''It Ain't Over 'til it's Over'': Rethinking the Darwinian Revolution

17 Pages·2005·0.22 MB·English

Journalof the History ofBiology (2005)38: 33–49 (cid:1)Springer2005
DOI 10.1007/s10739-004-6508-z
‘‘It Ain’t Over ‘til it’s Over’’: Rethinking the Darwinian Revolution
VASSILIKI BETTY SMOCOVITIS
Departmentof Philosophy andHistoryof Science
University of Athens, Athens
Greece
and
Departmentsof ZoologyandHistory
University of Florida
Gainesville, FL 32611
USA
E-mail: [email protected]fl.edu
Abstract. Thispaperattemptsacriticalexaminationofscholarlyunderstandingofthe
historicaleventreferredtoas‘‘theDarwinianRevolution.’’Inparticular,itconcentrates
onsomeofthemajorscholarlyworksthathaveappearedsincethepublicationin1979
ofMichaelRuse’sTheDarwinianRevolution:NatureRedinToothandClaw.Thepaper
closesbyarguingthatfruitfulcritical perspectivesonwhatcountsasthiseventcanbe
gainedbylocatingitinarangeofhistoriographicanddisciplinarycontextsthatinclude
the emergence of the discipline of evolutionary biology (following the ‘‘evolutionary
synthesis’’), the 1959 Darwin centenary, and the maturation of the discipline of the
history of science. Broader perspectives on something called the ‘‘Darwinian Revolu-
tion’’ are called for that include recognizing that it does not map a one-to-one corre-
spondence withthe historyof evolution, broadlyconstrued.
Keywords: Darwin centennial, Darwinian Revolution, discipline, evolutionary biology
evolutionarysynthesis, historiography
The Darwinian Revolution was probably the most significant revolution
that has ever occurred in the sciences, because its effects and influences
were significant in many different areas of thought and belief. The con-
sequenceofthisrevolutionwasasystematicrethinkingofthenatureofthe
world, of man, and of human institutions.1
It would seem to an outsider that the ‘‘evolutionary synthesis’’ that has
characterized recent evolutionary biology – a result of joint activity by
geneticists and naturalists – may well constitute a second Darwinian
revolution or a second stage of the Darwinian Revolution, or perhaps a
transformed Darwinian Revolution. But it should not be thought that the
revolution is over.2
1Cohen,1985, p. 299.
2Ibid, p. 297.
34 VASSILIKIBETTYSMOCOVITIS
Darwin as ‘‘Hollywood Epic’’: The ‘‘Darwinian Revolution’’ in the 1980s
By the time I actually encountered it formally sometime in mid-1980s
while still a graduatestudent, the ‘‘DarwinianRevolution’’ was not just
an established event, it had become, I would argue, a defining event for
historians of biology. A staggering number of books, articles, confer-
encesandexhibits,andevendocumentarydramasandfeaturefilmshad
been devoted to it directly, or to some related aspect exploring the
revolutionary Darwin and his world.
Revolutioninscienceas awholewasabigtopicfordiscussioninthe
mid-1980s, especially following the appearance of I. Bernard Cohen’s
popular Revolutions in Science in 1985.3 The ‘‘Darwinian Revolution’’
gotacompletechapterinabookotherwisedevotedtorevolutionsinthe
physical sciences. Considering the fact that the physical sciences had
until then dominated much of the history and philosophy of science,
and considering Cohen’s own predilections in that direction, the inclu-
sion of this chapter and effusive language describing it, indicated that
historians of science outside the smaller circles of the history of biology
were recognizing the event as being of prime importance. Darwin, the
token representative of the life sciences, finally seemed to join the
pantheon of those famous ‘‘dead white males’’ in the history of the
physical sciences like Copernicus, Galileo, and Newton, of course. It
also seemed to silence a range of queries that had begun early in the
1970s about whether or not the ‘‘Darwinian Revolution’’ constituted a
‘‘Kuhnian revolution’’ in science.4
A self-designated ‘‘outsider’’ (see the epigraph above), Cohen’s his-
toric appraisal was not exactly news to any of the ‘‘insiders’’ in the
history of biology. Almost from its start, the Journal of the History of
BiologyhadfeaturedDarwinorsomeaspectofthehistoryofevolution.
ThefoundingofthejournalhadbeenfacilitatedbyErnstMayr,keento
redress the imbalance generated by the new historiography of biology
stressinggenetics; asaresult ofhisinfluenceonthejournaland hisown
forays into both the history and philosophy of biology beginning in the
early 1960s, Darwin, evolution, and the history of natural history were
featured prominently.5 By the mid-1980s, the number of secondary
sources available to anyone interested in Darwin or the ‘‘Darwinian
Revolution’’ bordered on overwhelming. The occasion of the 100th
anniversary of Darwin’s death in 1982 served to generate a range of
3SeeCohen, 1985.
4 See Mayr, 1971, Greene, 1971, Ghiselin, 1971; and see Ruse 1970, 1971 for just a
quicksurvey ofsome ofthis literature.
5SeeSmocovitis, 1996for discussion ofthis.
RETHINKINGTHEDARWINIANREVOLUTION 35
commemorativeevents,lecturesandexhibits,whichculminatedwiththe
appearance of an enormous book titled The Darwinian Heritage in
1985.6 It was therefore no real surprise that a notorious review of the
volume in Quarterly Review of Biology charged that the plethora of
good works available on Darwin resembled a virtual ‘‘Holly-
woodEpic.’’7Thefollowingyear,acelebratedessayreviewappearingin
the Journal of the History of Biology, gave name to the glut of workers
devoting themselves to Darwin and his world as ‘‘the Darwin Indus-
try.’’8 Finally, in 1988, a new epoch in the history of the ‘‘Darwinian
Revolution’’ dawned when Darwin was relegated to an afterword in
what was a revolutionary work assessing the origins of evolutionary
thought in a 19th century radical political context. Adrian Desmond’s
proletarian-driven The Politics of Evolution was intended to shock
expectant readers by leaving the mighty figure of Darwin and his
Darwinian evolution until only the afterword of a very long book.9
Instead of diminishing the stature of Darwin, however, this organiza-
tional tactic only served to enhance the dramatic conclusion of an
otherwise conventional social history; the book only reified Darwin’s
towering stature in the history of 19th century evolutionary radicalism.
Like other impressionable graduate students, I was swept away by
the revolutionary fervor. I eagerly anticipated the new books and arti-
cles, sought out the critical reviews and especially enjoyed following the
public debates – and antics – of the Darwin Industry. In 1987 when the
JohnS. KnightWriting Programat Cornell asked me to designmy first
course in their ‘‘writing-across-disciplines’’ series to help initiate stu-
dentsinto the ‘‘disciplinary discourse’’ of myfield, Ididn’t evenneedto
think twice about it; my new course would be on ‘‘The Darwinian
Revolution.’’ It seemed timely, of general interest to young people, and
the perfect book existed to guide the course, Michael Ruse’s Darwinian
Revolution. Nature Red in Tooth and Claw.10 Appearing in 1979, Ruse’s
book had quickly become a classic on the subject and was available in
paperbackedition.Itwasusedwidelyinteachingcoursesjustlikemine.
Although there were other books on the topic,11 Ruse’s Darwinian
Revolution,came to besynonymouswith the historicalevent designated
as the ‘‘Darwinian Revolution.’’ This became so much the case, that
in my mind, any mention of the phrase ‘‘Darwinian Revolution’’
6Kohn, 1985.
7Ruse, 1986.
8Lenoir,1987.
9Desmond, 1989.
10Ruse, 1979; revised edition 1999.
11See forexample Oldroyd, 1980.
36 VASSILIKIBETTYSMOCOVITIS
immediately conjures up the visual image of an orange-colored skeletal
hand against a bright blue background (the cover of the book) and the
dramatic phrasing of the subtitle appropriated from Alfred Lord Ten-
nyson, ‘‘nature red in tooth and claw.’’
Michael Ruse’s The Darwinian Revolution: Nature Red in Tooth and
Claw
Michael Ruse’s book, which served as a popular textbook to many
courses taught by historians of biology thus serves as a good starting
point for historical understanding of something called the ‘‘Darwinian
Revolution.’’ It is vitally important to those of us who trained as his-
torians of biology after 1979. Ruse’s book concentrates on a now fairly
familiar story. It locates the origins of Darwinian evolution in the
context of Enlightenment views that included belief in progress, in
theological movements like natural theology, in the shifting views and
practices of traditional natural history, anatomy, and morphology, and
inemergingrelatedscienceslikegeology.Thestorygivespreeminenceto
theDarwinfigureandsomedetailsofhislifeareincluded,butthebook
captures more fully the ‘‘spirit of the times,’’ the Zeitgeist, or the
intellectualmilieuasembodiedbyagroupofplayerswhoseinteractions
gave rise to Darwinian evolution and which in turn was shaped by
something called Darwinism.
The book placed the revolution in Britain; it was unabashedly
Anglocentric, for the simple reason that Ruse argued that the British
natural theology tradition, which Darwin had imbibed during his
undergraduate Cambridge days had inspired his interest in adaptation.
It argued for the importance of Lyell’s uniformitarianism in shaping
Darwin’s natural history, and argued that Darwinian natural selection
with its non-purposiveness and its challenge to the argument for God’s
design, effectively killed Aristotelian teleology, though that did not
necessarilystopbeliefinahigherpurposeorinprogress(twoadditional
pointsRuse carefully pointed out).The relationship between something
called‘‘science,’’apracticeRusewasfollowingtheemergenceof,andits
interaction with prevailing religious views was one of the major themes
of the book as a whole.
Ruse’s book also did much to place Darwin in his philosophical
milieu, for example by considering the influence and effects of William
Whewell and J. F. W. Herschel on Darwin’s philosophy of science. It
also explored Darwin’s methodology, and his reasoning as a scientist.
It assessed the theory in terms of vera causa and whether or not it
RETHINKINGTHEDARWINIANREVOLUTION 37
demonstrated a ‘‘consilience of inductions,’’ which I took to mean that
Darwin drew his conclusions from a range of independent lines of
evidence. One reason Darwin’s Origin had to be taken seriously as a
work of science, while Chambers’s Vestiges could not, was due to the
fact that Darwin structured his argument according to contemporary
vera causa principles.
Althoughthebookstressedtheintellectualbackdropwhichgaverise
to Darwin’s theory, it also considered its social origins. The fact that
Darwinian evolution emerged within a community negotiating laissez-
faire economics that was witnessing the emergence of a modern indus-
trial capitalist state was included, as were nitty-gritty historical details
crucial to understanding British society like the enactment of the Corn
Laws in 1815 and the passing of the First Reform Bill in 1832. Malthus
was featured prominently as was Spencer and Tennyson; the very
subtitle ‘‘nature red in tooth and claw’’ directed readers to the fact that
a competitive view of nature and society undergirded something called
the ‘‘Darwinian Revolution.’’
In considering the social origins, Ruse’s book did one more thing: it
included the fact that the community of scientists – as something called
scientists – was in the process of emerging just at the same time. Thus,
muchaboutstatus,professionalconnections,employmentconditions,as
well as research program and what counted as legitimate methodology,
whatRusereferredtoasthe‘‘professionalization’’ofscience,werebeing
debated at the same time as Darwin’s actual argument. The book’s
approach thus included insights gleaned from the sociology of British
scienceandnotjustthesocialhistoryofmid-nineteenthcenturyBritain.
Acompactbook(some273pagesoftext),itwasclearlywritten,well
organized, and was generally a ‘‘no fuss’’ account of what had by 1979
becomeafairlycomplextopic.Re-readingitforthisretrospective,Iwas
struck by the breadth of literature that it synthesized and by a meth-
odology that was modest and ecumenical; it managed to combine
approaches from intellectual history, the philosophy of science, social
history and even the sociology of scientific knowledge in an unself-
conscious manner. Very much so, Ruse’s The Darwinian Revolution
included most everything that had accumulated on the nineteenth cen-
turyBritishevolutionaryscenebythelate1970s;itwasahighlyeffective
summaryofthe‘‘stateoftheart’’inDarwinandVictorianstudiesandit
achieved this in a manner that made it accessible to beginners.12 That is
12SeeGhiselin,1969(reprint1984)foranexampleofanunusuallyinfluentialbook
that explored Darwin’s reasoning and the ‘‘method in his madness’’; Ruse engaged
Ghiselin’s insightsdirectly in his book.
38 VASSILIKIBETTYSMOCOVITIS
why it became a popular textbook for instruction to something called
the ‘‘Darwinian Revolution.’’
Much has happened to our understanding of the ‘‘Darwinian Rev-
olution’’ since 1979, however. Only a quick survey of literature bearing
on the subject reveals a staggering range of approaches, some of which
enhances the perspective developed by Ruse and his predecessors, some
of which alters it, while some attempts to directly challenge the
‘‘eventfulness’’ or the very notion of a ‘‘Darwinian Revolution’’ as
whole. How do we make sense of this literature given the broad and
what appears at times to be conflicting approaches, methodologies and
interpretationsoffered?Whatkeypointscanweextractfromthisrecent
work? In short, how exactly, and to what extent, has the ‘‘Darwinian
Revolution’’actuallyalteredinourunderstanding?Andshouldweeven
believe in the notion of a ‘‘Darwinian Revolution’’ at all?
The ‘‘Darwinian Revolution’’: Here and Now
Assuming we choose to uphold the notion of the ‘‘Darwinian Revolu-
tion’’ (I have more to say about this below), the general historian can
extract some key points that have emerged from some recent accounts.
Many recent biographies, for example, have been especially useful to
rendering a more richly contextualized picture of key players like
Darwin himself,13 some of his adversaries like Richard Owen, some of
his advocates like Thomas Henry Huxley, as well as of course his
codiscoverer, Alfred Russel Wallace.14 What these biographies have
contributed is not just detailed knowledge of the lives of these key
individuals, but an enhanced appreciation of the sociopolitical and
professional context in which British evolutionism developed. It is
frankly astonishing to contrast the life of privilege led by more estab-
lished naturalists like Darwin to that of Huxley and Wallace, both of
whom faced financial exigencies that shaped the kinds of publications
and scientific programs that each selected.
Beginning with Desmond and Moore’s Darwin in 1991 and culmi-
nating with the splendid two-volume biography of Charles Darwin by
Janet Browne, furthermore, our sense of what counts as ‘‘context’’ has
appreciably widened. It isn’t just that we can now see Darwin as the
‘‘squarson-naturalist,’’ or as a rather upright or proper Victorian
gentleman, but as a spider-like being functioning as the center of an
13Desmond and Moore, 1991;Browne, 1995,2002.
14Rupke, 1994; Desmond, 1994;1997; Raby2001, Shermer,2002; Fichman, 2004.
RETHINKINGTHEDARWINIANREVOLUTION 39
elaborate network of naturalist-practitioners that encompassed the vast
reaches of the British empire. Browne especially makes us appreciate
that his theory was thus not just located in the laissez-faire economic
politics of his day, or even in the radical politics of his day, but in the
wider imperial ambitions of modern Britain. The extent to which the
voyage of the Beagle and other such expeditions that took Darwin’s
contemporaries like Joseph Hooker and Thomas Henry Huxley to the
far-flungreachesoftheBritishEmpireisnowpartofanycriticalhistory
of the ‘‘Darwinian Revolution,’’ as are vital questions about the prac-
tices of collecting and transporting specimens as vital objects of empire.
How institutions like gardens, zoos, and private collections selected
these specimens, and whether or not they were displayed, and in what
manner they were displayed, are all part of understanding the 19th
century context which fostered evolution, as is the interplay of local or
‘‘native’’ knowledge with ‘‘elite’’ scientific knowledge. Here too, the
recent emphasis on Wallace, who as commercial collector followed
establishedroutesalongthe‘‘plumetrade’’insearchofvaluablenatural
historyobjectshasgivenusanenhancedappreciationofthepracticesof
natural history that fed into the development of evolutionary theory
generally, and biogeographyin particular. Biographies – by definition –
are organized around a figure, but taken as whole, what these sophis-
ticated recent biographies of figures long part of the ‘‘Darwinian Rev-
olution’’ have done is to refashion these individuals as actors in vast
social or cultural networks.
Even more traditional areas of interest, like Darwin’s scientific
research efforts, have been explored in greater detail in some of these
biographies.Browne’spathbreakingVolumeII,ThePowerofPlace,for
example, goes far in examining Darwin’s research efforts after the
publication of Origin in 1859. What emerges is not only an enhanced
appreciation of the diversity of his interests in general, but how exten-
sive his botanical researches were in particular. That Darwin relied on
botanical knowledge and on examples from the plant world heavily has
beengenerallyrecognized,buttheextenttowhichheactivelyengagedin
variedbotanicalresearchesandtheextenttowhichhemaybeviewedas
a botanist has not been explored sufficiently. How botany – and bota-
nists – shaped Darwinian theory and what role they played in the his-
tory of evolution is yet another promising areaof inquiry opened up by
Browne’s treatment of Darwin.
Yet another aspectof the Darwin figure thatemerges fromBrowne’s
Volume II, is the fact that Darwin earned for himself celebrity status.
He was one of the first scientists in the history of science to have his
40 VASSILIKIBETTYSMOCOVITIS
portraits and photographs heavily recognized by lay audiences, and did
not shy away from lavish attention, signing autographs, and posing for
a number of photographers. Why is this important to the ‘‘Darwinian
Revolution?’’ItallowsustounderstandthatDarwin’sstatus,especially
following the publication of Origin, continued to gain with time, thus
promoting the credibility and popularity of his ‘‘theory’’ (more about
that later) and subsequent work. It also shifts attention to the public
face of Darwin, and Darwinian theory, and to the fact that the new
profession of ‘‘science’’ served legitimating functions (and was in term
legitimated by) popular audiences.
A series of related aspects which accompanies any consideration of
Darwin’s Origin is a closer consideration of authorship, readership,
publication history and popular culture as a whole. These are consider-
ations that I think are now critical for any modern treatment of the
‘‘DarwinianRevolution’’andcomemostlyoutofthescholarshipofsocial
historianslikeJamesSecord,andhisVictorianSensation.15Accordingto
Secord 19th century evolution (more correctly here transmutationism)
was embedded within sets of social, literary, and material practices that
resultedintextslikeRobertChambers’sVestigesoftheNaturalHistoryof
Creation. As text, it had a life of its own, independent of any authorial
intent, or even – radically enough to state – the ‘‘content’’ of the work.
What Secord has in mind here is no mere conventional literary analysis
that includes close readings (as generally performed by intellectual his-
torians)orrhetoricalornarrativeanalysis(usuallythedomainofliterary
historians),allofwhichhavebeenputtofruitfuluseinthescholarshipof
the‘‘DarwinianRevolution.’’16Heinsteadhasinmindan‘‘experimentin
a different kind of history,’’17 one which ultimately challenges the con-
ventional understanding of the ‘‘Darwinian Revolution’’ which empha-
sized the origin of high theory – and great theorists – but which largely
ignored the practical world that was embodied in the production, dis-
semination, and readership of popular 19th century texts. According to
Secordsuchanapproach‘‘…hasmadeitpossibletoescapetheoldimage
ofscienceasdominatedbyahandfulofgreattheoristsandsimultaneously
to understand theory making as a form of practice. Intellectual history,
whichusedtobewrittenasastoryofdramaticchangesinworldview(the
‘Darwinian Revolution’), can be recast by looking at the basic material
products of cultural life and drawing upon techniques developed for
studyingordinaryaction.’’18
15Secord, 2000.
16See Beer,1983, 1986; Levine,1988.
17Secord 2000, p. 518.
18Ibid, p. 520.
RETHINKINGTHEDARWINIANREVOLUTION 41
Secord is at pains to demonstrate much of this operating, by closely
followingtheproduction,dissemination,andthereadingofChambers’s
Vestiges. His book is an impressive work of scholarship that alters the
terrain of the ‘‘Darwinian Revolution,’’ considerably. It succeeds in
openingawindowintothereadingworldofthe19thcenturyandallows
us to gain deep appreciation of the publication, and more generally the
production, of scientific texts. It allows historians to appreciate how
diverse and vast the readership for popular science was, as well as to
appreciate the fact that there was a popular demand for cosmic trans-
mutationist theories well before Darwin’s Origin. Chambers’s Vestiges
was indeed an important ‘‘scientific’’ text in its time but that was also
because what counted as science was actively being negotiated; ulti-
mately,itsimpactinthehistoryofevolutionmayverywellhavebeento
serve as the ‘‘testing ground’’ for more conservative texts that would
come to represent modern science more closely like Darwin’s Origin.
There is therefore much to Secord’s conclusion that ‘‘[t]he Origin was
important in resolving a crisis, not in creating one.’’19
All of this is interesting and important to our understanding of the
19th century Victorian scientific context broadly construed. Certainly,
anyone interested in the history of evolution after ‘‘Secord, 2000,’’ will
take the 1844 publication of Vestiges very seriously, and of course
anyone assessing the impact of Darwin’s Origin will appreciate further
thefactthathiscelebratedtextwastheembodimentofasetofpractical
concerns. I am not so sure that Secord’s larger ambition to ‘‘decon-
struct’’ or perhaps to ‘‘decenter’’ something called the ‘‘Darwinian
Revolution’’ is as effective, however. It isn’t so much the theoretical
scaffolding that he has put to use (in my mind a very productive use),
that is so much the problem, but that his understanding of the ‘‘Dar-
winian Revolution’’ is, to put it mildly, a crude and dated caricature of
thecelebratedevent.Leavingittothefinalchapter,Secordbuildstothe
triumphalconclusionwhereherevealstheintentofhisanalysisandthat
ishisbeliefthathisapproachwill extinguishthe‘‘great manof history’’
approach that stresses ‘‘great works’’ and ‘‘great theories’’ which cul-
minate with ‘‘great events’’ like the ‘‘Darwinian Revolution.’’ The fact
ofthematteris,thatfewreputablehistoriansofsciencehaveupheldany
of those notions for a couple of generations, at least. As of the mid-
1980s, it was historians well ‘‘outside’’ the area like I. Bernard Cohen
who spoke of the ‘‘Darwinian Revolution’’ in such celebratory – and
na€(cid:1)ve – historical terms. Even the most hard-line of the ‘‘Darwin
Industry’’ in the 1980s (who I take to be some of the folks contributing
19Ibid, p. 514.
42 VASSILIKIBETTYSMOCOVITIS
to volumes like Kohn’s Darwinian Heritage), did not hold sucha heroic
view of Darwin or a simplistic view of the ‘‘Darwinian Revolution.’’
Thus, when Secord has in mind ‘‘an old image’’ of science (see the
quotation),I takeitto mean a very oldimage, and one in mymind that
ceasedtoexistfornearlyallhistoriansofevolutionby1979.Eveninhis
treatment of Chambers, Secord underestimates the importance given to
himbyhistoriansbefore2000.Althoughafullandnuancedpicturewas
lacking,Chambers–andhistext–wereactuallytakenveryseriouslyby
historians of evolution; he is after all a major player in Ruse’s 1979
textbook.
Secord’sattemptto‘‘decenter’’the‘‘DarwinianRevolution,’’andhis
organizational, (or perhaps rhetorical) tactic of leaving Darwin and the
‘‘Darwinian Revolution’’ until the final chapter was not the first such
effort. As noted, it followed the precedent established earlier by Adrian
Desmond in The Politics of Evolution who argued that transmutationist
theories were part of the radical politics that swept through Britain in
the 1930s. It is used again by another book – yet again published with
the same press (the University of Chicago) – that seeks to challenge the
‘‘DarwinianRevolution’’butwithadifferentprojectinmind.Itsgoalis
not so much to ‘‘decenter’’ the ‘‘Darwinian Revolution’’, but to geo-
graphically and therefore culturally relocate the origins of the theory
associated with it to a continental and Germanic philosophical tradi-
tion. Titled The Romantic Conception of Life, the book aims ultimately
torelocatenotjustevolutionarythought,buttheoriginsofthewholeof
biology to an earlier German philosophical context.
Anyonefollowingtheworkofitsauthor,RobertJ.Richards,should
have seen this coming. In the course of his career he has argued for a
broader definition of the term ‘‘evolution,’’ one that includes its devel-
opmental or embryological meanings.20 It is this ‘‘ecumenical’’ defini-
tionthattakeshimtoearlierandnon-Anglocentric,Germaniclocations
for understanding the origins of evolutionary thought; and it most
definitely is thought that interests Richards. Unlike Secord (and even
Desmond)Richardsupholdsthenotionofthoughts,ideasandtheories,
though they are held by frail human creatures seeking such romantic
ideals like love and meaning in life. The title of this book The Romantic
Conception of Life, is thus a deliberate word play on life, love, biology,
andtheattempttofindameaningfulworldviewforagroupofamorous
turn-of-the-nineteenth century Germanic thinkers. It is a rich and
impressive work of scholarship that makes a convincing case that the
word‘‘evolution’’hasbeenmuchmorevariedthanthenarrowerviewof
20See Richards, 1992a, b.