language development from an evolutionary perspective [michael studdert-kennedy 1990]

8/12/2019 Language Development From an Evolutionary Perspective [Michael Studdert-Kennedy 1990]

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Haskins Laboratories tatus Report on Speech Research1990 SR 101 /102 14 27

Language evelopment from an Evolutionary Perspective

Michael Studdert-Kenned

EVOLUTION N EVELOPMENT

PreliminaryIn his famous course of lectures a t th e

Univers ity of Geneva (1906-1911), de Saussuredistinguished langue language as a system, acultural insti tution, from parole language asspoken and heard by individuals: .. .language is

no t complete in any speaker; it exists only withina collectivity...only by virtue of a sort of contrac tsigned by members of acommunity (de Saussure,1966, p. 14). L an gu ag e w as thus seen as anabstract property ofa group, related to it s variableindividual speakers somewhat as a species is to it svariable individual members.

Nineteenth century attempts to apply evolutionary principles to language did indeed vieweach language as a species. For example, Darwinwrote: The formation of different languages an dof distinct species...[is] curiously paralleI...We findin dis tinc t languages s t riking homologies due tocommunity of descent, an d analogies due to a s ~ilar process of formation (1871, pp. 465-466).Here, Darwin was following in the s teps of AugustSchleicher (1821-1868), who drew on t he ear li erwork of Darwin himself an d on th e vast scholarship of 19th cen tu ry European philology, to construct a taxonomy of Indo-European languages.Later work has continued to draw on Darwinianprinciples to construct evolutionary trees of language families (Lehmann, 1973).

However, a strict analogy between languagesan d species i s untenable for both linguistic an dbiological reasons. F or t he historical linguist th emost obvious difficulty is that a biological speciesis a reproductively isolated population: propertiesof one sp ecies c annot pa ss to another. Yetlanguages in contact clearly do influence oneanother, even a ft er t he y have become discretesociopolitical entities. Often we cannot then knowhow far shared prope rt ie s have arisen fromcontact, how fa r from common descent.

A more serious difficulty follows from twoc en tr al t en et s of neo- Dar winian th eo ry ( them od er n s yn th es is o f M en de lia n genetics withDarwinian natural selection). First, in th eevolution of a species, th e principal unit ofvariation and selection is no t th e species, but theindividual organism. Individuals within a speciesdiffer in the number of offspring they produce;

genes whose express ion increases th e relativenumber of an individual s offspring will increasetheir own relative number in th e speciesgene pool. The characterist ics of a species ar ethe re fo re det ermined b y comp etition amo ngindividuals of that species. And, by analogy, thecharacteristics of a language ar e formed bycompetition among speakers of that language. Sofar, so good.

Th e difficulty arises when we combine th eprinciple of individual selection with the so-canedcentral dogma of modem biology, the Weismannbarrier, insulating germ cells from body cells:genes alter body cells, but body cells cannot altergenes. In other words, biological evolution doesno t proceed by t he t ransmiss ion of acqui redcharacters across generations, and this isprecisely what an evolutionary model of languagechange requires. We must therefore distinguishth e cultural, or Lamarckian, evolution oflanguage, a concern of historical linguistics, fromi ts biological, or neo-Darwinian , evolu tion , aconcern of developmental biology.

This distinction was first clearly formulated byChomsky (1965, 1986), who recognized that deSaussure s definition of language as a property orproduct of a social group did no t lend itself totreatment in biological evolutionary terms.Chomsky made the necessary move byreformulating the langue parole distinction ascompetence (what a speaker-listener knows) andperformance (what a spe aker-listener does inimplementing knowledge). He thus se t the locus ofhuman l ang uag e cap acity in th e individual s


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anguage evelopment from an volution ry Perspective 15

mindlbrain. He also s et t he stage for th e modernstudy oflanguage acquisition.

Form an d functionNonetheless, th e competence-performance dis

tinction as usually formulated also does not lenditself to evolutionary treatment. Competence isknowledge of a particular language, formed by interaction between an innate schema, the universal grammar, a nd the gram mar of th elanguage being learned. Universal grammar is ...a theory of t he i ni ti al state of th e languagefaculty, prior to a ny l ing ui st ic experience(Chomsky, 1986, pp. 3-4). Performance is thussaid to be a product of a partial ly innate competence. In what follows, I shall argue that this precisely reverses th e true course of development.Universal grammar is no t a prescription, or program, for development, but a part ial and poste-riori description of th e phenotypic product of th e

d ev el op me nt al s ys te m (cf. Oyama, 1985).Universal grammar is a consequence , no t acondition of development.

What is a t s take here is the relation betweenform (competence) an d function (performance).Zoologists have traditionally stressed th e harmonious m at ch b et we en form an d function, asexpressed in an animal s mode of life, but theyhave disagreed on how th e match comes about.From classical times down into th e 19th century,t he s tandard belief was that species an d generahad fixed, unchanging forms or essences:departures from th e species p ro to ty pe w er e unreal, an d structure took precedence overfunction. For example, according to Mayr (1982),Georges Cuvier (1769-1832), th e great Frenchzoologist, held that ... structure has primacy overfunction and habit, an d ... only a change instructure might necessitate a change in function(p. 367). Certain non-Darwinian French zoologistssti ll hold such views: ...evolution originates inparent forms; if t hes e a re a bs en t, new types oforganization never appear (Grasse, 1977, p. 75).Thus, Chomsky and his s t ruc tural is t forebears inlinguistics align themselves with th e essentialisttradition in biology by asserting th e primacy ofform over function.

For Darwin an d modern evolutionary biologists,working within th e British empiricist tradition,th e form-function relation is reversed. Species ar enot e te rnal ly fixed. A species is a geneticallyvariable population of individuals, adapted to aparticular ecological niche an d thereby reproductively isolated. No aspect of an animal's structure

determines a unique function. Rather, a structuredetermines an unbounded range of functions, tosome of which i t is more nicely ta ilored thanothers. We can use a screwdriver to drive screws,or as a dagger, a lever, a drumstick, a fork ... . Ifcertain members of a species are forced bycompetition with their fellows, or by an environmental change, into a new mode of life, a newh ab it at , t he y m ay call on some hitherto unusedpotential function of their structure. The newmode of life then confers a reproductive advantageon those individuals whose structure is marginallybetter suited (due to small differences in theirgenetic make-up) to the new mode. Little by little,as with th e b eak s of Darwin s famous finches(Lack, 1947), th e new selection pressures reshapeth e old structure to it s new function. Behavior isthus the great pacemaker of evolutionary change(Mayr, 1982, p. 612), th e cause not the consequence of speciation and of species form.

In short , a commitment to gradual evolution bynatural selection entails a commitment to th eprimacy of function over form. My central assumption, then, is that language competence (theneural substrate oflanguage form) was shaped inphylogeny, and is still shaped in ontogeny, by language performance (function, behavior).

Behavior as the pacemaker ofdevelopment

If we ext end Mayr s dictum on evolution todevelopment, we see development in a new light.We are freed from preoccupation with the initialstate as an index ofgenetic endowment, and fromth e nativist-empiricist controversy that hasdominated studies of language developmentalmost since their inception.

At birth, an organism suffers an abrup t changein th e quality a nd q ua nt it y of environmentalconditions that may affect it s growth. Bu t thediscontinuity does no t change th e developmentalsystem. Gene action does no t cease at birth, nordoes regulation of gene act ion by th e cellularenvironment. Rather, th e developmental circlewidens. The new external environment affords abroader range of stimulation, eliciting a broaderrange of response. Changes in th e organism'senv ironment and behav io r now mediate, to asteadily increasing degree, changes in th e cellularenvironment that controls genetic action.

The developmental course is not one of simplematuration cf. Borer Wexler, 1987). Languagedevelopment will no t go forward merely becauseth e env ironment mee ts general conditions of


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16 Studdert ennedy

survival-air, food, other people, an d so on.Specifically linguistic input and, probably, outputar e cal led for so that if either is se t to zero,development stops. Specific extralinguisticprocesses of perceptuomotor an d cognitivedevelopment-a growing grasp on th e physicalan d social modes of being an d acting thatlanguage represents-must also feed into th el anguage sys tem. Thus, the proper study oflanguage growth is the sequence of behavioral andcognitive conditions, linguistic an d extralinguistic,that precipitate language change. The task of this post-natal embryology, as we might call it, is tochart th e course by which perceptual and motoricfunctions induce structure, from undifferentiatedinfant p er fo rm an ce to differentiated adultcompetence.

In short, I am proposing that p rinc ip le s o fgrowth, generally accepted in th e development ofth e peripheral anatomy, also apply to th e central

nervous system. Fo r example, writing of theplasticity of growing bones, D Arcy Thomps.on(1917/1961) remarks ... th e very importantphysiological truth that a condition of str in th eresult of a stress is a direc t stimulus to growthitself. This indeed is no less than one of th ecardinal facts of theoretical biology. Th e soles ofou r boots wear thin, bu t the soles of our feet growthick th e more we walk upon them: for i t wouldseem that th e living cells a re s timu la ted bypressure, or by what w call exercise, to increasean d multiply (p. 238, italics in the original).Bosma (1975) invokes this principle of exercise indescribing the developm ent of th e vocalapparatus. Perhaps the development of the neuralrepresentations of speech an d language is also aninstance of .....one of the cardinal facts oftheoretica l biology.

Ontogeny parallelsphylogeny

Complex functions, an d the physical structuresthat support them, arise in evolution by gradualdifferentiation from simpler forms. Everyevolutionary change is a change in developmenttha t is preserved in la ter generations.Accordingly, when a complex function is anevolutionarily coherent, hierarchically developedsystem, ontogeny ma y parallel, or recapitulate,phylogeny. I should emphasize that I am notproposing to re ins ta te the discredited biogeneticlaw of Ernst Haeckel (1834-1919). I am merelydrawing at tention to developmental facts wellknown since th e embryological studies of Karl

Ernst von Baer (1792-1876) in th e early 19thcentury (Gould, 1977).

Of course, if we lack, as we largely o forlanguage, precursor forms that confirm th esequence, w cannot be sure that th e or der ofdevelopment we now observe was the actual orderof evolution. Such prec ursors as we mayestablish-for example, lateralized systems forneural control of body p ostu re a nd manualfunction (MacNeilage, in press), or certaincapacities for symbolic representation (SavageRumbaugh, in press)-do not help us in th epresent context. Some stages may have beeninserted into th e evolutionary sequence later thanothers that now follow them in development. Thismay be true, for example, of lef t hemispheresensitivity to speech at , or soon after, birthbecause i t is unlikely that a specialized neuralsubstrate for speech evolved before speech itself.

Yet other processes may have evolved indepen

dently, parallel to processes with which they werelater integrated. For example, some form of segmental phonology, affording at least a modest lexicon, would seem necessarily to have evolved before syntax began to take shape, an d we sti ll observe this sequence in development. Bu t laterstages of phonology and syntax may have evolved,as they sti ll seem to develop, more or le ss independently. Similarly, prosodic variations in pitch,amplitude an d duration, characteristic of bothh um an a nd non-human sys tems o f communication,perhaps first followed an independent courseof evolution, to be modified and integrated into th elinguistic system only as longer utterances an dmore finely differentia ted syntact ic functionsemerged. The double dissociation of r ight and lefthemispheres for emotional an d linguistic uses ofprosody ma y reflect such a course of evolution.

Yet further limits on a phylogenetic interpretation of language ontogeny may seem to arisebecause th e child is bo rn i nt o a communi ty ofcompanions who a lr ea dy sp eak a language.Moreover, th e child is born with a vocal tract an da pair of hands that soon mature into formsadaptable for spoken or signed language, and witha rich, plast ic neural substrate fit for shaping bycognitive and linguistic function. The exogenousan d endogenous condi ti ons of developmenttherefore differ radical ly from those that musthave prevailed in a hominid community wherelanguage was not ye t fully formed.

N on et he le ss , l an gu ag e a cq ui si ti on is no tinstantaneous. The child does not have immediateaccess to the full adult language that surrounds it.


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Language evelopment from an volutionary Perspective 17

The effective linguistic environment changes, stepby step, as the child comes into possession of newcognitive an d linguistic capacities. The problem ofhow linguistic input i s ordered so as to ensurecoherent development is solved by the child's ownincreasingly differentiated linguistic attention.With each new step the child finds t he n ex t stepwaiting, as i t were, i n t he a du lt language becauseth e adult language is adapted to th e child no lessthan th e child to th e language. The reason for thisis simply that language, like every evolved form,is th e product of successive ontogenies, it sstructure a record of it s own evolution cf Locke,1983). Looked at in this way, language is no t anobject, or even a skill, that lies outside th e childand ha s somehow to be acquired or internalized.Rather, it is a mode of action in to which th e childgrows because th e mode is implicit in t he humandevelopmental system.

We conclude that language, as a complex,

hierarchical, behavioral structure with a lengthycourse of development, is a good candidate for(circumspect) study in a recapitulatory frameworkbecause it s development is rich in sequentialdependencies: syllables an d formulaic phrasesbefore phonemes and features (as I shall arguebelow), holophrases before words, words beforesimple sentences, simple sentences before lexicalcategories, lexical categories before complexsentences, an d so on. Thus, if we assume that eachof th e subsystems, phonology an d syntax, evolvedhierarchically by repeated cycles of differentiationan d integration, we may recover their course of

evolution by t racing th e course of their growth.The general heuristic value of th e assumption ist ha t i t not only throws light on evolution, but alsopromises an under st anding of development infunctional terms.

Development is no t teleologicalIf we combine the principles of gradual evolution

and of a limited, functional recapitulation, we ar efreed from th e temptation to assign purpose todevelopment. At each point in its development anorganism is already complete, adapted and

adapting, as best i t can, to present conditions,internal and external. Just as earlier evolutionaryforms existed for themselves, no t for an y laterforms to which they might give rise, so th e presentform of a developing organism ha s it s own presentfunction. A child does not l ea rn i ts first words sothat it may later combine them into sentences.First words have their own economy. Moregeneral ly, a child 's grammar, at each stage ofdevelopment, is a possible adult grammar.

This l as t s ta temen t is limit ed exactly to th eextent that th e princ ip le of recapi tu la tion islimited. Some stages may have been inserted intoth e developmental sequence relatively late inevolution. Others may now serve a new function,having lost th e function for which they originallyarose. Such stages, perhaps part icularly duringth e early months of rapid growth before linguisticfunctions have begun to differentiate, would havesurvived through successive generations becausethey facilitated later adaptive changes, while notinfringing on present functions. I n t hi s sense, anearly form may preadapt for a later. However, itwould be a n e rr or to see such preadaptations asliteral preformations. Development, like evolution,is a tinker, putting to present use whateverchance ha s laid to hand cf. Jacob, 1977).

Individuals reach the same developmentalends different routes an d at different

ratesMany linguists believe that universal aspects oflanguage ar e innate. Universals ar e said to bepurely genetically determined, an d either to bepresent at birth (Chomsky, 1986) or to maturewithout contribution from th e environment (Borer Wexler, 1987). Aspects that vary acrosslanguages, though perhaps const ra ined to a fewvalues by principles of universal grammar, aresaid to be learned from, or fixed by, th esurrounding language. This is th e model of parameter setting in syntactic theory (Chomsky,1981; Roeper Williams, 1987) and, in effect, ofJakobson s (1968) account of phonologicaldevelopment cf. Goad Ingram, 1987).

I do not challenge th e descriptive adequacy ofsuch theories. What I quest ion is the proposed developmental mechanism, the attribution of speciesinvariance to th e genes, within-species variabilityto th e environment. For, in fact, both invariancean d variability arise from both genes and environment. The point is important because individual similarities in language development are oftentaken as support for a biological account of th eprocess, while individual differences are seen as a

threat (Goad Ingram, 1987).Consider, first, that identical phenotypes may beinduced either by a g en et ic c ha ng e in anu nc ha ng ed e nv ir on me nt or by a changedenvironment acting on an unchanged genome. Forexample, a yellow (as opposed to grey) body in thefruit fly Drosophila melanogaster can be produced either by mutation or by feeding the flylarva on fruit jelly impregnated with yellow dye(Dobzhansky, 1957). Similarly, th e lack of a


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18 Studdert-Kennedy

posterior cross-vein in fruit fly wings can resulteither from mutation or from subject ing th e flypupa to a brief high-temperature shock early indevelopment (Waddington, 1975, Chapters 7 and8). Another example, no t due to experimentalintervention comes from Piaget (1978) whostudied fresh water sna il s in Swiss lakes duringth e late 1920's. He describes three species. Onespecies Limnaea stagnalis , found in deep, calmwaters, ha s an elongated shell; two other species L lacustris an d L. bodamica , found in shallow,rough waters have almost indis tinguishablestubby, contracted shells. If th e two stubby speciesar e bred in th e calm water of a laboratoryaquarium L. bodamica r et ains i ts stubby shellover many generations while L. l custrisgradually takes on th e elongated form characteristic of L. st gn lis Had Piaget simplydescribed th e two stubby species, he might haveattributed their similarity to shared genes. Bu t we

cannot reliably infer genotype from phenotype, asdiscussions of la nguage development o ft enassume. Without controlled breeding studies, wecannot determine whether an invariant speciestypical form reflects genetic constancy orenvironmental constancy.

Notice, moreover, that even when we haveis ol ate d ge ne tic o r environmental factors thatcontribute to some aspect of phenotypic variation,we have no t demonstrated that th e variable heldconstant ha s no effect. The snail species that takesan elongated form after generations in calmwater a contracted form after generat ions inrough water ca n adapt to the environmentalchange because it is genetically equipped to o so.The snail species that retains its contracted form,w he th er b re d i n rough waters or in calm, lacksthis genetic potential. Bu t we cannot concludefrom this that th e environment ha s no effect onthe form of it s shell. We can conclude only that thetwo environments to which th e snail was exposeddid no t suffice to se lec t for a change in formalthough other environments might have done so

In short, phenotypic form is both genetically andenvi ronmental ly determined a fact of someimportance to language development. If we cannotass ign pheno typi c invar iance even in these simple examples, either to genes alone or toenvironment alone, we are surely no t justified indoing so for language universals, or for the withinlanguage constancies in l inguist ic developmentthat ar e often cited as evidence for an innatelanguage competence

By th e same token, variabi li ty in developmentamong chi ld ren learning th e s am e l an gu ag e

cannot be assigned solely to th e environment.Certainly different chi ldren, even within th esame social class , must be exposed to wide lyvarying p at te rn s a nd frequencies of l inguist icinput, and such differences ar e likely to affect thecourse and rate of development. At th e same time,children differ genetically, and these differencestoo affect development (Locke Mather, in press).Th e problem is not to assign variabil i ty to agenetic or environmental source, but to understand how children resist th e effects of genetic andenvironmental variation so as to arrive a t acommon language.

Development is buffered against extremevariation

The problem of uni formity within an d acrosslanguages is, in some respects, the obverse of th eproblem of language diversi ty. Languages, l ikespecies, differ because individuals differ. If al l

members of a species were genetical ly identical ,followed an identi.cal course of development, an darrived at an identical developmental term, newspecies could never arise. Genetic variation withina species, affording subtle individual differencesin adaptive response to new environments, is th ebasis for th e origin of species by natural selection.Similarly, if all speakers ofa language followed anidentical course of development to an identicallinguistic term, new languages could never arise.

Of course, differences among speakers that leadto language change ar e unlikely to rest on geneticdifferences because speakers of differe nt

languages o no t differ systematically in theirlanguage-related genetic endowment: childrenlearn any first language to which they areexposed. In fact, from a genetic point of view,language differences rest on commonalties ratherthan differences: th e shared capacity to adapt toan y linguistic environment and to learn th esurrounding l ang uag e. In other words, i t isprecisely because language is supported by (wemust presume) many thousands of genes whichdepend on t he e nv ir on me nt to trigger theirexpression, that l anguages can be learned-andthat languages differ. Subtle environmentallyinduced individual differences in languagedevelopment, selected and transmitted to otherindividuals by sociocul tu ral forces of which weunderstand at present very little, are one sourceoflanguage diversity.

What then are th e forces that resist change,guiding speakers into a common language? Theproblem is a special case, analogous to th e generalproblem in developmental biology of how species


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maintain their identities across generations. Wecan gain conceptual leverage on th e question byinvoking the principle of canalization(Waddington, 1957, 1975). Th e term is purelydescriptive, intended to capture ...a large numberof well-known facts in genetics an d embryology,all of which ar e summarized in the statement thatth e development of any pa rt icu la r phenotypiccha ract er i s to some extent modifiable, an d tosome extent resistant to modification, by changeseither in the genotype or in the environment(Waddington, 1975, p. 72). In other words, bothgenetic an d environmental variations ar e bufferedagainst extreme divergence, canal ized towardequifinality by const ra in ts from th e developingsystem. Genes an d environment act in reciprocal,mutually constraining concert to assure a s tabletrajectory of growth.

To characterize the constraints on languagegrowth in an y detail is well beyond th e scope ofpresent

knowledge.There

would seem, however,to be two main sources of constraint, one in thechild itself, one in the physical, social an dlinguistic environment. Within th e child, eachstep in cognitive and linguistic development opensnew paths and closes others, an automaticconsequence of th e selection, an d increasingdifferentiat ion, of th e neu ral structures thatsupport language an d cognition, continuing th eprocesses of tissue differentiation in th e embryo.Once launched, th e learning of a particularlanguage becomes increas ingly const rained bystructural changes in th e child's bra in . These

changes open paths into th e language beinglearned, an d perhaps into other languages of th esame general type. Presumably, developmentalong these paths will cease if the child is abruptlytransferred to a markedly di fferent l inguis ticenvironment. (Studies of savings, that is, of th ehead-start or lack of it, displayed by children whohave switched l anguages o r been exposed to asecond language, might then throw light oncognitive an d linguistic commonalties between the

.languages and on their inferred neuralsubstrates.)

The external constraints on the child arise, first,from a general social context that invites th e childto engage with it s companions and to match it sbehavior to theirs. The second source of constraintis the language that the child's companions speak.Every language is a solution, one of anuncountable, but presumably limited, se t ofsolutions to th e problem of developing acommunicative system within the perceptuomotor,memorial and cognitive limits of humans (cf.

Lindblom, 1983). The child is then easily guidedinto it s language because, as we have alreadyremarked, every language has evolved underconstraints very like those that limit the child.

Given the increas ing strength of endogenousan d exogenous constraints as the child grows, wewould expect phenotypic variation in phonologyand syntax, if no t in lexicon, to d im in is h aschildren come into possession of their language.Nonetheless, individual variability, far from beingevidence against a biological account of languagedevelopment, is a hallmark o f d ev elo pi ngbiological systems.

In th e following section, I will attempt to illustrate some of th e princ ip les sketched above asthey might apply to the development ofphonological form over the first two years oflife.

TH M RG N OF PHON TIS GM NTS

PerceptionThe first systematic studies of infant capacity to

perceive speech derived from th e well-knownstudies of adult categorical perception. In aseminal se t of experiments, th e model for manyothers, Eimas and his col leagues E imas ,Siqueland, Jusczyk, Vigorito, 1971) demonstrated that 1 an d 4-month old infants displayedth e same pattern of d iscr iminat ion betweensyllab les on a synthetic voice onset t ime (VaT)continuum as adults: they discriminatedsignificantly better between syllables belonging to

d if fe re nt E ng li sh phoneme cat egor ie s thanbetween syllables belonging to th e same phonemecategory. Much th e same result came from laterinfant studies of stop consonant place ofa rt i cu l at ion , consonan t manner, th e [r]-[1]distinction, and almost every other syntheticcontinuum on which infants were tested (seeAslin, Pisoni, Jusczyk, 1983, for a comprehensive review).

Fr om t his ac cu mu lat ed wealth of evidenceEimas (1975) concluded tha t categoricalpercept ion ref lect s th e oper ation of innatemechanisms, specialized for processing speech,an d that ... t he se ear ly categories serve as th ebasis for future phonetic categories p. 342). Thisconclusion is often cited by students of languageacquisition whose primary interest is in syntax.Fo r example, Gleitman and Wanner (1982)summarily dismiss th e problem of th e origin ofdiscrete phonetic segments (a problem no lesssevere than th e problem of th e origin ofmorphemic units to which they devote much


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attention) by citing the work of Eimas and othersto support th e claim that ...no learning apparatusis required for an in it ial segmentation of th eacoustic wave into discrete phones. Th esegmentation ha s been provided in th e nervoussys tem (p. 16). Exact ly how this capacity forsegmentation came to be evolutionarily, or comesto be ontogenetical ly, provided in the nervoussystem, they do not consider.

In any event, many reasons to doubt Eimas sinterpretation of th e infant data have been givenby Jusczyk (1981), Kuhl (1987), Studdert-Kennedy(1986), and Walley, Pisoni an d Aslin (1981). I willnot rehearse th e reasons in detail, but here brieflynote three. First, we now know that categoricalperception is peculiar to neither speech . no raudition (see Hamad (1985) for several relevantpapers) . Second, Kuhl and her colleagues havedemonstrated categorical effects on labial,alveolar a nd v el ar VOT continua in chinchillas(Kuhl Miller, 1975), and on a place continuumin macaques (Kuhl Padden, 1982, 1983). Thus,whatever th e bearing of categorical studies on th edevelopment of speech perception, they clearly dono t reflect a perceptual specialization. Third, th eclaim that categorical perception reveals th e basisfor future phonetic categories confuses two typesof category. Th e categories mimicked by asyn thet ic ser ie s vary along a single acousticdimension in a fixed context; they comprise, atmost, th e random variations that we mightobserve in a single sy llable, spoken repeatedlywith identical stress an d a t an identical rate by

th e same speaker. However, th e phonologicalcategories that a child must form ar e equivalenceclasses of intrinsic and extrinsic allophonicvariants, formed by execution of a particularphoneme in a range of phonetic contexts, spokenwith varying degrees of stress, at different rates,and by different speakers.

For study of th e infant's potential grasp on theseequivalence classes, we must turn to another bodyof research. In a systematic series of studies Kuhlan d he r colleagues have demonstrated that 3- to6-month old i nf an ts c an l ea rn to recognize th eequivalence of: (i ) isolated vowels spoken by amale, a female or a child; (ii) syllables with thesame consonantal onset before different vowels;(iii) syllables with a particular consonantalacoustic pattern in initial, medial or final position;(iv) syllables that share an initial feature (stop,nasal ). (For a comprehens ive review, see Kuhl,1987). Yet we should be cautious in interpretingeven these s tudies as evidence that infants can

recognize p ho ne ti c ( as opposed to acoustic)invariants, if only because Kluender, Diehl an dKilleen (1987) have successfully trained Japanesequail to form equivalence classes across syllablesspoken with different pitch contours, by differentspeakers or with th e same consonant beforedifferent vowels. In short , we have no grounds forclaiming that perceptual analysis of th e acousticstructure of a spoken syllable, an d th e formationof categor ies from th e resulting components,engage distinctively human capacities, whetherinfant or adult.

In fact, these infant studies seem to have nomore than a general bearing on th e specialized development of language. They are psychophysicalstudies, demonstrating that i nf an ts a t, or soonafter, birth have the capacity to discriminate an dcategorize certain acoustic patterns which occur inspeech. They provide detailed support for a general observation, suggested by th e fact that speechsounds ar e concentrated in th e few octaves of th eacoustic spectrum to which humans (and manyother animals) ar e most sensitive: spoken lan-guage ha s evolved an d develops within th e constraints of prelinguistic auditory capacity. Surely,it would be surprising if this were no t so. We dono t expect a n a ni ma l to have a communicationsystem that is not matched to it s sensory capacities.

However, th e most serious objection to th e standard interpretation of these studies is that byassuming the child to be innately endowed withsensitivity to the smallest phonetic units of which

spoken utterances are composed, they implicitlyadopt a view of development as proceeding fromth e specific to th e general rather than th e reverse.Yet, as we have already seen, it is a general ruleof both phylogeny an d ontogeny that complexstructures evolve by differentiation of sma ll erstructures from larger. Accordingly, we should notexpect children to build words from phonemes asadults do; rather, we should expect phonemes toemerge from words. We may note, in passing, thata similar principle must apply to the developmentofword classes an d syntactic structures, a fact notgenerally recognized in developmental psycholinguistics.

From this vantage we can take a different viewof infant speech perception than has hithertoprevailed, one that emphasizes development function rather than psychophysical capacity. Oneaspect of this work will entail charting the processof attunement to the surrounding language, th ecourse by which t he i nfa nt le arns to perceive


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speech, bringing to bear capacities evidenced inth e laboratory on th e speech it hears at home.Work along these lines has, in fact, already begun,with studies of infants loss of sens it iv i ty tophonetic cont rasts not deployed in their nativelanguage e.g., Best, McRoberts, Sithole, 1988;Werker Tees, 1984). Other studies by HirshPasek and her colleagues e.g., Hirsh-Pasek Golinkoff, in press; Hirsh-Pasek, Nelson, Jusczyk,Cassidy, Druss, Kennedy, 1987) ar e tracing thedevelopment of sensitivity to prosodic patternsthat specify clausal units, perhaps the thin end ofan infant wedge into syntactic structure.

Here, however, I wish to pursue another aspectof perceptua l function: it s role in guiding th edevelopment of production. Fo r I assume that thedevelopment of speech depends no t only on thematuration an d use of th e vocal motor system, bu talso on th e infant s gradual discovery of structurein the speech it hears : By learning how to listenthe infant learns

how to speak. The speech signaltherefore comes to specify for the infant theactions-or, more exactly, th e motoric componentsof articulatory act ion-by wh ich sp eech isproduced. Yet neither phonemes no r features, th eperceptual units typically posited in infantstudies, ca n be defined either acoustically ormotorically. They are abst rac t units beyond th ereach of an infant who does not yet know alanguage. we ar e to understand speechdevelopment, we must couch ou r descriptions andframe our experiments i n t er ms of auditory an dmotoric units to which an infant might reasonably

be expected to have access. what follows I willbriefly sketch a speculative account of the processby which infants gradually harness prelinguisticmotoric elements to communicative use.

ProductionWe may discern at least four broad stages in the

early development of speech, successive cycles ofdifferentiation an d integration that carry theinfant from prelinguistic cries an d mouthings toth e emergence of phonetic segments and thebeginnings of phonology in early words. Thestages form a necessary hierarchical sequencefrom th e general to th e specific, from th e nonlinguistic to th e linguistic. However, t hey a re no tsharply delimited: processes that begin in onestage may cont inue more or less unchanged intoseveral later stages-for example, a child ma ycontinue to babble long after it has begun toproduce words. Also, we must be cautious incharacterizing a stage as differentiative or

integrative for two reasons. First, becausedifferentiation at one level for example, breakinga syllable into it s component gestures) entailsintegration at another coordinating movements toform gestures), and vice versa. Second, becauseeven within a level both differentiat ion an dintegration go on during a single stage: forexample, i nt eg ra ti ng g es tu ra l p at te rn s intoconsonants and vowels entails simultaneouslydifferentiating a syllable into it s segmentalcomponents.

Nonetheless, since even a coarse an d tentativetaxonomy ofdevelopment may lend insight into itsprocess, le t us consider th e following four stages: 1) early vocalizing: differentiation of respiratoryan d vocal tract activities into patterns ofsoundmaking associated with different nonspeechactions 0-7 months ; 2) canonical babbl ing:integration of nonspeech movement patterns intorhythmic syllabic structures 7-10 months); 3

variegated babbling an d early words: differentiation of the syllable into it s component gestures 10-15 months ; 4) integration of recurrentgestura l pat terns into canonical phonetic segments 15-24 months). The suggested time periodsare, of course, approximate, because childrendiffer widely in the time courses of theirdevelopment.

ifferentiation Early vocalizingOver th e first half year of life infant sounds

progress systematically from clearly nonspeechvocalizations to canonical babbling that invites

phonetic transcription. Th e changing forms ar epresumably determined, a t least in part, bygeneral maturation an d by exercise of the vocalapparatus and of i ts neura l control structures forreview, see Kent, 1981). Stark 1986) divides thedevelopment of soundmaking over th e f irst 7-8months of life into three stages: reflexive cryingan d vegetat ive sounds 0-8 weeks), cooing andlaughter 8 to 20 weeks), an d vocal play 16-30weeks). These stages ar e of interest in the presentcon text because they reflect changes in thetopography of vocal tract activities and in theforms of th e resulting sounds by whichprelinguistic elements ar e differentiated an dmarshalled for protolinguistic use cf. Oller, 1986).

Reflexive crying, like th e distress calls of otheranimals, has, of course, a communicative thoughnon-l inguistic) function. Cries, executed with ar el at iv el y u n co n st ri ct ed vocal tract, arepredominantly voiced, often with th e formantstructure of low to mid front vowels. Vegetative


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22 Studdert ennedy

sounds-the grunts, sighs, clicks, stops and pops,associated with brea th ing and feeding-may be.either voiced or voiceless, formed with either anopen, vowel-like or a constricted, consonant-likeconfiguration of th e vocal tract. The cooing, orcomfort, sounds of Stark s second stage tend tooccur in a ser ies of 3-10 segments, each of about500 ms, separated by voiceless intakes of breathan d glottal stops. Their energy is concentrated infrequencies below about 1500 Hz, a pattern thatOller (1980) has termed a quasi-resonantnucleus, having the form of a nasal consonant ornasalized vowel.

Thus by th e end of th e fifth month, th e infant ssound r eper to ir e a lr eady contains a varietyprotoconsonantal and protovocalic elements. Theimportant changes in th e n ex t stage ar e not somuch in th e size and quality ofthe repertoire as init s function an d organization. First, sounds beginto lose their original functional moorings: they

become ...d ivorced from their previous cry,vegetative or comfort sound contexts, and a re usedin a var ie ty of communicative situations (Stark,1986, p. 159). Second, sounds become longer (7001500 ms), an d form longer , more complexsequences. The infant emits repetitious strings ofconsonant-like clicks, trills, f ri ct ion noi se s,syllabic nasals with a constrict ion at the f ront ofth e mouth, an d lip smackings. Vowel-like sounds,now with a fully resonant nucleus (Oller, 1980)often carrying extreme pitch glides, ar e executedwith increased variat ion in tongue h ei gh t a ndfront-back placement.

However, th e key change dur ing thi s stage iscombinatorial: t he infan t begins to superimposemovements of tongue, ja w and lips on th elaryngeal actions associated with cry (KoopmansVan Beinum Van der Ste lt , 1986), so that theproportion of supraglottal to glottal articulationsg ra du al ly i nc re as es (Holmgren, Lindblom,Aurelius, JaIling, Zetterstrom, 1986.) Even atthi s ear ly stage, children may differ quite sharplyin their preferred types of vocalization (Stark,1986). Toward th e end of th e stage increasinglylong and complex c om bi na ti on s o f t ractconstrictions and openings appear, formingsequences that Oller (1980) terms marginalbabQle.

The functional value of th e differentiation ofearly soundmaking into these diverse patterns isno t obvious. Teleologically, of course, exercise ofth e vocal apparatus must contribute to it s neuraland anatomical development, laying th e basis forlater integration of sounds into syllables.However, the immediate function may simply lie

in the increased range of emotional expressionthat it affords, with a consequent tightening ofdyadic social bonds cf Stern, 1985; Trevarthen Marwick, 1986). Interestingly, MacNeil ag e(personal communication) ha s noted a possibleprecursor of such differentiation in th e repetitivegirneys (lip smacks , accompanied by a low murmur) exhibited by Japanese macaques in intimate,affiliative situations (Green, 1975).

In any event , I have dwelt on this early stagebecause it is here that we can see most clearly th etinkering together of disparate non-linguistic patterns of vocal tract activity into th e beginnings ofphonological structure cf Bates et aI., in press).

Integration: Canonical babbleWith th e onset of canonical babble, often a

sudden event over a few days in the 7th or 8t hmonth, t he infan t begins to integrate patterns ofvocal tract constriction an d opening into unitary,

cohesive syllables. Rhythmic, reduplicated soundsequences ar e common: [ bababa ], [ ne nene ],[dIChdI] However, th e convenient use of phonetictranscription should not mislead us into supposingthat the infa nt has independent control oversegments within a syl labl e. In fact, rhythmiclowering a nd ra isi ng of th e ja w in canonicalbabble seems to occur with little or noindependent movement of th e tongue (Davis MacNeilage, 1990). This rhythmic jaw oscillationoften begins about the same time as rhythmicmovements of th e legs and arms (Thelen, 1981; inpress), and perhaps facilitates th e integrat ion of

vocal tract a ct iv it ie s i nt o cohesive syllabicpatterns.

We owe th e clear distinction between early vocalization an d canonical babble to Oller (1980,1986). He d es crib es some half dozen acousticproperties of structures that listeners recognize ascanonical, adu l t consonant-vowel (CV)(constricted-to-open) or CVC syllables. Of theseproperties th e most important in the present context (because t he y ar e th e only ones that appearrarely, if ever, in early vocalizations) are temporal. The canonical syllable ha s a duration of noless than 100, no more than 500 ms; onset (and,when present, offset) formant transitions displaysmooth changes in frequency an d amplitude withdurations between 25 an d 120 ms. These acousticpatterns reflect the infant s increasing skill in integrating th e closing an d opening phases of ja wmovements.

Patterns ofmovement in babble a re not random.During th e const riction phase of th e syllable,complete closure, as in stops, is favored over


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anguage evelopment from an Evolutionary Perspective 23

partial closure, as in fricatives. Points of closureare biased toward th e front of th e mouth,engaging l ip an d tongue tip muscles acti ve insucking. During th e open phase, th e favoredtongue positions ar e those associated with lowfront vowels, indicating that th e tongue tends toride up an d down on the jaw with little or no

active movement of it s own (Davis MacNeilage,1990). Th e glottis is typically approximatedthroughout th e syllable, giving the impression ofvoiced consonants at syllable onset.

These phonetic biases have been reported forinfants growing in a number of languageenvironments (Locke, 1983). The biases are alsopresent in many adult languages, perhapsreflecting th e infant proclivities from whichlanguages have evolved (Locke, 1983; cf.Lindblom, 1989). Here we have a tangle thatcross-linguistic data on babble are still too sparseto resolve. How fa r do th e perhaps universal

b ia se s of infant babble reflect th e naturationals ta te of the infant vocal apparatus, an d how fa r dothey reflect th e surrounding language? We mayask much th e same question concerning individualdifferences in babbling r eper to ir e within alanguage: do they ref lect d iffe rences in th edevelopment of th e vocal apparatus or differencesin th e language patterns which th e infantshappen to have heard? Whatever th e answers tothese questions, recent work with deaf infants ha sshown that th e emergence of canonical babble isnot purely an effect ofmaturation.

Deaf infants were once said to exhib it th e samebabbling patterns as hearing infants, at least overth e first year of life (Mavilya, 1969; cf. Locke,1983). However, recent comparisons havedemonstrated that canonical babble does no tappear on schedule i n dea f infants (Oller Eilers,1988; Oller, Eilers, Bull, Carney, 1985; StoelGammon Otomo, 1986). Whether this is due toth e lack of auditory input from the infant s ownvocalizations, from those of it s adult companions,or both, we do no t know. If self-stimulation wereth e only essential , a purely maturational accountcould still hold. To th e extent that communicativeinterchange, or th e impulse to imitate th e actionsof conspecifics plays a role, th e onset of canonicalbabble would be determined, at least in part, byexperience of a surrounding language. Persuasiveevidence for th e role of the surrounding languagecomes from reports that deaf children exposed tosign language begin to babble with their fingersat about the same age as hearing infants begin tobabble with their mouths (Newport Meier, 1985;Laura Petitto, personal communication).

Finally, what is th e function of canonical babble? None ha s been th e answer of some (e.g.,Jakobson, 1968; Lenneberg , 1967) who viewedbabble as random mouthing, sharply discontinuous from truly linguistic utterance. However,Locke and Pearson (1988) have recently reported asevere (though no t i rremediable) delay in the de

velopment of speech in a tracheostomized child,deprived of th e opportunity to babble from 5 to 2months. This suggests that babble is a necessarystep in normal linguistic development. Moreover,several s tu di es h av e now shown that babblemerges smoothly into early words an d that th ephonetic structure ofa child's f irst words tends toreflect it s babbling preferences (Locke, 1983;Oller, Wieman, Doyle, Ross, 1975; Vihman,Macken, Miller, Simmons, Miller, 1985; Vihman Miller, 1988). Th e immediate function wouldseem then to be to continue th e imitative processof aligning th e infant's communicative skills more

closely with those ofits adult companions. ifferentiation Variegated babble and

early wordsToward the end of th e first year full syllable

reduplication fades. Th e infant begins todifferentiate th e closing an d opening gestures ofsuccessive syllables so that variegated sequences(Oller, 1980) appear, giving the impression ofvariations in consonant and/or vowel (e.g., Inem/,Im ;jnw Idedi/). Also, at about this t ime the childproduces it s first recognizable attempts at adultwords or phrases All gone, What s that?), u s ~ y in English children) single syllables ordisyllables. The two modes of output then proceedconcurrently, often for many months, with wordsgradually coming to predominate. Over this periodth e child is gradually forging links between it sperceptual and productive capacities.

The perceptua l g round for this developmentseems to be laid by th e child's growing attention towords in the surrounding language. Data on earlycomprehension are scarce, but children evidentlyaccumulate sizeable receptive lexicons beforeattempting their first words. Benedict (1979) hasreported a longitudinal s tud y o f comprehensionand production in eight children from th e age o f9or months to a point where they ha d achieved aproductive lexicon of50 words (between 5 and 22months) . In every child comprehension comfortably outstripped production. On th e average, thechildren understood more than 60 words by thet ime they could produce 10, with a range from 30to 182 words; an d on th e average, their receptivelexicon over th e period of the study was three


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4 Studdert Kennedy

times their productive lexicon (Benedict, 1979,Table I

The ga p between perception an d productiondemonstrates that a perceptuomotor l ink is no t innate, as some have proposed (Liberman Mattingly, 1985), bu t must develop. Many studieshave shown that th e phonetic forms of early wordsa re s imil ar to those of concurrent babble (seeVihman, in press, for review). A child selects forimitation words that match vocal motor schemes(McCune i h m ~ 1987) already present in it sbabble and avoids words that do no t match (Menn,1983). Thus, th e child initially grows into alexicon, as i t were, by discovering correspondencesbetween adult words an d auditory feedback fromit s own babbled output. Because th e structure ofauditory feedback must correspond, in somefashion, to th e structure of motor controls thatproduced it , th e child's recognition of babble-toword correspondences presumably faci l ita tes

growth of perceptuomotor links, and their gradualextension to new words.Each word (or formulaic phrase) seems to be a

prosodic uni t (Macken, 1979), it s productionplanned as a whole (Menn, 1983). Evidence forthis comes from gestural interactions. A childoften fails to execute two different places or manners of articulation within th e same word, thusmaintaining some of th e reduplicative tendenciesof canonical babble: o g [gag], l y [jeiji],duck [tAt]. Closing an d opening gestures also interact. For example, Davis and MacNeilage (1990)report an extensive study of a child 's concurrent

babbling and speech over th e period from to 20months of age. Their data ar e rep le te with instances no t only of consonant, but of vowel an deven consonant-vowel assimilation. The latter isrevealed by th e child's preference for high frontvowels following alveolar closures an d for low,front-central vowels following labial closures. Atth e same time, these authors also report an inverse relation between consonant an d vowel reduplication: where t he child succeeds in combatingassimilation in th e open phases of a disyllable, sheoften fai ls to do so in th e closing phases, and viceversa. This demonstrates an incipient segregationofconsonants an d vowels into phonetic classes.

Th e study by Davis an d MacN eilage isparticularly important because th e child deployedan unusually large lexicon, growing from about 25words at 14 months to over 750 words a t 20months. Evidently, a child may have a substantiallexicon long before i t has fully masteredsegmental structure. The principal phonologicalachievement of this period, then, is internal

modification of th e integrated syllable bydifferentiation of it s gestural components.

Integration: From gestures to phonemesThe final step in the path from mouth sounds to

segments is the integrat ion of gestural patterns ofsyllabic constriction and opening into the coherentperceptuomotor structures we know as consonantsand vowels (Studdert-Kennedy, 1987). As i s wellknown, t he s ta tu s of th e segment is problematic.For, on th e one hand, we can neither specify th einvariant articulatory-acoustic properties sharedby all instances ofa particular consonant or vowel,n or i so lat e an y given segment as a discretearticulatory-acoustic entity within a syllable. Onthe other hand, across-word metathetic errors inadult speaking ( Spoonerisms ) attest to th efunctional role of segments in th e planning an dexecution of an utterance Shattuck-Hufnagel,1983). Moreover, such errors typically entail

exchanges between consonants an d vowels thatoccupy corresponding slots in their respectivesyllables. Since th e exchanging elements may bephysically quite disparate, it is evident that theirexchange is premised on shared function (onset,nucleus, coda) in th e formation of a syllable.

There ar e therefore two aspects to th ee me rge nce o f segments as elements of wordformation in a child's lexicon. Fir st i s t he groupingof a ll i ns tances of a particular gesture-soundpattern into a single class presumably on the basisof their perceptuomotor, or phonetic, similarity(e.g., grouping th e initial or final patterns of daddog bed etc. into th e class /dJ). Second is th edistributional analysis and grouping o f th esegesture-sound patterns into higher-order classes(consonants, vowels) on the bas is of their syllabicfunctions. These processes of phonetic categoryformation ar e perhaps analogous to thoseproposed by Maratsos an d Chalkley (1980) for th eformation of syntactic categories.

Evidence from across-word metathetic errors forth e formation of these classes in young children issparse. Th e only systematic data kno wn to mecome from Jeri Jaeger (personal communication).She reports her daughter s first across-wordmetathesis as occurring in her 27th month: ummytakes for tummy aches. Thi s was followed in he r30th month by fritty pace for pretty f ce sea tet fortea set and Bernie n Ert for Ernie n Bert.Jaeger did n ot r ep or t data on th e size of herdaughter s lexicon a t this time, no r on th ecomplexity orher multi-word utterances. B ut t hecollection of errors suggests that both were welladvanced.


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Language Development from an Evolutionary Perspective 25

Two possible selection pressures may precipitateformation of consonant an d vowel classes. Onepressure is toward economy of storage as th elexicon increases in size. We have seen that achild may accumulate an appreciable lexicon ofsome 750 words without showing signs ofindependent segmental control (Davis MacNeilage, inpress). But thi s lexicon is roughly one hundredthof th e size th at it will eventually become; an d i tseems reasonable to suppose that, as the lexiconincreases, words should organize themselves onth e basis of their shared g es tu ra l a nd soundpropertieso Recurrent patterns of laryngeal an dsupralaryngeal gesture would thus formthemselves into unitary classes of potential utilityfor recognition an d activation of lexical items cf.Lindblom, 1988; Lindblom, MacNeilage, Studdert-Kennedy, 1983).

A second possible selection pressure is towardrapid lexical access in the formation of multi-word

utterances. Several authors (e.g., Branigan, 1979;Donahue, 1986) have argued that the form of earlymulti-word combinations may be constrained bythe child's limited ability to organize an d executet he r equi red a rt icul atory sequences. One suchconstraint might be a child 's inabili ty to producetwo successive words with different initial placesof art iculat ion . Donahue (1986), for example ,describes two strategies adopted by he r son in hi sfirst two-word utterances. One strategy was toattempt only those words that conformed to hispreexisting rule of labial harmony: big book bigbird big ball were all attempted, bu t big dog an d

big cooky were adamantly refused (p.215). Th esecond strategy was to circumvent th e consonantharmony rule by adopting vocalic words lackingconsonants (e.g., where e j ~ ]w t [wac.] as pivotsthat could be comfortably combined with many ofthe words already in his vocabulary. Such findingsimply tha t t he i nt eg ra ti on of gestures intoindependent phonemic control structures, orarticulatory routines (Menn, 1983), may serve toinsulate them from articulatory competition withincompatible gestures and so facilitate their rapid,successive activation in multi-word utterances'.

ON LUS ON

I have proposed that th e study of languagedevelopment might be fruitfully cast in anevolutionary and recapitulatory framework, as asort of post-natal embryology. Language universals (like other species characters) are theendpoint of development and evolu tion), aconsequence, not a condition, oflearning a naturallanguage. Language function thus determines

language form, an d form is viewed as an aposteriori description of, rather than an a prioriprescription for, development. We ar e thus freedfrom th e habit of viewing universals as innate,language-specific properties as learned. Both setsof characters ar e t he product of a species-specificdevelopmental system, in which genet ic an denvironmental conditions cannot be separated.

I have attempted to illustrate th e approach witha sketch of th e process by which th e universalphonological categories of consonants an d vowelsmight emerge. They ar e viewed as deriving bysuccessive cycles of differentiation and integrationfrom prior non-linguistic perceptual and motorcapacities.

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language development from an evolutionary perspective [michael studdert-kennedy 1990]

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