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Title: Form and Function

A Contribution to the History of Animal Morphology

Author: E S (Edward Stuart) Russell Release Date: January 23, 2007 [EBook

#20426]

Language: English

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A CONTRIBUTION TO THE

HISTORY OF ANIMAL MORPHOLOGY

By E S RUSSELL,

M.A., B.Sc., F.Z.S

have been corrected, all

other inconsistencies in

spelling and punctuation

are as in the original

PREFACE

This book is not intended to be a full ordetailed history of animal morphology: acomplete account is given neither ofmorphological discoveries nor ofmorphological theories My aim has beenrather to call attention to the existence ofdiverse typical attitudes to the problems

of form, and to trace the interplay of thetheories that have arisen out of them.

The main currents of morphologicalthought are to my mind three—thefunctional or synthetic, the formal ortranscendental, and the materialistic ordisintegrative

The first is associated with the greatnames of Aristotle, Cuvier, and von Baer,and leads easily to the more open vitalism

of Lamarck and Samuel Butler Thetypical representative of the secondattitude is E Geoffroy St Hilaire, and thishabit of thought has greatly influenced thedevelopment of evolutionary morphology.The main battle-ground of these twoopposing tendencies is the problem of the

relation of function to form Is function themechanical result of form, or is formmerely the manifestation of function oractivity? What is the essence of life—organisation or activity?

The materialistic attitude is notdistinctively biological, but is common topractically all fields of thought It datesback to the Greek atomists, and thetriumph of mechanical science in the 19thcentury has induced many to acceptmaterialism as the only possible scientificmethod In biology it is more akin to theformal than to the functional attitude

In the course of this book I have nothidden my own sympathy with thefunctional attitude It appears to meprobable that more insight will be gained

into the real nature of life and organisation

by concentrating on the active response ofthe animal, as manifested both inbehaviour and in morphogenesis,particularly in the post-embryonic stages,than by giving attention exclusively to thehistorical aspect of structure, as is thecustom of "pure morphology." I believe

we shall only make progress in thisdirection if we frankly adopt the simpleeveryday conception of living things—which many of us have had drilled out ofus—that they are active, purposefulagents, not mere complicated aggregations

of protein and other substances Such anattitude is probably quite as soundphilosophically as the opposing one, but Ihave not in this place attempted anyjustification of it I have touched very

lightly upon the controversy betweenvitalism and materialism which has beenrevived with the early years of the presentcentury It hardly lends itself as yet tohistorical treatment, and I could hardlyhope to maintain with regard to it thatobjective attitude which shouldcharacterise the historian.

The main result I hope to have achievedwith this book is the demonstration,tentative and incomplete as it is, of theessential continuity of animal morphologyfrom the days of Aristotle down to ourown time It is unfortunately true thatmodern biology, perhaps in consequence

of the great advances it has made incertain directions, has to a considerableextent lost its historical consciousness,

and if this book helps in any degree tocounteract this tendency so far as animalmorphology is concerned, it will haveserved its purpose.

I owe a debt of gratitude to my friends DrJames F Gemmill and Prof J ArthurThomson for much kindly encouragementand helpful criticism The credit for theillustrations is due to my wife, MrsJehanne A Russell One is from Nature;the others are drawn from the originalfigures

E S R

C HELSEA , 1916.

V Etienne Geoffroy St Hilaire 52

VI The Followers of Etienne

England—Richard Oven 102

X The Embryological Criterion 133

XII The Close of the

4 Ideal Typical Vertebra (Owen.) 102

5 Natural Typical Vertebra

Vertebrate Embryo (Von

8 Gill-slits of the Pig Embryo

9 Meckel's Cartilage and

Ear-ossicles in Embryo of Pig

10 Cranial Vertebræ and Visceral

Arches in Embryo of Pig

15 The Five Primary Stages of

of the soul, and he made some goodguesses at the mechanism of the organs ofspecial sense He showed that, contrary tothe received opinion, the seminal fluid didnot originate in the spinal cord Twocomparisons are recorded of his, one thatpuberty is the equivalent of the floweringtime in plants, the other that milk is theequivalent of white of egg.[1] Both showhis bias towards looking at the functionalside of living things The lattercomparison reappears in Aristotle.

A century later Diogenes of Apolloniagave a description of the venous system

He too placed the seat of sensation in thebrain He assumed a vital air in all livingthings, being in this influenced byAnaximenes whose primitive matter was

infinite air In following out this thought hetried to prove that both fishes and oystershave the power of breathing.[2]

A more strictly morphological note isstruck by a curious saying of Empedocles(4th century B.C.), that "hair and foliageand the thick plumage of birds are one."[3]

In the collected writings of Hippocrates

Hippocraticum, of which no part is later

than the end of the 5th century, there arerecorded many anatomical facts Theauthor of the treatise "On the Muscles"knew, for instance, that the spinal marrow

is different from ordinary marrow and hasmembranes continuous with those of thebrain Embryos of seven days (!) have all

the parts of the body plainly visible Work

on comparative embryology is contained

in the treatise "On the Development of theChild."[4]

The author of the treatise "On the Joints,"which Littré calls "the great surgicalmonument of antiquity," is to be creditedwith the first systematic attempt atcomparative anatomy, for he compared thehuman skeleton with that of otherVertebrates

Aristotle (384-322 B.C.)[5] may fairly besaid to be the founder of comparativeanatomy, not because he was speciallyinterested in problems of "puremorphology," but because he describedthe structure of many animals and

classified them in a scientific way Weshall discuss here the morphological ideaswhich occur in his writings upon animals

—in the H ISTORIA A NIMALIUM , the De

P ARTIBUS A NIMALIUM , and the De

Generatione Animalium.

T h e H ISTORIA A NIMALIUM is a mostcomprehensive work, in some ways thefinest text-book of Zoology ever written.Certainly few modern text-books take such

a broad and sane view of living creatures.Aristotle never forgets that form andstructure are but one of the manyproperties of living things; he takes quite

as much interest in their behaviour, theirecology, distribution, comparativephysiology He takes a special interest in

reproduction The H ISTORIA A NIMALIUM

contains a description of the form andstructure of man and of as many animals asAristotle was acquainted with—and hewas acquainted with an astonishingly

large number The later D E P ARTIBUS

Animalium is a treatise on the causes of

the form and structure of animals Owing

to the importance which Aristotleascribed to the final cause this workbecame really a treatise on the functions

of the parts, a discussion of the problems

of the relation of form to function, and theadaptedness of structure

Aristotle was quite well aware that each

of the big groups of animals was builtupon one plan of structure, which showedendless variations "in excess and defect"

in the different members of the group But

he did not realise that this fact ofcommunity of plan constituted a problem

in itself His interest was turned towardsthe functional side of living things, formwas for him a secondary result of function.Yet he was not unaware of facts of formfor which he could not quite find a place

in his theory of organic form, facts of formwhich were not, at first sight at least, facts

of function Thus he was aware of certainfacts of "correlation," which could not beexplained off-hand as due to correlation ofthe functions of the parts He knew, forinstance, that all animals without frontteeth in the upper jaw have cotyledons,while most that have front teeth on both

jaws and no horns have no cotyledons (De

Gen., ii 7).

Speaking generally, however, we find inAristotle no purely morphologicalconcepts What then does morphology

owe to Aristotle? It owes to him, first, a

great mass of facts about the structure of

a ni ma l s ; second, the first scientific

classification of animals;[6] third, a clear

enunciation of the fact of community of

plan within each of the big groups; fourth,

an attempt to explain certain instances of

the correlation of parts; fifth, a pregnant

distinction between homogeneous and

generalisation on the succession of forms

in development; and seventh, the first enunciation of the idea of the É CHELLE DES

êtres.

(1) What surprises the modern reader of

t h e H ISTORIA A NIMALIUM perhaps morethan anything else is the extent and variety

of Aristotle's knowledge of animals Hedescribes more than 500 kinds.[7] Notonly does he know the ordinary beasts,birds, and fishes with which everyone isacquainted, but he knows a great dealabout cuttlefish, snails and oysters, about

crabs, crawfish (Palinurus), lobsters,

shrimps, and hermit crabs, about urchins and starfish, sea-anemones andsponges, about ascidians (which seem tohave puzzled him not a little!) He hasnoticed even fish-lice and intestinalworms, both flat and round Of the smallerland animals, he knows a great manyinsects and their larvæ The extent of hisanatomical knowledge is equally

sea-surprising, and much of it is clearly theresult of personal observation No one canread his account of the internal anatomy of

the chameleon (H IST A NIM , ii.), or his

description of the structure of cuttlefish

(H IST A NIM , iv), or that touch in the

description of the hermit crab (Hist.

Anim., iv.)—"Two large eyes not .

turned on one side like those of crabs, butstraight forward"—without beingconvinced that Aristotle is speaking ofwhat he has seen Naturally he could notmake much of the anatomy of small insectsand snails, and, to tell the truth, he doesnot seem to have cared greatly about theminutiæ of structure He was too much of

a Greek and an aristocrat to care aboutlaborious detail

Not only did he lay a foundation forcomparative anatomy, but he made a realstart with comparative embryology.Medical men before him had known manyfacts about human development; Aristotleseems to have been the first to study in anydetail the development of the chick Hedescribes this as it appears to the nakedeye, the position of the embryo on theyolk, the palpitating spot at the third day,the formation of the body and of the largesightless eyes, the veins on the yolk, theembryonic membranes, of which hedistinguished two.

(2) Aristotle had various systems ofclassifying animals They could beclassified, he thought, according to theirstructure, their manner of reproduction,

their manner of life, their mode oflocomotion, their food, and so on Thus

y o u might, in addition to structuralclassifications, divide animals intogregarious, solitary and social, or landanimals into troglodytes, surface-

dwellers, and burrowers (Hist Anim., i.).

He knew that dichotomous classifications

were of little use for animals (De

Partibus, i 3) and he explicitly and in so

many words accepted the principle of all

"natural" classification, that affinities must

be judged by comparing not one but thesum total of characters As everyoneknows, he was the first to distinguish thebig groups of animals, many of whichwere already distinguished roughly by thecommon usages of speech Among his

Sanguinea he did little more than definewith greater exactitude the limits of thegroups established by the popularclassification Among the "exsanguineous"animals, however, corresponding to ourInvertebrates, he established a much moredefinite classification than the popular,which is apt to call them indiscriminately

"shellfish," "insects," or "creeping things."

He went beyond the superficialities ofpopular classification, too, in clearlyseparating Cetacea from fishes He hadsome notion of species and genera in oursense He distinguished many species of

cuttlefish—O CTOPUS (P OLYPUS ) of which

there were many kinds, Eledone

(Moschites) which he knew to have only

one row of suckers while Octopus has

t w o , A RGONAUTA , N AUTILUS , S EPIA, and

apparently L OLIGO MEDIA (= his Teuthis)

and L VULGARIS (or forbesii) which seems

to be his Teuthos He had a grasp of theprinciples which should be followed injudging of the natural affinities of species.For example, he knew that the cuckooresembles a hawk "But," he says, "thehawk has crooked talons, which thecuckoo has not, nor does it resemble thehawk in the form of its head, but in theserespects is more like the pigeon than thehawk, which it resembles in nothing but itscolour; the markings, however, upon thehawk are like lines, while the cuckoo is

spotted" (H IST A NIM , Cresswell's trans.,

p 147, London, 1862)

The groups he distinguished were—man,viviparous quadrupeds, oviparous

quadrupeds, birds, fishes, Cetacea,Cephalopoda, Malacostraca (= higherCrustacea), Insecta (= annulose animals),Testacea (= molluscs, echinoderms,ascidians) A class of Acalephæ,including sea-anemones and sponges, wasgrouped with the Testacea The first fivegroups were classed together assanguineous, the others as exsanguineous,from the presence or absence of redblood.

Besides these classes "there are," he says,

"many other creatures in the sea which it

is not possible to arrange in any classfrom their scarcity" (Creswell, LOC CIT ,

p 90)

(3) Aristotle's greatest service tomorphology is his clear recognition of the

unity of plan holding throughout each ofthe great groups.

He recognises this most clearly in the case

of man and the viviparous quadrupeds,with whose structure he was best

acquainted In the H ISTORIA A NIMALIUM hetakes man as a standard, and describes hisexternal and internal parts in detail, thenconsiders viviparous quadrupeds andcompares them with man "Whatever parts

a man has before, a quadruped hasbeneath; those that are behind in man form

the quadruped's back" (Cresswell, loc.

cit., p 26) Apes, monkeys, and

Cynocephali combine the characteristics

of man and quadrupeds He notices that allviviparous quadrupeds have hair.Oviparous quadrupeds resemble the

viviparous, but they lack some organs,such as ears with an external pinna,mammæ, hair Oviparous bipeds, or birds,also "have many parts like the animalsdescribed above." He does not, however,seem to realise that a bird's wings are theequivalent of a mammal's arms or fore-legs Fishes are much more divergent; theypossess no neck, nor limbs, nor testicles(meaning a solid ovoid body such as thetestis in mammals), nor mammæ Instead

of hair they have scales

Speaking generally, the Sanguinea differfrom man and from one another in theirparts, which may be present or absent, orexhibit differences in "excess and defect,"

or in form Unity of plan extends to all theprincipal systems of organs "All

sanguineous animals have either a bony or

a spinous column The remainder of thebones exist in some animals; but not inothers, for if they have the limbs they havethe bones belonging to them" (Cresswell,

LOC CIT , p 60) "Viviparous animals with

blood and feet do not differ much in theirbones, but rather by analogy, in hardness,softness, and size" (Cresswell, LOC CIT ,

p 59) The venous system, too, is builtupon the same general plan throughout theSanguinea "In all sanguineous animals,the nature and origin of the principal veinsare the same, but the multitude of smallerveins is not alike in all, for neither are theparts of the same nature, nor do all

possess the same parts" (Cresswell, loc.

cit., p 56) It will be noticed in the first

and last of these three quotations that

Aristotle recognises the fact of correlationbetween systems of organs—betweenlimbs and bones, and between blood-vessels and the parts to which they go.Sanguineous animals all possess certainorgans—heart, liver, spleen, kidneys, and

so on Other organs occur in most of theclasses—the œsophagus and the lungs

"The position which these parts occupy isthe same in all animals [sc Sanguinea]"

(Cresswell, loc cit., p 39).

Unity of plan is observable not only in theSanguinea, but also within each of theother large groups Aristotle recognisesthat all his cuttlefish are alike in structure.Among his Malacostraca he comparespoint by point the external parts of the

carabus (Palinurus), and the astacus

(Homarus), and he compares also the

general internal anatomy of the various

"genera" he distinguishes As regardsTestacea, he writes, "The nature of theirinternal structure is similar in all,especially in the turbinated animals, forthey differ in size and in the relations ofexcess; the univalves and bivalves do not

exhibit many differences" (Cresswell, loc.

cit., p 83) There is an interesting remark

about "the creature called carcinium"(hermit-crab), that it "resembles both theMalacostraca and the Testacea, for this inits nature is similar to the animals that arelike carabi, and it is born naked"(Cresswell, LOC CIT , p 85) In the last

phrase we may perhaps read the firstrecognition of the embryological criterion

With the recognition of unity of planwithin each group necessarily goes therecognition of what later morphology callsthe homology of parts The parts of ahorse can be compared one by one withthe parts of another viviparous quadruped;

in all the animals belonging to the sameclass the parts are the same, only theydiffer in excess or defect—these remarks

are placed in the forefront of the Historia

Animalium Generally speaking, parts

which bear the same name are forAristotle homologous throughout the class.But he goes further and notes the essentialresemblance underlying the differences ofcertain parts He classes together nailsand claws, the spines of the hedgehog, andhair, as being homologous structures Hesays that teeth are allied to bones,

whereas horns are more nearly allied to

skin (H IST A NIM , iii.) This is an

astonishingly happy guess, consideringthat all he had to go upon was theobservation that in black animals the hornsare black but the teeth white One cannotbut admire the way in which Aristotlefixes upon apparently trivial andcommonplace facts, and draws from themfar-reaching consequences He often goeswrong, it is true, but he always errs in thegrand manner

While Aristotle certainly recognised theexistence of homologies, and even had afeeling for them, he did not clearlydistinguish homology from analogy Hecomes pretty near the distinction in thefollowing passage After explaining that in

animals belonging to the same class theparts are the same, differing only in excess

or defect, he says, "But some animalsagree with each other in their parts neither

in form nor in excess and defect, but haveonly an analogous likeness, such as a bonebears to a spine, a nail to a hoof, a hand to

a crab's claw, the scale of a fish to thefeather of a bird, for that which is afeather in the bird is a scale in the fish"(Cresswell, LOC CIT , p 2) One of these

comparisons is, however, a homology not

an analogy, and the last phrase throws alittle doubt upon the whole question, for it

is not made clear whether it is position orfunction that determines what areequivalent organs

In the D E P ARTIBUS A NIMALIUM there