AMER. ZOOL., 23:829-843 (1983)
T. H. Morgan and the Influence of Mechanistic Materialism
on the Development of the Gene Concept 1910-1940'
GARLAND E. ALLEN
Professor of Biology, Washington University, St. Louis, Missouri
nia Institute of Technology (1928) as a
monument to experimental, quantitative
and physico-chemical biology.
In the present paper I would like to
explore more deeply one aspect of Morgan's work that I only touched upon in my
book of 1978: his changing conception of
the fundamental unit of heredity, the gene.
In the period between 1900 and 1926 Morgan's view of hereditary units underwent
a profound shift: from opposition to any
kind of fundamental particles of heredity
(1900) to championing of the discrete,
atomistic, Mendelian gene that existed as
a definable segment of a chromosome
(1925). Behind this change lies an even
more profound reorganization of Morgan's thought, a reorganization embodied
in his changing conception of the field of
heredity itself. In this paper I will explore
the significance of that change—its biological, philosophical, and sociological
implications—and the effect which this
changing conception has had on contemporary biology. In particular, I would like
to argue that the hardening of the conception of heredity championed by Morgan
and especially his followers, had profound
implications for the fields of genetics,
embryology and evolution right down to
the present day.
Behind Morgan's changing conception
of heredity, and of the nature of the gene,
lay an important philosophical issue: the
growth and spread of mechanistic materialism in biology during the early 20th
century. Morgan played an important role
in the development of mechanistic thought
and its application for a variety of biological problems, particularly in embryology
and genetics. In the following section I will
discuss briefly Morgan's background and
the general outlines of his career. This will
the stage for a discussion of his move
set
1
From the Symposium on The Place of Thomas Hunt from descriptive to experimental and
Morgan in American Biology presented at the Annual mechanistic biology in the period between
Meeting of the American Society of Zoologists, 271891 and 1900.
30 December 1982, at Louisville, Kentucky.
INTRODUCTION
I am grateful for the opportunity
afforded by this symposium to review the
role of T. H. Morgan in the development
of modern biology. As is almost always the
case with such a retrospective glance, I find
that I agree with many of the points which
I made in earlier writings on Morgan, as
well as disagree—sometimes alarmingly—
with others. In general, however, I believe
that the major thesis of an earlier series of
papers on Morgan, as well as of my fulllength biography (Allen, 1978) remains
substantially correct. In brief, the thesis
was that Morgan and a number of his contemporaries revolted from the older
descriptive and speculative biology of the
late 19th century as exemplified by the
work of Ernst Haeckel (1834-1919), and
August Weismann (1834-1914). The
younger biologists embraced a new mechanistic and physico-chemical approach to
biology championed by Julius Sachs (18321897) and especially Jacques Loeb (18591924). In making this shift, Morgan and
his contemporaries not only sought out new
biological problems but also new methods
for approaching them. They tried to free
biology from the overweening influence of
morphology, that all-encompassing area of
research which attempted to use anatomy,
physiology, and embryology as a means of
deducing phylogenetic relationships. The
younger generation's aim was to make biology experimental, quantitative, and rigorous—in Morgan's words to put biology
"on the same footing as physics and chemistry." Morgan himself never tired of trying
to achieve this goal. Indeed, the final major
accomplishment of his career was to establish the Division of Biology at the Califor-
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830
GARLAND E. ALLEN
MORGAN'S BACKGROUND AND CAREER:
A BRIEF SUMMARY
Morgan was born in Lexington, Kentucky on September 25, 1866 of a distinguished southern family. His maternal
great-grandfather was Francis Scott Key,
and his paternal great-grandfather was
John Wesley Hunt (1773-1849), one of
Kentucky's first millionaires. His uncle was
Colonel John Hunt Morgan (1825-1864),
whose escapades as leader of the famous
"Morgan's Raiders" during the Civil War
has become legendary in the South. His
father, Charlton Hunt Morgan (18391912) was U.S. Consul at Messina, Italy,
during the days of Garibaldi, and openly
admired the guerilla leader's "Red Shirts."
The gentility of Morgan's background is
visible in the now-restored Hunt-Morgan
House (built 1814) in Lexington. Morgan
himself was neither pretentious, nor given
to much discussion of his family background. Yet the influences of his upbringing are everywhere apparent in his persistent self-confidence, his wisdom and
judiciousness in considering all sides of
issues, and his ability to treat all people
with whom he came in contact, regardless
of social or scientific standing, as equals.
After finishing his undergraduate work
at the State University of Kentucky (now
the University of Kentucky) in Lexington,
Morgan headed for graduate school at the
then relatively young Johns Hopkins University in Baltimore. At Hopkins Morgan
studied primarily under William Keith
Brooks (1848-1908), a student of Louis
Agassiz (1807-1873) and later his son
Alexander Agassiz (1835-1910) at Harvard. Like the elder Agassiz, Brooks was a
morphologist. In addition, however, he was
interested in a variety of broader issues
within biology. He had a strong philosophical bent, and loved to discourse at length
on topics such as the biogenetic law and
evolutionary perfectionism (McCullough,
1969). Brooks also had the ability to relate
the most minute observation to broad biological principles, and to see major areas
of biology as problems to be solved rather
than fixed and final " t r u t h s " already
revealed. To Brooks the main problems of
biology in the late 19th and early 20th centuries were those of heredity, evolution,
and especially embryonic growth and differentiation. Above all, he thought that
these problem areas could be probed
"without undertaking to resolve biology
into physics or chemistry" (Brooks, 1902,
p. 710). Morgan always found Brooks' tendency toward speculation and philosophical thinking annoying and ultimately
unproductive for furthering biological
research (Allen, 1978, p. 45).
More positive in many ways was the direct
influence of the other major professor at
Hopkins, H. Newell Martin (1848-1896),
a former student of Sir Michael Foster
(1836-1907) at Cambridge, and highly
recommended to Hopkins by T. H. Huxley
(1825-1895) for whom he had worked as
an assistant. Martin was a physiologist, an
advocate of the experimental method in
biology, and well grounded in chemistry
and physics (Geison, 1978, pp. 140ff.). It
was to Martin that Morgan and others of
Brooks' students often turned for the
experimental or physiological insights
which Brooks could not provide.
Morgan's doctoral work, under Brooks,
involved a morphological study of the
embryology and phylogeny of the Pycnogonids, a group of arthropods known as
"sea spiders" (Morgan, 1891). This work
involved a detailed microscopical study of
the very earliest stages of cleavage of sea
spider embryos. Morgan challenged the
then-dominant theory of Anton Dohrn
(1840-1909), which claimed that the Pycnogonids were derived from the annelids
long before that group gave rise to the
arthropods. In his thesis, Morgan argued
on embryological considerations that the
Pycnogonids were descended from the
arthropods, specifically the arachnids after
that group split off from the annelids, thus
providing sound evidence against Dohrn's
interpretation. Even as a doctoral student
Morgan showed a propensity to challenge
established ideas which remained with him
throughout his life.
After leaving Hopkins in 1891, Morgan
took up the position at Bryn Mawr College
recently vacated by E. B. Wilson (18561938), who had been called to head the
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
Zoology Department at Columbia University in New York. At Bryn Mawr the other
member of the biology department, was
Jacques Loeb (1859-1924), newly arrived
from Germany. Loeb was a thoroughgoing
materialist, grounded in 19th century German materialistic philosophy, and a strong
advocate of the mechanistic conception of
life (Loeb, 1911). Morgan and Loeb became
close friends, and it was through this association that Morgan first began to reject
seriously the descriptive and morphological tradition in which he had been trained.
A second influence in this same direction
came from Morgan's work, at the famed
Stazione Zoologica in Naples in the summer of 1892 and for most of a full academic
year, 1894-1895. Here he met the young
and enthusiastic experimental embryologist Hans Driesch (1867-1941) with whom
he also became lifelong friends. Driesch
was at that time involved in the heated controversy with Wilhelm Roux (1850-1924)
over the mosaic theory of development.
Despite the differences in interpretation of
their experiments, Driesch and Roux
agreed on one thing: the importance of
experimental and mechanistic analysis of
embryological problems. Both also agreed
on the importance of studying embryological problems for their own intrinsic value,
and not as mere avenues for reconstructing
phylogenies. Morgan was greatly excited
by the atmosphere of experimental and
mechanistic work being carried out by
Driesch and others at the Naples station.
His experiences there became a profound
turning point in his career, catalyzing him
to become a convinced and thoroughgoing
experimentalist. From that point onward,
Morgan waged all-out war against speculative and idealistic biology, such as that
found in the writings of Ernst Haeckel
(1834-1919), August Weismann (18341914), and his teacher Brooks. As he came
to oppose speculative biology, Morgan
increasingly championed not only experimentalism but at a deeper level the mechanistic philosophy of life.
Between 1891 and 1910 Morgan established himself as one of the foremost
experimental embryologists in the United
States, and the world. He carried out
831
research on regeneration, early cleavage
and differentiation, and wrote articles and
books on evolution, large-scale mutations
(such as those proclaimed by de Vries),
heredity, sex determination, and the chromosome theory. In 1908 he began the
search for large-scale de Vriesian mutations in animals, employing the small fruit
fly Drosophila melanogaster which he began
growing in his laboratory (Allen, 1975).
Although he did not find the large-scale
mutations for which he was searching,
Morgan did find one small mutation early
in 1910, the white-eyed male fly which
launched a new era in experimental
genetics. From 1910 through the late
1920s, Morgan and a small group of enthusiastic co-workers, A. H. Sturtevant(18911971), Calvin B. Bridges (1889-1938), and
H. J. Muller (1890-1968), among others,
actively elucidated the relationship between
genes and chromosomes of the fruit fly. As
is familiar to most biologists, this was the
period of the "classical school" of genetics.
Morgan and his group demonstrated that
genes could be regarded as discrete segments of chromosomes, arranged in a linear fashion. They were able to map the
genes, and eventually demonstrate a close
correlation between genetic and chromosomal maps. They uncovered many facts
about gene interaction, and demonstrated
the complexity of organization of the
genome of diploid, sexually reproducing
organisms. Along with work on plant
genetics at such centers as Cornell (R. A.
Emerson) and the University of Missouri
(L. J. Stadler and Barbara McClintock) the
Morgan school established clearly and
unequivocally the material nature of
hereditary units, and their structural organization in the chromosome.
After the heyday of Drosophila work in
the early to mid-1920s, Morgan gradually
returned to the real love of his youth:
experimental embryology. He never lost
sight, however, of his commitment to
mechanistic and physico-chemical biology.
When he was invited to form the Division
of Biology at the California Institute of
Technology in 1928, Morgan set out to
design a department, in composition of faculty and in physical structure, that would
832
GARLAND E. ALLEN
emphasize the relationship between biol- physics. It is the interaction of material
ogy, physics, chemistry and mathematics. entities which produces all change and
Morgan's Division of Biology became a motion in the universe. 4) All non-physical
model for the "new" biology of the mid- forces or mystical causes are inadmissible
20th century. And while Morgan himself as explanations of any phenomena. For
saw the work progress further and further example, the vital force or elan vital of late
from that with which he was directly famil- 19th century and early 20th century bioiar, he could rest satisfied that the new work logical science would be excluded as an
pointed in the direction in which biological unknowable and therefore unallowable
investigation must go. It is no accident that explanation for the special properties of
out of Morgan's laboratory at Cal Tech in living things. Some materialists would even
the years between 1928 and 1945 came a reject the use of concepts such as energy
number of investigators who pioneered the or gravity because they are not themselves
new biochemical and molecular genetics: directly measurable. For most, however,
Milislav Demerec, Boris Ephrussi, George such terms are allowable because their effects
Beadle, Albert Tyler, and E. L. Tatum. can be measured in the actual motion of
After retiring from active Chairmanship material objects.
of the Division in 1940, Morgan continued
Mechanistic materialism is a special form
research on several embryological prob- of materialism which can be summarized
lems, and died after a short illness in Pas- as five propositions. 1) The parts of a comadena on December 4, 1945.
plex whole are distinct and separate from
We turn now to a brief examination of one another: for example, the atoms in a
mechanistic biology—in more conven- molecule or the gears and levers in a clock.
tional philosophical terms known as mech- 2) If (1) above is true, then it follows that
anistic materialism—as a basic view of the the proper method for studying the whole
is to break it down into its component parts,
functioning of the natural world.
each of which can be investigated indeMECHANISTIC MATERIALIST PHILOSOPHY:
pendently of its more complex involveSOME DEFINITIONS
ment with other parts. This method of
Philosophical materialism can be defined investigation is often referred to as analysis,
by four general characteristics. 1) Material that is, the breaking apart of the whole into
reality exists outside of human perception its component parts. 3) Behind the method
and has existed prior to our knowledge of of analysis lies the general assumption that
it. While we may never be able to perceive the whole is equal to the sum of its parts
that material reality perfectly, such a real- and no more. There are no mystical or
ity does exist independent of our existence. "emergent" properties arising out of the
2) Ideas about the world are derived from association of the parts in the whole. Thus,
our interaction with material reality, and if we know all about each part it should be
not from some a priori source. Despite many possible to reconstruct the whole in its
19th century variations of this theme, most totality; nothing more is needed. 4) Sysversions of materialistic philosophy adhere tems change over time due largely to conto the idea that we derive our ideas largely stant forces impressed on them from the
from experience with reality, and not the outside. For example, the planets move in
definable orbits because of the gravitaother way around. This is not to say, of tional attraction of other bodies; organcourse, that once we have developed ideas isms die through accident or through the
we do not impress them on reality and try accumulation of waste metabolites or
to change aspects of the real world. None- chance mutations; populations evolve
theless, the main direction of such activity because organisms are constantly prepasses from material reality to human per- sented with challenges from a changing
ception. 3) All change in the universe is a environment. It is important to point out
result of matter in motion—that is, the that in this view change is seen as everaction of one material entity on another. present: it does not, however, arise necThe classic examples of matter-in-motion essarily out of conditions existing within
are the atomic theory and billiard-ball
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
the system (organism, population) itself, but
through changes presented to the organism by its external environment. 5) The
mechanistic world view is basically atomistic. Mechanists tend to see all phenomena
in terms of a mosaic of separate, interacting, but ultimately independent parts.
In the late 19th and early 20th centuries
a second view of materialism, which I will
call holistic materialism, began to emerge
in the writings of physiologists such as
Claude Bernard, and of naturalists such as
Darwin and Ernst Haeckel. Holistic materialism shares all the basic premises of
materialism itself, but differs significantly
from mechanistic materialism in several
ways. Its basic propositions are: 1) The parts
of a complex whole are interconnected, and
cannot be studied only in isolation from
each other. One of the major characteristics of any part is its interactions with the
whole. It is therefore not enough to study
the part by itself, but the part must be studied in its dynamic interaction with other
parts to make the whole. 2) This means
that in addition to the methods of analysis,
it is necessary to devise new techniques for
studying the interactions of parts. 3) It follows, then, that the whole is more than the
mere sum of its component parts—it is composed of the parts and their interactions with
one another. The emergent properties
which result from the interaction of parts
are nothing mystical or abstract. It is simply that the whole of a process, for example, a functioning organism, is more than
the additive values of its separate organs
and tissues. 4) Processes in the world are
dynamic and developmental. Change is a
fundamental part of any system, built into
the interaction of the parts within the
whole. The simplest forms of change are
wear and tear, and deterioration, which
mechanists recognize also. But most systems change in other ways as well, for
example, the development of an embryo
from egg to adult, or the evolution of a
population from one species to two or more
over time. Thus, holistic materialism sees
change as something more than merely the
response of a system to its external environment, recognizing that the system contributes something itself to its own developing and changing state. The key term
833
here is development—that is, change produced by the regular interaction of certain
known or knowable elements within a complex system.2
Although he was a strong advocate of
mechanistic biology, Morgan was never a
rigid and hard-line mechanist such as
Loeb. He was too good a naturalist and
too well-rounded a biologist to accede to
statements such as Loeb's claim that phototropic insects were "photochemical
machines enslaved to the light" (Fleming, 1964, p. xxii). Morgan did join with
Loeb, however, (and with physiologist W.
J. V. Osterhout), to edit a series of books
for Lippincott and Co. under the title,
Monographs on Experimental Biology, which
focused on topics dealing with physicochemical biology. The significance of this
series has been discussed elsewhere (Allen,
1969) but I should emphasize that Morgan's endorsement of a mechanistic biology was always guided on the one hand by
expediency in terms of definable problems,
and on the other by an appreciation of
organic complexity in such issues as embryonic development and differentiation.
We turn now to an examination of Morgan's early views of heredity in the period
1895-1912—that is, prior to but including
the very early years of his genetics work
with Drosophila.
MORGAN, HEREDITY, AND THE
GENOTYPE-PHENOTYPE DISTINCTION
Around the turn of the century many
biologists were aware that the term
"heredity" was used in several different and
ambiguous ways. Danish botanist Wil2
Historically and philosophically oriented readers
will recognize that a specific form of holistic materialism, known as dialectical materialism, makes many
of these points more explicit and develops a more
rigorous view of the factors which bring about and
guide change within complex systems. The term
dialectic, of course, refers to the view that two opposing tendencies are the driving force behind all developmental change. In addition, dialectical materialists
hold that quantitative changes lead inevitably to qualitative changes—that is, that evolution leads inevitably to revolution. True dialectical thinking played
only a small part in the history of genetics that
will be discussed in this paper; a more thorough presentation of the subject can be found in two other
papers (Allen, 1980, 1983).
834
GARLAND E. ALLEN
helm Johannsen (1857-1927) pointed out
the ambiguities in his now-famous paper
of 1911, "The Genotype Conception of
Heredity" (Johannsen, 1911), while William Bateson, in his Materials for the Study
of Variation, had voiced many of the same
complaints some 15 years earlier (Bateson,
1894; also Coleman, 1970, p. 294). More
recently historians of science Jan Sapp at
the University of Montreal and Scott Gilbert at Bryn Mawr College, have dissected out the various ways in which the
term heredity was understood by workers
in different fields of biology in the early
decades of the present century. Sapp has
identified at least five different contexts in
which heredity was used, each with its own
special slant and meaning: embryological,
Mendelian, biometrical, evolutionary, and
cytological (Sapp, 1982, pp. 27-33). Gilbert has also pointed out that with the rise
of Mendelian genetics, and especially of
the Morgan school, the term heredity
evolved from its earlier embryological, to
its more modern, genetic meaning (Gilbert, 1978). What do these authors mean
by distinguishing between an embryological and Mendelian view of heredity? And
how, and in what context, did the meaning
undergo a change in the biological community as a result of the work of Morgan
and his colleagues?
To begin with, it is useful to view genetics
and embryology in terms of vertical and
horizontal theories of transmission, a concept first discussed some years ago by Frederick Churchill (Churchill, 1974). In
Churchill's view, Mendelian genetics is a
vertical theory of heredity: that is, it is concerned with the transmission of elements
from parent to offspring through time,
from one generation to the next. Embryonic development, on the other hand, represents a horizontal view of heredity, one
which involves transformation of hereditary potentials into structural and physiological actualities during the course of a
single generation, that is, the development
of the individual. It was precisely this distinction which Johannsen was trying to
make when he wrote about the genotype
and the phenotype as different aspects of
the hereditary process. The vertical con-
ception of heredity encompasses not only
traditional Mendelian genetics, but also
cytology and evolution. From the very start,
the embryological conception of heredity,
as a horizontal view, was set apart from the
others. What, then, was the embryological
meaning of the term "heredity," and what
part did it play in early 20th century biology?
From the 1850s onward embryologists
defined heredity in an increasingly epigenetic fashion. This meant they emphasized
that what was passed from parent to offspring were not traits or adult characters
in the final and formed sense, but the potentiality to produce traits. Since environmental factors of all sorts can influence the
actual course of development, the interesting problem of heredity to many
embryologists was not the transmission of
potentialities, but the development of
potentialities into the actuality of adult
anatomy and physiology. As a consequence
of this view, embryologists around the turn
of the century focussed far more of their
interest and attention on the cytoplasm
than on the nucleus of the egg cell, and
subsequent embryonic cleavage products.
Embryologists, then, saw the most interesting problems of heredity as (1) the conversion of hereditary potentialities into
embryological actualities, and (2) the interaction of various parts: the cytoplasm and
the nucleus, the egg and its environment,
the embryo and the external world. This
view is plainly visible in the many theories
of sex determination current in the period
1880-1910, emphasizing such factors as
temperature, humidity, availability of food,
or other climatic triggers for differentiation into male or female embryos (Allen,
1966). The embryological view of heredity
is captured nicely by Edwin Grant Conklin
in 1908 when he wrote:
Indeed heredity is not a peculiar or
unique principle for it is only similarity
of growth and differentiation in successive generations . . . . In fact, the whole
process of development is one of growth
and differentiation, and similarity of
these in parents and offspring constitutes
hereditary likeness. The causes of hered-
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
ity are thus reduced to the causes of successive differentiation and development,
and the mechanism of heredity is merely
the mechanism of differentiation.
(Conklin, 1908, p. 90)
Morgan said it even more bluntly two years
later: "We have come to look upon the
problem of heredity as identical to the
problem of development" (Morgan, 1910,
p. 449). In short, then, embryologists
tended to see heredity as what we would
call today the translation of genetic potentialities into adult traits. Given this view,
the interesting aspect of heredity was how
specific traits materialized epigenetically,
during embryonic development. This was,
indeed, Morgan's view of heredity up
through the very early years of his Drosophila work. Morgan was very much an
epigeneticist, and strongly opposed to preformationist views. In fact, prior to 1910
Morgan opposed both the Mendelian theory of heredity, and the chromosome theory of heredity, on the grounds that both
were preformationist views, relegating the
most interesting questions of biology to
hypothetical, pre-formed particles. Morgan stated that the Mendelian theory was
essentially a preformationist doctrine:
The nature of Mendelian interpretation
and description inextricably commits to
the "doctrine of particles" in the germ
and elsewhere. It demands a "morphological" basis in the germ for the minutest phase (factor) of a definitive character. It is essentially a morphological
conception with but a trace of functional
feature. With an eye seeing only particles
and a speech only symbolizing them,
there is no such thing as a study of a
process possible . . . . It has been possible, I think, to show by means of what
we know of the genesis of these color
characters that the Mendelian description—of color inheritance at least—has
strayed very wide of the facts; it has put
factors in the germ cells that it is now
quite certainly our privilege to remove;
it is declared a discontinuity where there
is now evident epigenesis. (Morgan, 1909,
p. 509)
To Morgan the particular conception of
835
heredity, whether applied to chromosomes
or to the more abstract Mendelian factors,
appealed to a frame of mind which sought
finalistic answers, rather than the probing
of complex processes. In 1910, the same
year that he published his first account of
the white-eyed male Drosophila, Morgan
wrote:
It may be said in general that the particulate theory is the more picturesque
or artistic conception of the developmental process. As a theory it has in the
past dealt largely in symbolism and is
inclined to make hard and fast distinctions. It seems to better satisfy a class or
type of mind that asks for afinalisticsolution, even though the solution be truly
formal. But the very intellectual security
that follows in the train of such theories
seems to me less stimulating for further
research than does the restlessness of
spirit that is associated with the alternative [that is epigenetic or embryological] conception. (Morgan, 1910, pp.
451-452)
From the embryological point of view, then,
paniculate theories were equivalent to preformationism: they pushed the interesting
problem of the development of adult traits
back into the reservoir of preformed entities, and consequently obscured any investigation of developmental processes. This
was the same objection which Morgan had
made to the paniculate theories of Darwin,
Weismann, Haeckel or Nageli: that by postulating the existence of an adult trait in
an hereditary particle, the developmental
issues were ignored.
As evidence of the prevalence of this tendency, Morgan pointed out that many
authors actually used the term "unit character" to refer to the Mendelian factor. By
so doing, they confused heredity potentiality with adult actuality. As Morgan
emphasized, the germ cells do not contain
miniature wings, eyes, or pigments, but only
the potential under appropriate circumstances, for producing these traits. Prior to
1919 Morgan himself consistently avoided
the term "unit character," substituting for
it the term "unit factor."
Given this general history of ambiguity,
836
GARLAND E. ALLEN
it is no wonder that Morgan and others
seized quickly upon Wilhelm Johannsen's
distinction between the genotype and phenotype, first put forward in his paper of
1911 (Johannsen, 1911). Johannsen separated out the several meanings of the term
heredity which he found in current use
among biologists (Johannsen, 1911, p. 130).
As Johannsen emphasized, there were two
components to the development of adult
traits in organisms: the genotype, that is,
the hereditary elements passed vertically
from parent to offspring, and the phenotype, the horizontal development of those
genotypic capabilities into adult realities.
Heredity, as a field of study, Johannsen
argued, should be solely concerned with
the vertical process—that is, the passing
on of the genotypic elements. The study
of the development of phenotype belonged
more properly to the realm of embryology.
The genotype, Johannsen emphasized,
was purposely an ahistorical concept,
whereas the phenotype, especially under
the influence of Haeckel's "biogenetic law,"
was a truly historical concept. The genotype conception to Johannsen was as ahistoric as the chemist's conception of atoms
combining to make molecules. H2O is
always water, regardless of where the
hydrogen and oxygen atoms have been
prior to their combination. The hydrogen
and oxygen atoms recovered from water
can be recombined into other elements and
behave exactly as every other hydrogen and
oxygen atom. The same was true, in
Johannsen's view, of Mendelian factors or
what he called "gens." The fundamental
basis of heredity, he stated, is hidden deep
within the gametes. Only through the analytical methods of Mendelian theory, coupled with experimentation and mathematical formulation, could the process of
heredity be understood. The term "phenotype" was coined by Johannsen as a
polemical word for the morphological,
descriptive view of heredity characteristic
of the old natural history tradition, including the phylogenetic, biometrical, and
especially embryological schools. Thus,
Johannsen's view rigorously separated the
field of Mendelian genetics from all earlier
studies of heredity, that is, those originating in evolutionary, biometrical, or embryological theory. In so doing, Johannsen
purposefully gave a new meaning to the
term "heredity".
The genotype conception became a
watershed for Morgan. It helped him redefine his view of heredity from a primarily
phenotypic (that is, embryological) to a primarily genotypic (that is, Mendelian) conception. It helped him move from a process-oriented epigenetic view of heredity,
to a morphological conception based on
the transmission of material particles from
parent to offspring through the germ cells.
Finally, it helped draw his attention away
from the cell cytoplasm, the realm of concern of most embryologists, to the cell
nucleus, the concern of students of chromosome and Mendelian theory.
Primed as he was from the point of view
of mechanistic philosophy, Morgan found
the genotype conception to be the perfect
analytical tool for focussing the many questions of heredity which were emerging from
his Drosophila work. We know that Morgan
was very impressed with Johannsen's ideas
which he must have encountered as early
as December or January 1910-1911.
Johannsen's paper was first presented orally
at the Christmas meeting of the American
Society of Naturalists in Ithaca, New York,
in December 1910. Morgan attended that
meeting and must have heard Johannsen's
talk. Also, Johannsen had been invited to
spend the winter term 1911 at Columbia
University, and it is highly unlikely that the
two men did not discuss their respective
views on heredity at great length. At any
rate is it reasonable to assume that by the
time Johannsen's paper was published in
the American Naturalist Morgan was already
familiar with the importance of this clearcut distinction.
But how did the genotype-phenotype
distinction actually influence the development of Morgan's work in heredity? What
is the relationship between the development of this concept, and the application
of mechanistic materialism to biological
problems, particularly the theory of the
gene, in Morgan's mind?
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
MORGAN AND THE THEORY OF THE
GENE, 1910-1926: THE IMPACT OF
MECHANISTIC MATERIALISM
To understand Morgan's changing view
of the gene we need to examine several
important issues in the history and sociology of science. One is the notion of what
a "research program" is and how it functions in guiding the development of an academic discipline. Thus, for example, Irmre
Lakatos has suggested that by developing
not only intellectual concepts, but also
methods of research, standard protocols,
and even philosophical positions, researchers establish a program which defines the
direction of future research (Lakatos,
1970). Another is what philosopher Lindley Darden calls interfield as compared to
intrafield theories: that is, the importance
of bringing together and showing the correspondence between events (or interpretations) in one field to similar or parallel
events in another (Darden and Maull,
1977). Third, is Jan Sapp's notion of
fields of research in struggle for authority,
meaning, among other things, for access
to research funds, to students, and to jobs
(Sapp, 1982). Building on the work of
French sociologist of science, Pierre Bourdieu, Sapp has argued that the struggle for
authority has done much to shape the kinds
of research questions and intellectual
frameworks established within various
fields (Bourdieu, 1975). I will try to use
these conceptions, at least sketchily, as a
way of understanding not only how the
study of heredity developed to its prominent place in biology between 1915 and
1940, but also how it became redefined as
a different set of problems and questions
from what it had been at the time Morgan
entered the field in the first decade of the
century.
I will argue that by redefining heredity
from an embryological to a genetic context, Morgan established a research program with distinct boundaries, a process
which helped attract students and gain
research funds for specific projects. At the
same time such a redefintion made heredity amenable to experimental and mecha-
837
nistic analysis to a degree that had not been
possible under the older, embryological
conception. That is, the redefinition of
heredity narrowed down the kinds of questions which Mendelian geneticists asked.
The redefinition also introduced a new terminology that set the new field of
"genetics" apart from its embryological
past. Most important, by treating Mendelian heredity in terms of material elements
within the cell—that is, the chromosomes—Morgan created a very powerful
interfield theory. Evidence from the cytological study of chromosome behavior in
meiosis strongly paralleled that obtained
from breeding data. The chromosome theory gave a material basis to the hypothetical, and thus speculative, Mendelian factors. The mechanistic philosophy so
prominent in biology at this time emphasized the necessity of a material basis for
all biological processes. The chromosomes
provided just the material base for which
the Mendelian "factors" asked.
It is important to keep in mind that prior
to 1910 Morgan's view of both the chromosome and the Mendelian theories was
highly skeptical. While the details of his
skepticism on both theories have been discussed elsewhere (Allen, 1966, 1978, pp.
125-144) I want to reemphasize that Morgan argued that both the Mendelian theory
and especially the chromosome theory were
preformationist—repositing in hypothetical or real particles power to control the
very process that it was the function of biology to study. In addition, to Morgan the
Mendelian theory required too many subhypotheses for comfort: for example, alternative factors "fleeing" from each other
during meiosis; or selective fertilization to
explain the inheritance of sex on a Mendelian basis. Morgan even argued that the
Mendelian theory focussed too much
attention on the cell nucleus, when the
cytoplasm was, to the embryologist, the seat
of all the really interesting events (Morgan,
1897, pp. 121, 135).
Once he began his experiments in earnest on Drosophila, Morgan rapidly accepted
both the Mendelian and chromosome theories (Allen, 1966). There seemed to be
838
GARLAND E. ALLEN
concrete evidence from both his own and
from E. B. Wilson's work, that chromosomes were determining factors in heredity. At the same time, Morgan saw the parallel and possible physical relationship
between Mendel's factors and the chromosomal structures. However, Morgan's
conversion was by no means rapid. For
example, in his first truly Mendelian paper,
"Sex Limited Inheritance in Drosophila"
(Morgan, 1910), Morgan treated the inheritance of eye color in terms of color "factors." Sex "limited" meant to Morgan
"associated with the X-chromosome," so
at that time it is clear that he saw a connection between the Mendelian factor for
eye color and the X-chromosome. However, in this paper Morgan avoids saying
openly that factors are parts of chromosomes—although, as Scott Gilbert has
pointed out, Morgan really leaves the
reader no alternative, since he states that
"the fact is that the R [the factor for eye
color] and X are combined and have never
existed apart" (Morgan, 1910, p. 122).
Nonetheless, Morgan is extremely cautious
in assigning the Mendelian factors any
material reality. The aversion to particles
was still too strong to allow Morgan to make
an immediate transition.
By the period 1914-1915, however,
Morgan was becoming more definite. He
still saw that Mendelian factors could be
regarded as a formality without any necessary reference to the chromosomes as the
bearers of hereditary particles (Morgan el
al., 1915, p. viii). In the preface to The
Mechanism of Mendelian Heredity Morgan,
Sturtevant, Muller, and Bridges state first
that the Mendelian theory stands on its own
and can be approached solely from the
point of view of breeding data. But then
they go on:
Why then, we are often asked, do you
drag in the chromosomes? Our answer
is that since the chromosomes furnish
exactly the kind of mechanism that the
Mendelian laws call for; and since there
is an ever-increasing body of information that points clearly to the chromosomes as the bearers of the Mendelian
factors, it would be folly to close one's
eyes to so patent a relation. (Morgan et
al., 1915, pp. viii-ix)
Even in 1915, the authors do not use the
term "gene" anywhere in The Mechanism
ofMendelian Heredity, although it was introduced by Johannsen as early as 1909 in his
Elemente der Exakten Erblichkeitslehre
(Johannsen, 1909, p. 143). I suggest that
by 1915 Morgan had still not clearly and
completely separated the genetic from the
embryological meanings of heredity, and
was still maintaining considerable caution
in defining the unit of heredity as a discrete
particle (thus stripping it of its embryological connotation). The old embryological
preformationism still had a dominant influence on Morgan's thinking. I should point
out that the experimental evidence available by 1915 only showed the possible parallel in activity between chromosome
behavior during meiosis and the supposed
segregation, crossing-over, and recombination of Mendelian factors. There was no
direct visible proof of actual correspondence between sections of chromosomes
and actual gene locations (that would only
come considerably later—in the early
1930s).
By 1919, however, in the publication of
The Physical Basis of Heredity, Morgan had
made more firm the material basis of the
gene, and thus his commitment to the Mendelian interpretation of "heredity." For
one thing, he defines the gene itself for the
first time:
The Drosophila evidence shows at least
several hundred independent elements,
and as new ones still appear as frequently
as at first, the indications are that there
are many more such elements than those
as yet identified. These elements we call
genes, and what I wish to insist on is that
their presence is directly deducible from
the genetic results, quite independently
of any further attributes or localizations
we may assign to them. It is this evidence
that justifies the theory of paniculate
inheritance. (Morgan, 1919, p. 237)
However, Morgan goes on to state explicitly the relation of the gene to the chromosomes:
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
[The analysis of crossing over] leads then
to the view that the gene is a certain
amount of material in the chromosome
that may separate from the chromosome
in which it lies, and be replaced by a
corresponding part (and by none other)
of the homologous chromosome. It is of
fundamental significance in this connection to recognize that the genes of the
pair that interchange do not jump out
of one chromosome into the other . . .
but are changed by the thread breaking
as a piece in front of or else behind them,
but not in both places at once. . . . (Morgan, 1919, p. 237)
Morgan realized that while genes interact
to produce adult characters, they are, and
can be regarded as, independent units so
far as the hereditary transmission process
is concerned. In The Physical Basis of Hered-
ity, Morgan wrote:
839
istic way. The gene as a mechanistic element had come into its own.
At the same time that Morgan defined
genes atomistically, he also began to
sharpen the distinction between nucleus
and cytoplasm on the one hand, and
between embryology and genetics on the
other. In a letter to his friend Jacques Loeb
in 1919, Morgan wrote that he was particularly anxious to counteract the notion that
the cytoplasm transmits any fundamental
characteristics to the next generation:
It is this point [that the cytoplasm transmits fundamental properties] that I am
anxious to go for, because of its widespread belief among biologists in general
for which I can find absolutely no real
basis except an emotional one. It is for
this reason mainly that I have not hesitated to hold up as examples two of my
best friends and a very famous German
investigator.
The essential point here is that even
although each of the organs of the body Then Morgan adds with his usual flair for
may be largely a product of the entire humor:
germ-plasm, yet this germ-plasm is made
I hope the former will forgive me and I
up of units that are independent of each
shall take my chances with the latter if
other in at least two respects, viz, in that
we meet, as seems unlikely, in another
each one may change (mutate) without '
world. (Morgan to Loeb, May 14, 1919;
the other's changing, and in segregation
and in crossing-over each pair is sepaLoeb Papers, Library of Congress, Box
rable from the others. (Morgan, 1919,
9)
pp. 240-241)
Morgan made the same point in his Nobel
Morgan even speaks of the genes as molec- Prize speech, delivered in Stockholm in
ular entities, acting perhaps in an enzyme- June, 1934 and published the next year
like manner. The most reasonable position (Morgan, 1935). The man who had once
to take at the time regarding the molecular stated that the cytoplasm was the seat of
structure and function of genes, Morgan the really interesting activities relating to
wrote, was that of Loeb and Chamberlain: heredity (Morgan, 1910, p. 453) now
argued that nothing of fundamental
The hereditary factor in this case must importance for heredity was carried out
consist of material which determines the within the cytoplasm, and that it was the
formation of a given mass of these nucleus on which the geneticists' attention
enzymes, since the factors in the chro- should be focussed. The nucleus and cytomosomes are too small the carry the plasm, once considered so vitally linked in
whole mass of the enzymes existing in Morgan's mind, had become separate and
the embryo or adult. (Morgan, 1919, p. distinct by 1919.
245)
The distinctions between the realms of
Thus, by 1919, Morgan not only used the embryology and of genetics as fields of
term "gene" directly, but also appears to investigation were made even more sharply
have thought of genes as specific, discrete, by Morgan in the early and mid-1920s. In
and independent units acting in an atom- The Theory of the Gene (1926), Morgan put
840
GARLAND E. ALLEN
it explicitly: embryology was the study of
the development of genetic potentialities
into adult realities; genetics was the study
of transmission of hereditary elements from
parent to offspring.
Between the characters, that furnish the
data for the theory [that is, Mendelian
theory] and the postulated genes, to
which the characters are referred, lies
the whole field of embryonic development. The theory of the gene, as here
formulated, states nothing with respect
to the way in which the genes are connected with the end-product or character. The absence of information relating
to this interval does not mean that the
process of embryonic development is not
of interest for genetics . . . but the fact
remains that the sorting out of the characters in successive generations can be
explained at present without reference
to the way in which the gene affects the
developmental process. (Morgan, 1926,
p. 26)
Thus, by the mid-1920s, the theory of the
gene was established on several grounds:
(1) As a materialistic view of heredity, in
which hereditary potentialities are
regarded as discrete, material units, the
genes;
(2) As a mechanistically established concept, in which the genes are regarded
as independent units shuffled and
reshuffled in successive generations; the
fundamental characters of these units
are unaffected by their recombination
with other units;
(3) As an experimentally established science based on the correspondence of
two distinct lines of investigation:
experimental breeding and the cytological observation of chromosome
structure;
(4) As the foundation for a new field of
research, called genetics, with its own
problems and methods; genetics is
related to, but set apart from both
embryology and evolution.
Between 1910 and the mid-1920s, therefore, Morgan the embryologist had become
Morgan the geneticist. He had shifted his
concern from the problem of "translation," to that of "transmission." His strong
interest in the cell (egg) cytoplasm had given
way to a predominant interest in the
nucleus, and particularly the chromosomes. His earlier interest in heredity as a
process of realizing hereditary potentialities in embryonic development had given
way to a focus on the material structures
and units of the germ plasm. And last, and
perhaps most ironically, Morgan's strong
interest in epigenesis had given way to what
he himself had once called the preformationism associated with paniculate theories
of heredity.
ANALYSIS AND CONCLUSION
How, then, are we to analyze and understand Morgan's change of view with regard
to the nature of heredity? Was it merely
the pragmatic realization of a good experimentalist confronted with new facts and
data? Or was there a deeper aspect involved,
a change of philosophical position?
Obviously, it was both. Morgan was eminently pragmatic—unusual in the degree
to which he could change his mind, if need
be, when confronted by the data. At the
same time, Morgan's change also resulted
from his adherence to a strong mechanistic
materialist view of the organism and a
mechanistic method of analysis, applied
especially after 1911 through the genotype-phenotype distinction of Wilhelm
Johannsen. Let me explain how I think
these two components interacted to help
Morgan redefine the nature of heredity.
Because of his mechanistic materialist
bias, Morgan was not content with merely
a numerically valid, even highly predictable, theory of heredity based on the inheritance of abstract entities. He sought a
material basis for the genetic process; and
that material basis turned out to reside in
the chromosomes, and further in the discrete atomistic units, or genes, of which
the chromosomes were composed. Adhering to mechanistic materialism also meant
adhering to the analytical method: the process of breaking complex processes down
inlo their separate components. This
included everything from analyzing the
chromosome into its linear array of genes,
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
the cell into its major components, the
nucleus and cytoplasm, and the organism
into its historical (phenotypic) and ahistorical (genotypic) dimensions. In the social
realm of scientific practice, it also meant
breaking the study of heredity, as viewed
in the late 19th and early 20th centuries,
into its two component concerns: genetics
(the study of genotypes) on the one hand,
and embryology (the study of phenotype,
or how phenotype develops), on the other.
Without the ability to respect data, and
a strong commitment to the "mechanistic
way of thought," however, Morgan would
perhaps never have been able to make such
a shift in position as that represented
between his view of heredity in 1909 and
that in 1926. He might otherwise have
remained always critical and skeptical of
the chromosome theory, like a Bateson or
a Goldschmidt, both of whom retained a
more abstract—some have said even
"idealistic" (Coleman, 1970; Allen, 1974)
—and formalistic approaches to heredity.
In other words, neither Bateson nor Goldschmidt ever completely abandoned the
older, embryologically conditioned views
of heredity and development as inseparable. Now, an element in Morgan's transition was his ability to apply Johannsen's
genotype-phenotype distinction in a mechanistic/analytical way to resolve the complex adult organism into separate components. Morgan was thus able to push aside
a whole set of problems (the causes of differentiation) which in the early decades of
the century were proving refractory to
experimental and analytical methods.
Despite his strong mechanistic bias, however, Morgan himself was more holistic
than many of his followers in genetics. For
one thing, he was always keenly aware of
gene interactions, even though he often
applied the concept only in the most mechanistic, rudimentary way. Nonetheless, his
constant reference to epistasis or/and
pleiotropy was more than mere lip-service.
To Morgan those interactions were fundamental, even though he could not get a
handle on understanding them experimentally. Furthermore, in his 1919 book, The
Physical Basis of Heredity, Morgan devoted
a number of pages to the concept of what
841
he called the "organism as a whole" (Morgan, 1919, pp. 24Iff.). Morgan addressed
this issue because many non-mechanistic
biologists at the time were embracing a kind
of mystical "holism" which postulated the
existence of a vital force (elan vital), or a
guiding and directive power, the entelechy
(Morgan's friend Driesch was a chief proponent of this idea), in order to explain the
highly complex processes associated with
embryonic differentiation. Morgan argued
that the organism is a whole, that is, is made
up of many parts which do interact. However, he said that the only way to understand the whole was first to understand its
individual parts. That was where mechanistic materialism came in. Morgan was
too good a biologist to think of the organism as only a mosaic of independent traits.
Yet, in his analysis of the hereditary process from 1910 or 1911 onward, Morgan
increasingly focussed on experimental
methods and data which lent themselves to
an increasingly mosaic view of the heredity
process. And although Morgan never really
gave up on the holistic side of biology, and
especially on embryology (for example see
Jane Oppenheimer's essay in this same
issue) the school of which he became the
titular head (the classical school of genetics,
that is, the Mendelian-chromosome theory) carried out the fractionation of the
whole into its component parts to a striking
degree. The field of genetics came to regard
heredity as the study of what genes do—as
if in a sense genes act independently of
their environments or of each other. Our
genetic vocabulary reflects this partitioning—we still talk about this or that gene
for wing shape, or diabetes, etc. We have
yet to give the gene its proper environment
back—that is, to restore its interactive,
embryological dimension, a dimension it
held prior to the rise of the classical school
of genetics, and of Johannsen's genotypephenotype concept in particular.
What, then, can we learn from this case
study of T. H. Morgan? We can, of course,
learn many things about Morgan himself:
his strong commitment to mechanistic
materialism as a philosophy of science, his
belief in the experimental method and in
the accumulation of quantitative data, his
842
GARLAND E. ALLEN
dislike of mystical explanations, his hardheadedness about empirical results, and his
willingness to change his own mind when
confronted with new evidence or new viewpoints. Beyond throwing further light on
Morgan himself, however, this study
emphasizes some more general principles
of scientific work, and the nature of science
itself. First of all it is important to recognize that everyone has a philosophical position, whether explicit or not, and that this
position affects the kinds of questions a
person asks and the kinds of answers he
accepts. It would thus seem reasonable for
all of us to examine continuously our philosophical assumptions, to decide if we want
to change them or not. There is nothing
more blinding, however, than an unexamined, but yet omni-present, philosophical bias.
Second, I think we can see both some of
the advantages and some of the disadvantages of the mechanistic materialist
approach as applied by Morgan and his
contemporaries in the early to mid parts
of the century. At the time, it helped
researchers focus their attention on which
questions to investigate. It emphasized
seeking material corroboration of biological components, rather than relying upon
the speculative units or abstract particle
theories which abounded in the late 19th
and early 20th centuries. It also emphasized the importance of experimental work,
and of the collection of quantitatative data
in doing scientific work. More important,
the mechanistic materialist method represented an important step in the transition of biology from a largely descriptive
science to one that was more rigorous, analytical and experimental. At the same time
the mechanistic materialist philosophy left
a legacy that hindered the development of
many aspects of biological theory in the
second half of the 20th century. By its analytical approach it deemphasized many special kinds of biological interactions:
between organisms and their environment,
between components of individual organisms, for example, between nucleus and the
cytoplasm of cells. Much of the study of
cytoplasmic inheritance in the 1940s and
1950s was retarded, even stifled, by the
predominant mechanistic view which held
that only the cell nucleus was of any importance in the study of heredity. Most important, the mechanistic legacy separated the
problems of heredity and development into
fields, producing a breach which has not
healed to this day. It even affected the
development of evolutionary theory, producing what Ernst Mayr calls the "beanbag
genetics" approach of the 1930s and 1940s.
It allowed ahistorical evolutionists such as
R. A. Fisher to eliminate the phenotype
entirely from the study of evolution, portraying not populations of organisms, but
pools of genes endlessly recombining in
successive generations, with only their frequencies changing (Allen, 1983).
In conclusion, I would like to suggest
that we do not have to live with the analytical legacy of mechanistic materialism
unaltered into the 1980s. It is time to recognize the limitations of this method, even
as much as we recognize the important
advances to which it gave rise in the past.
Not resorting to mysticism or vague speculations, we can still search for the proper
experimental methods for investigating
interactive processes, for understanding
that the whole is more than the sum of its
parts, and for understanding some of the
delusive questions—for example, causes
and control of embryonic differentiation—that have tantalized biologists for
hundreds if not thousands of years. New
methods and technologies, such as cybernetics or computer simulations of complex
systems, are steps in that direction. We have
only to recognize the ways in which our
unexamined philosophical biases may hinder our full utilization of new concepts and
new methodologies in the future.
ACKNOWLEDGMENTS
I have gained many ideas over the past
several years from papers by, and conversations with Jan Sapp, now of the University of Arizona and from Lindley Darden
of the University of Maryland. I have also
learned much by discussions of the general
subject of materialism in biology with Ernst
Mavr, Dick Lewontin, Frank Sullowav,
T. H. MORGAN, MECHANISTIC MATERIALISM AND THE GENE CONCEPT
Steve Gould and John Beatty. I am grateful
for the ideas and enthusiasm which they
have all so willingly shared with me. Part
of this paper was written under the auspices of NSF grant #SES-8026095.
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