THOUGHTS TOWARDS A THEORY OF THE DEVELOPMENT
OF ORGANISMS 101
THE BIOLOGICAL PROBLEM OF TO-DAY
INTRODUCTION.
What is development? Does it imply preformation or epigenesis? This
perplexing question of biology has reappeared recently as a problem of the
day. Of late years there have been set forth contradictory doctrines, each
seeking to explain the process by which the fertilised egg-cell, an
apparently simple beginning, gives rise to the adult organism, which often
is exceedingly complicated, and which has the capacity of producing new
beginnings like that from which it itself arose.
The opposing views of to-day were in existence centuries ago, and they are
known in the history of science as the theory of preformation or evolution,
and the theory of epigenesis. That most of the great biologists of the
seventeenth and eighteenth centuries were decided upholders of evolution
was the natural result of the contemporary knowledge of facts. For they
knew only the external signs of the process of development. All they saw
was the embryo becoming adult, the bud growing out into a blossom, as the
result of a process in which nutrition transformed smaller to greater
parts. And so they regarded development as a simple process of growth
resulting from nutrition. Their mental picture of the germ or beginning of
an organism was an exceedingly reduced image of the organism, an image
requiring for its development nothing but nutrition and growth. That the
material eye failed to recognise the miniature they attributed to the
imperfection of our senses, and to the extreme minuteness and resulting
opacity of the object.
That it might satisfy our human craving for final causes, the theory of
preformation had to be accompanied by a corresponding explanation of the
origin of the miniatures. Biologists had already abandoned the error of
such spontaneous generation as the origin of flies from decaying meat, and,
in its place, had accepted the doctrine of the continuity of life,
formulating it in the phrase, _Omne vivum e vivo_ (Each life from a life),
and in the similar phrase, _Omne vivum ex ovo_ (Each life from an egg). One
creature issued from another, within which it had lain as a germ, and the
series was continuous. Thus, the theory of preformation gave rise to the
conception that living things were a series of cases or wrappings, germ
folded within germ. The origin of life was relegated to the beginning, at
the creation of the world: it became the work of a supernatural Creator,
who, when He formed the first creatures, formed with them, and placed
within them, the germs of all subsequent creatures.
To reckon at their proper value the theory of preformation, and, still
more, the doctrine of enfolded germs, the standard of appreciation must not
be the present range of our knowledge. They must be viewed historically, in
the light of the knowledge of these days.
Nowadays it is not so much pure reason as a wider empirical knowledge of
nature, with its consequent transformation of ideas, that makes the
doctrine of enfoldment difficult. Abstract thought sets no limit to
smallness or greatness; for mathematics deals with the infinitely small and
with the infinitely great. So long as actual observation had not determined
the limits of minuteness in the cases in question, there were no logical
difficulties in the doctrine of enfolded germs. The biology of earlier
centuries had not our empirical standard. What appeared then to be a simple
organic material we have resolved into millions of cells, themselves
consisting of different chemical materials. The chemical materials have
been analysed into their elements, and chemistry and physics have
determined the dimensions of the ultimate molecules of these. It is only
because the minute constitution of matter is no longer a secret to us that
the theory of germ within germ now touches the absurd.
It was very different in earlier days; the acutest biologists and
philosophers were evolutionists, and an epigenetic conception of the
process of development could find no foothold alongside the apparent
logical consistency of the theory of preformation.
Wolff's _Theoria Generationis_ (1759) failed to convince his
contemporaries, because he could bring against the closed system of the
evolutionists only isolated observations, and these doubtful of
interpretation; and because, in his time, on account of the rudimentary
state of the methods of research in biology, men attached more importance
to abstract reasoning than to observation. His effort was the more
praiseworthy in that it was observation bearing witness against abstract
and dogmatic conceptions. By means of actual observation he tried to expose
the fallacy in preformation, to show that the organism was not fully formed
in the germ, but that all development proceeded by new formation, or
epigenesis; that the germ consisted of unorganised organic material, which
became formed or organised only little by little in the course of its
development, and that Nature really was able to produce an organism from an
unorganised material simply by her inherent forces.
It is interesting to display the essential contrast between preformation
and epigenesis in the poetical words of Wolff himself. 'You must remember,'
so run his words in the second argument against the probability of
preformation, 'that an evolution would be a phenomenon formed in its real
essence by God at the Creation, but created in condition invisible, and so
as to remain invisible for long before it would become visible. See, then,
that a _phenomenon_ of enfolding is a miracle, differing from ordinary
miracles only in these: first, it was at the creation of the world that God
produced it; second, it remained invisible for long before it became
visible. In truth, therefore, all organic bodies would be miracles. Would
not this change for us the presence of Nature? Would it not spoil her of
her beauty? Hitherto we had a living Nature, displaying endless changes by
her own forces. Now it would be a fabric displaying change in seeming only,
in truth and essence remaining unchanged and as it was constructed, save
that it gradually becomes more and more used up. Formerly it was a Nature
destroying herself and creating herself anew, only that endless changes
might become visible and new sides be brought to light. Now it would be a
lifeless mass shedding off piece after piece until the stock should come to
an end.'
None the less, who seeks in Wolff's 'Theoria Generationis' an account of
the means or forces by which Nature builds up organic forms will seek in
vain. The _vis essentialis_ (inherent force) with which Wolff endowed his
plastic organic material, or the _nisus formativus_ (formative force),
afterwards suggested to science by Blumenbach--what are they but empty
words by which men seek to grasp in thought what has eluded them? Wolff's
epigenesis was not a complete explanation--indeed, from its fundamental
conception it could not possibly be such. For investigation of the natural
forces by which development proceeds can advance only slowly and step by
step, and for long will constitute the foremost task of biology. The
prosecution of biological investigation will continuously endow the theory
of epigenesis with a fuller and fuller meaning, but will never transform it
into a solution final in the sense of the theory of preformation.
It seems to me that the significance of Wolff's doctrine lies in this: it
rejected the purely formal theory of preformation because actual
observations were against it. Thereby Wolff freed research from the
straitened bonds of prejudice, and entered the only possible path by which
science can advance--the path along which the biology of our century has
made so great advances.
Biologists of to-day approach the problem of organic development equipped
with incomparably greater knowledge and with more delicate methods of
research. But in our thoughts to-day, as we discuss the essential nature of
the process of organic development and the mutual causal relations between
rudiments and their products, the same contradictory views are present,
altered only as our methods of expression have altered.
In a striking fashion Roux[1] has contrasted the opposing ideas inherent in
our modern conception of development, but yet identical with those which
formerly found expression in the theories of preformation and epigenesis.
By the term "embryonic development," in its ordinary acceptation, we
understand the appearance of visible complexity. But when we speak of the
visibility of the resulting complexity, we use a subjective term, the value
of which is relative to the human eye. Going further into the matter, we
must break up the conception into two parts, and distinguish between the
actual production of complexity and the mere transformation of complexity
from a condition invisible to us into complexity visible to our senses.
'The two kinds of development I have indicated bear a relation to each
other that recalls the old opposing doctrines of preformation and
epigenesis, the alternatives of a time when it was a task--perhaps the only
possible task--to record the completed results of the stages in development
as they became complete--in fact, to record the externally visible changes
of shape. In this descriptive investigation of the development of external
form, epigenesis, the successive formation of new shapes, gained a complete
victory over evolution, the mere becoming visible of pre-existing details
of shape.
'The closer investigation of embryonic development that is necessary in a
search for causes brings us once more against the old alternatives, and
compels us to a closer scrutiny of them.
'In this, if we still retain the old terms, epigenesis would mean not
merely the building up of complicated form through the agency of a
substratum, apparently simple, but perhaps with an extraordinarily
complicated, minute structure, but, in the strictest sense of the term, the
new formation of complexity, an actual increase of complexity. Evolution,
on the other hand, would imply the mere becoming visible of pre-existing
latent differentiation. Clearly, according to these general definitions,
occurrences which outwardly exhibit epigenesis may be in reality partial or
complete evolution. In fact, the deepest consideration leads us again to
the original question: Is embryonic development epigenesis or evolution? Is
it the new formation of complexity, or is it the becoming visible of
complexity previously invisible to us?'
Thus, in our own days, after the controversy has been at rest for long,
biologists are assembled in opposing groups, one under the standard of
epigenesis, another under that of preformation.
Weismann[2] leads the van for preformation; for the last ten years he has
occupied himself with the theoretical discussion of the questions set forth
above; and now, in a recent treatise, _The Germplasm_, he has combined his
views, already many times modified, in a coherent theory. Now he explains
candidly that he has been driven to the view that epigenetic development
does not exist. 'In the first chapter of my book,' he remarks, 'will be
found an actual proof of the reality of evolution, a proof so simple and
obvious that I can scarcely understand to-day how it could have escaped my
notice so long' (_Germplasm_, p. 14). Elsewhere he writes: 'I believe that
I have established that ontogeny can be explained only by evolution, and
not by epigenesis.'
A mental process, which consciously or unconsciously plays a great part
with evolutionists, and helps to determine their conclusions, is
characteristic of the direction of their inquiries. They set out from the
fact that the characters of the parents, often to the smallest detail, are
transmitted to children by means of the germ or rudiment; they conclude
that the active causes of all the complexity that arises must be contained
in the apparently homogeneous germ, embryological differentiation being a
spontaneous process. It follows that the apparent homogeneity is, in
reality, latent complexity which becomes patent during the progress of
ontogeny. Latent complexity implies a material substratum, consisting of
actual particles for which many different names have been found. As our
senses can give us no experimental knowledge of these particles, which are
so small as to be invisible, modern evolutionists attempt to picture them,
in imagination, by reflecting all the visible characters of the perfected
organism upon the undivided egg-cell, so peopling that globule of yolk with
a system of minute particles corresponding in quality and in spacial
arrangement with the larger parts of the adult.
Weismann has practised this art in the true spirit of a virtuoso, and has
elaborated it into a novel mode of biological investigation. Take an
example;--'It would be impossible,' he says in _The Germplasm_ (p. 138),
'for any small portion of the human skin to undergo a hereditary and
independent change from the germ onwards, unless a small vital element
corresponding to this particular part of the skin existed in the germ
substance, a variation in this element causing a corresponding variation in
the part concerned. Were this not the case, birth-marks would not exist.'
Thus, in a slightly altered fashion, we come again to the position of the
evolutionists of last century, for whom the germ was an extremely small
miniature of the adult creature. The new evolution, as Weismann in especial
has established it, seems to me to differ from the old doctrine only in two
important points; and these must be placed to the credit of the greater
scientific knowledge of our century. The first point concerns the relative
positions of the parts in the patent and latent conditions. The older
evolutionists assumed that these were identical, that the germ was a true
miniature. It is true that Weismann regards his almost countless germinal
particles as being held together in an architectural structure of almost
inconceivable complexity. For him the germ is an exceedingly complicated
living being, a microcosm in the truest sense, in which every independently
variable part that ever appears throughout the whole life is represented by
a living particle, and in which each of the living particles is endowed
with a definite, inherited position, a constitution, and the power of rapid
multiplication. It is upon the qualities of these ultimate particles that
he makes depend the qualities of the corresponding parts of the adult, the
parts that are cells as well as the parts built of many cells. As, however,
during visible development the parts of the embryo undergo many changes of
position and metamorphoses, Weismann is compelled to make the assumption
that the germ, as a micro-organism, is not simply a miniature of the adult,
but that its minute particles have an arrangement totally different from
that of the corresponding parts in the adult organism.
The second point is the origin of each new generation. To explain the
continuity of development, the old evolutionists held that the generations
lay enfolded one within another. Weismann avoids this difficulty by
endowing his germs with divisibility, but he gives us no proof that
division could possibly take place in the case of structures composed of
innumerable particles built up into a definite and most complicated
architectural system.
Although the new evolution differs from the old in the points mentioned
above, the two theories obviously agree very closely in the nature of their
arguments and conclusions. When, to satisfy our craving for causality,
biologists transform the visible complexity of the adult organism into a
latent complexity of the germ, and try to express this by imaginary tokens,
by minute and complicated particles cohering into a system, they are making
a phantasmal image which, indeed, apparently may satisfy the craving for
causality (to satisfy which it was invented), but which eludes the control
of concrete thought, by dealing with a complexity that is latent, and
perhaps only imaginary. Thus, craftily, they prepare for our craving after
causality a slumbrous pillow, in the manner of the philosophers who would
refer the creation of the world to a supernatural principle.
But their pillow of sleep is dangerous for biological research; he who
builds such castles in the air easily mistakes his imaginary bricks,
invented to explain the complexity, for real stones. He entangles himself
in the cobwebs of his own thoughts, which seem to him so logical, that
finally he trusts the labour of his mind more than Nature herself.
'Experiment,' says Weismann in _The Germplasm_, 'is not the only way to
reach general views, nor is it always the safest means of discrimination,
although at first it seems conclusive....[3] It seems to me that in this
case we can draw more prudent conclusions from the general facts of
inheritance than from the results of experiments that are neither quite
clear nor undubious, although in themselves they are most valuable, and
deserve the most careful consideration. If one remembers what was said in
my section on the architecture of the germplasm as the basis of the theory
of determinants, it will be agreed with me that ontogeny must find its
explanation in evolution, and not in epigenesis.'[4]
I take up a more epigenetic position, and years ago I attacked evolutionary
doctrines in many of their modifications.[5] Thus, in the _Studien zur
Blaetter Theorie_, published by Richard Hertwig and myself, I combated the
supposed law that the germinal layers histologically were primitive organs.
Next, in a pamphlet entitled _The Problem of Fertilisation: a Theory of
Heredity_, I attempted to disprove the principle of His that there were
organ-building foci in the germ. In my treatise _On Ovogenesis and
Spermatogenesis in the Nematodes_, I declared against the suppositions
involved in Weismann's doctrine of the germplasm, and sharply distinguished
the theory, simultaneously propounded by Strasburger and myself, that the
nucleus is the bearer of the hereditary material, from the evolutionistic
interpretation given it by Weismann.
A paper on 'The Blastopore and Spina Bifida,' and an occasional lecture on
'Old and New Theories of Development,' gave me the opportunity of dealing
with Roux's mosaic theory, although that not only shows learning, but
apparently is the outcome of experiment. I advocated in its place the
theory that 'the embryological development of an organism is no mosaic
work. The parts of an organism develop in relation to each other, the
development of a part depending upon the development of the whole.' The
labours of Roux, as well as the valuable researches of Driesch, induced me
to carry out a series of experiments with the object of getting a surer
basis for my epigenetic conception of development. The results of these
were published recently under the title, _On the Value of the First
Cleavage-cells in the Formation of the Organs of Embryos_.
In the latter treatise I confined myself advisedly to the exposition and
interpretation of the results of my investigations, having in view a
subsequent discussion of the more theoretical bearings of my results. It is
this that sees the light in the present book.
As for many years I have occupied myself with the problem of development,
pursuing observation and framing theory, there is due to myself and to
others an exposition of the position I have assumed in many of my
treatises, but in a more connected and elaborated fashion than has been
possible hitherto. This course is the more imperative, as in his recent
_magnum opus_ on the germplasm Weismann has propounded a theory of
evolution wrought with the greatest care and acuteness, and totally
irreconcilable with my conclusions. The chief differences between my views
and those of Weismann have now become clearer and more tangible than ever.
It is true that in my text-book, _On the Structure and Function of
Cells_,[6] published in the autumn of 1892, I gave a short account of my
theory of heredity in chapter ix., 'The Cell as the Material Beginning of
the Organism.' But in that I could not deal with Weismann's work, which
appeared simultaneously, and, moreover, in a text-book it was impossible to
do more than sketch my views.
My present task is twofold; it has both a positive and a negative side.
First, I have to examine the arguments recently alleged in favour of the
theory of preformation, testing them to reveal their inherent weaknesses,
and to controvert their fallacies. As Weismann unquestionably is the chief
of those who have advocated preformation, and has made a closed system of
it again, it is necessary for me to take special notice of his conception
as it is set forth in _The Germplasm_. Although I am no friend of polemic,
the case demands it. For the decision of a question so momentous as the
relative scopes of evolution and epigenesis in embryology must have an
important bearing on the future of biology, upon its aim and the method of
research.
But criticism of Weismann's hypothesis is not to be an end in itself; I am
more anxious to show the lines upon which, as I think, the real meaning of
the process of organic development will come to be learned. In a second
section, therefore, I shall explain my own views in greater detail, and, as
I hope, place them on a firmer foundation than formerly was possible.
FOOTNOTES:
[1] Wilhelm Roux in _Zeitschrift fuer Biologie_, vol. xxi. (1885): _Zuer
Orientirung ueber einige Probleme der Embryonalen Entwicklung._
[2] See Weismann's _Collected Essays_, Clarendon Press (2nd edit.), vol.
i., 1891, vol. ii., 1892; and Weismann's _Germplasm_, Walter Scott's
Contemporary Science Series, 1893. The references in this translation are
to the latter volume.
[3] _The Germplasm_, p. 137.
[4] _Ibid._, p. 138.
[5] The ideas expressed in this book may be found, in an elementary
condition, in various publications of my own, and written in conjunction
with my brother, Richard Hertwig: Oscar and Richard Hertwig, _Die
Actinien_; Jena, 1879 (pp. 203-217). Oscar Hertwig, _Das Problem der
Befruchtung und der Isotropie des Eies, eine Theorie der Vererbung_; Jena,
1884. Oscar Hertwig, _Vergleich der Ei- und Samenbildung bei Nematoden,
Arch. f. Mikrosk. Anatomie_, vol. xxxvi., 1890, pp. 77-128. Oscar Hertwig,
_Urmund und Spina bifida, Arch. f. Mikrosk. Anatomie_, vol. xxxix., 1892,
pp. 476-492. Oscar Hertwig, _Aeltere und neuere Entwicklungstheorien_;
Berlin, 1892. Oscar Hertwig, _The Cell_: Sonnenschein; London, 1895. Oscar
Hertwig, _Ueber den Werth der ersten Furchungszellen fuer die Organbildung
des Embryo, Arch. f. Mikrosk. Anatomie_, vol. xlii., 1893. The chief other
writers to whom I refer are: Herbert Spencer, _Principles of Biology_.
Darwin, _Pangenesis, a Provisional Hypothesis (in Variation of Plants and
Animals under Domestication)_. Haeckel, _Die Perigenesis der Plastidule_.
Weismann, _loc. cit._, p. 8. Naegeli, _Mechanisch-physiologische Theorie
der Abstammungslehre_; Muenchen, 1884. Strasburger, _Neue Untersuchungen
ueber den Befruchtungsvorgang bei den Phanerogamen als Grundlage fuer eine
Theorie der Zeugung_, 1884. H. de Vries, _Intracellulare Pangenesis_. W.
His, _Unsere Koerperform und das physiologische Problem ihrer Entstehung_,
1874. W. Roux, _loc. cit._, p. 6. Driesch, _loc. cit._, p. 48.
[6] An English translation, _The Cell_, was published by Swan Sonnenschein
and Co. in 1895.