still retain the
complete female sexual apparatus, because colonies containing males
still occasionally occur in these species. Although the majority of
colonies are now purely female, the occasional appearance of males
points to the fact that the unisexuality of the majority cannot have
been of very long duration. The process of transformation of the
species from a bisexual into a unisexual form, only composed of
females, is obviously incomplete, and is still in process of
development. The case is similar with several species of _Cynipidae_,
which reproduce by the parthenogenetic method. In these cases the
occurrence of a very small proportion of males is the general rule, and
is not confined to single colonies. Thus Adler[195] counted 7 males and
664 females in the common _Cynips_ of the rose.
In some Ostracodes, on the other hand, the males appear to be entirely
wanting: at least, I have tried in vain for years to discover them in
any locality or at any time of the year[196].
_Cypris vidua_ and _Cypris reptans_ are such species. Now, although the
transformation of these formerly bisexual species into purely unisexual
female species appears to be complete[197], yet the females still
possess a large, pear-shaped _receptaculum seminis_, with its long
spirally twisted duct, which is surrounded by a thick glandular layer.
This is the more remarkable as the apparatus is very complicated in the
Ostracodes, and retrogressive changes could be therefore easily
detected. Furthermore among insects, in the genus _Chermes_ the
_receptaculum seminis_ of the females has also remained unreduced,
although the males appear to be entirely wanting, or at least have
never been found, in spite of the united efforts of several acute
observers[198]. The case is quite different in species which retain
both sexual and parthenogenetic reproduction. Thus, the summer females
of the _Aphidae_ have lost the _receptaculum seminis_; and in these
insects sexual reproduction has not ceased, but alternates regularly
with parthenogenetic reproduction.
Certainly this proof of the truth of my theory as to the significance
of sexual reproduction is far from settling the question: it only
renders the theory highly probable. At present it is impossible to do
more than this, because we do not yet possess a sufficient number of
facts, for many of them could not have been sought for until after the
theory had been suggested. We are here concerned with complicated
phenomena, into which we cannot acquire an immediate insight, but can
only attain it gradually.
But, nevertheless, I hope to have shown that the theory of natural
selection is by no means incompatible with the theory of ‘the
continuity of the germ-plasm;’ and, further, that if we accept this
latter theory, sexual reproduction appears in an entirely new light: it
has received a meaning, and has to a certain extent become intelligible.
The time in which men believed that science could be advanced by the
mere collection of facts has long passed away: we know that it is not
necessary to accumulate a vast number of miscellaneous facts, or to
make as it were a catalogue of them; but we know that it is necessary
to establish facts which, when grouped together in the light of a
theory, will enable us to acquire a certain degree of insight into some
natural phenomenon. In order to direct our attention to those new facts
which are of immediate importance, it is absolutely necessary to seek
the aid of some general theory for the arrangement and grouping of
those which we already possess. This has been my object in the present
paper.
But it may be perhaps objected that these phenomena are far too
complicated to be attacked at the present time, and that we ought to
wait quietly until the simpler phenomena have been resolved into their
components. It may be asked whether the trouble and labour involved in
the attempt to solve such questions as heredity or the transformation
of species are not likely to be wasted and useless.
It is true that we sometimes meet with such opinions, but I believe
that they are based upon a misunderstanding of the method which mankind
has always followed in the investigation of nature, and which must
therefore be founded upon the necessary relations existing between
mankind and nature.
Science has often been compared to an edifice which has been solidly
built by laying stone upon stone, until it has gradually risen to
greater height and perfection. This comparison holds good up to a
certain point, but it leads us to easily overlook the fact that this
metaphorical building does not at any point rest upon the ground, and
that, at least up to the present time, it has remained floating in the
air. Not a single branch of science, not even Physics itself, has
commenced building from below; all branches have begun to build at
greater or less heights in the air, and have then built downwards: and
even Physics has not yet reached the ground, for it is still very
uncertain as to the nature of matter and force. In no single group of
phenomena can we begin with the investigation of ultimate causes,
because at this very point our means of reasoning stop short. We cannot
begin with ultimate phenomena and gradually lead up to those which are
more complicated: we cannot proceed synthetically and deductively,
building up the phenomena from below; but we must as a rule proceed
analytically and inductively, proceeding from above downwards.
No one will dispute these statements, but they are often forgotten, as
is proved by the above-mentioned objection. If we were only permitted
to attack the more complicated phenomena after gaining a complete
insight into the simpler ones, then all scientists would be physicists
and chemists, and not until Physics and Chemistry were done with should
we be permitted to proceed to the investigation of organic nature.
Under these circumstances we ought not to possess now any scientific
theory of medicine; for the study of pathological physiology could not
be commenced until normal physiology was completely known and
understood. Yet how great a debt is owing by normal to pathological
physiology! This is an example which enforces the conclusion that it is
not only permissible, but in the highest degree advantageous, for the
different spheres of phenomena to be attacked simultaneously.
Furthermore, if we had been compelled to proceed from the simple to the
complex, what would have become of the Theory of Descent, the influence
of which has advanced our knowledge of Biology to an altogether
immeasurable extent?
But in this often repeated criticism that we are not yet ready to
attack such complicated phenomena as heredity, is hidden still another
fallacy, for it is implied that facts become less certain in proportion
to the complexity of their causes. But is it less certain that the egg
of an eagle developes into an eagle, or that the peculiarities of the
father and mother are transmitted to the child, than that a stone falls
to the ground when its support is taken away? Again, is it not possible
to draw a perfectly distinct and certain conclusion as to the relative
quantity of the material basis of heredity, present in the germ-cells
of either parent, from the fact that the father and mother possess an
equal or nearly equal share in heredity? But it is really unnecessary
to argue in this way: why should we do more than re-affirm that such a
method of procedure in scientific investigation is the only way by
which we can gradually penetrate the hidden depths of natural phenomena?
No! Biology is not obliged to wait until Physics and Chemistry are
completely finished; nor have we to wait for the investigation of the
phenomena of heredity until the physiology of the cell is complete.
Instead of comparing the progress of science to a building, I should
prefer to compare it to a mining operation, undertaken in order to open
up a freely branching lode. Such a lode must not be attacked from one
point alone, but from many points simultaneously. From some of these we
should quickly reach the deep-seated parts of the lode, from others we
should only reach its superficial parts; but from every point some
knowledge of the complex _tout ensemble_ of the lode would be gained.
And the more numerous the points of attack, the more complete would be
the knowledge acquired, for valuable insight will be obtained in every
place where the work is carried on with discretion and perseverance.
But discretion is indispensable for a fruitful result; or, leaving our
metaphor, facts must be connected together by theories, if science is
to advance. Just as theories are valueless without a firm basis of
facts, so the mere collection of facts, without relation and without
coherence, is utterly valueless. Science is impossible without
hypotheses and theories: they are the plummets with which we test the
depth of the ocean of unknown phenomena, and thus determine the future
course to be pursued on our voyage of discovery. They do not give us
absolute knowledge, but they afford us as much insight as it is
possible for us to gain at the present time. To go on investigating
without the guidance of theories, is like attempting to walk in a thick
mist without a track and without a compass. We should get somewhere
under these circumstances, but chance alone would determine whether we
should reach a stony desert of unintelligible facts or a system of
roads leading in some useful direction; and in most cases chance would
decide against us.
In this sense I trust that the sign-post or compass which I offer may
be accepted. Even though it should be its fate to be replaced by a
better one at a later period, it will have fulfilled its object if it
enables science to advance for even a short distance.
------------------------------------------------------------------------
Footnotes for Essay V.
Footnote 176:
C. Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’
München u. Leipzig, 1884.
Footnote 177:
‘Ueber die Berechtigung der Darwin’schen Theorie.’ Leipzig, 1868, p.
27.
Footnote 178:
l. c., Preface, p. vi.
Footnote 179:
Since the above was written many other morphological peculiarities of
plants have been rightly explained as adaptations. Compare, for
instance, the investigations of Stahl on the means by which plants
protect themselves against the attacks of snails and slugs (Jena,
1888).—A. W., 1888.
Footnote 180:
l. c., pp. 117, 286.
Footnote 181:
Compare the second and fourth of the preceding Essays, ‘On Heredity’
and ‘The Continuity of the Germ-plasm as the Foundation of a Theory
of Heredity.’
Footnote 182:
Compare Rauber, ‘Homo sapiens ferus oder die Zustände der
Verwilderten.’ Leipzig, 1885.
Footnote 183:
‘Sitzungsberichte der baierischen Akademie der Wissenschaften,’ vom
18 Nov. 1865. Compare also his ‘Mechanisch-physiologische Theorie der
Abstammungslehre,’ p. 102, etc.
Footnote 184:
Jordan, ‘Remarques sur le fait de l’existence en société des espèces
végétales affines.’ Lyon, 1873.
Footnote 185:
S. Hermann’s ‘Handbuch der Physiologie,’ Theil II; ‘Physiologie der
Zeugung,’ by V. Hensen.
Footnote 186:
E. van Beneden, ‘Recherches sur la maturation de l’œuf, la
fécondation et la division cellulaire.’ Gand u. Leipzig, 1883, pp.
404 et seq.
Footnote 187:
Rolph, ‘Biologische Probleme.’ Leipzig, 1882.
Footnote 188:
Cienkowsky, ‘Arch. f. mikr. Anat.,’ ix. p. 47. 1873.
Footnote 189:
Hensen, ‘Physiologie der Zeugung,’ p. 139.
Footnote 190:
Coalescence takes place in the so-called bud-like conjugation of
_Vorticellidae_ and _Trichodinidae_, etc.
Footnote 191:
Compare (1) Bardeleben, ‘Zur Entwicklung der Fusswurzel,’
Sitzungsber. d. Jen. Gesellschaft, Jahrg. 1885, Feb. 6; also
‘Verhandl. d. Naturforscherversammlung zu Strassburg,’ 1885, p. 203;
(2) G. Baur, ‘Zur Morphologie des Carpus und Tarsus der
Wirbelthiere,’ Zool. Anzeiger, 1885, pp. 326, 486.
Footnote 192:
In frogs the sixth toe exists in the hind legs as a rudimentary
prehallux. Compare Born, Morpholog. Jahrbuch, Bd. I, 1876.
Footnote 193:
I here make use of the same illustration which I employed in my
first attempt to explain the effects of _panmixia_. Compare the
second Essay ‘On Heredity.’
Footnote 194:
[E. Ray Lankester has suggested (Encycl. Britann., art. ‘Zoology,’
pp. 818, 819) that the blindness of cave-dwelling and deep-sea
animals is also due to the fact that ‘those individuals with perfect
eyes would follow the glimmer of light and eventually escape to the
outer air or the shallower depths, leaving behind those with
imperfect eyes to breed in the dark place. A natural selection would
thus be effected.’ Such a sifting process would certainly greatly
quicken the rate of degeneration due to panmixia alone.—E. B. P.]
Footnote 195:
Adler, ‘Zeitschrift f. wiss. Zool.,’ Bd. XXXV, 1881.
Footnote 196:
Compare my paper, ‘Parthenogenese bei den Ostracoden,’ in ‘Zool.
Anzeiger,’ 1880, p. 82. Purely negative evidence, unless on an
immense scale, is quite rightly considered to be of no great value in
most cases. But the condition of these animals renders the
accumulation of such evidence unusually easy, because the presence of
males in a colony of Ostracodes can be proved by a very simple
indirect test. Thus if a colony contains any males the _receptacula
seminis_ of all mature females are filled with spermatozoa, and on
the other hand we may be quite sure that males are absent, if after
the examination of many mature females, no spermatozoa can be found
in any of their _receptacula_.
Footnote 197:
We cannot, however, be absolutely certain of this, for it is
conceivable that males may still occur in colonies other than those
examined.
Footnote 198:
It has now been shown by Blochmann that males appear for a very short
time towards the close of summer, as in the case of _Phylloxera_.—A.
W., 1888.
------------------------------------------------------------------------
APPENDICES.
Appendix I. Further considerations which oppose Nägeli’s
explanation of transformation as due to internal causes[199].
When I describe Nägeli’s theory of transformation as due to active
causes lying within the organism, as a phyletic force of
transformation, I do not mean to imply that it is one of those
mysterious principles which, according to some writers, constitute the
unconscious cause which directs the transformation of species. Nägeli’s
idioplasm, which changes from within itself, is conceived as a
thoroughly scientific, mechanically operating principle. This cause is
undoubtedly capable of theoretical conception: the only question is
whether it has any real existence. According to Nägeli, the growing
organic substance, the idioplasm, not only represents a _perpetuum
mobile_ rendered possible as long as its substance continually receives
from without the matter and force which are necessary for continuous
growth, but it also represents a _perpetuum variabile_ due to the
action of internal causes[200]. But this is just the doubtful point,
viz., whether the structure of the idioplasm itself compels it to
change gradually during the course of its growth, or whether it is not
rather the external conditions which compel the ever slightly varying
idioplasm to change in a certain direction by the summation of small
differences. It has been shown above that we do not gain anything by
adopting Nägeli’s theory, because the main problem which organic nature
offers for our solution, viz. adaptation, remains unsolved. Hence this
theory does not explain the phenomena of nature, and I believe that
there are also certain facts which are directly antagonistic to it.