THE AUK

A QUARTERLY JOURNAL OF

ORNITHOLOGY

Vo•,. 76 JULY, 1959 No. 3

THE STATUS OF AVIAN SYSTEMATICS AND ITS UNSOLVED PROBLEMS*

BY ERWIN STRESEMANN BIOLOGISTSall over the world have devoted the 1958 to the mernoriesof Darwin and Wallace. We ornithologistsalso should call to our attentionthe powerful impetuswhich we have receivedfrom the intellectualwork of thesegreat men, to whom we owe a better understandingof the origin of the great diversityamong . Two factorsare responsible,according to Darwin's theory, for the ever greater perfection of the living world: variation and selection. I suggestthat thesesame factors are of major importancein the interplay of ideas and concepts. Permanent advancesin our scientificunder- standingwe owe likewiseto variation and selection. Variation is the consequenceof the individuality of thosewho work on the sanhetopic. Zoologistsdiffer in their philosophicalbackground, in the extent of their knowledge,in their thoroughness,and in their gift of combination. This variability is certainlydisplayed to a high degreeby the avian systematists.What Max Fiirbringer has written about them, will forever remain true: "At various times a few fortunate individualshave existedwho were gifted with such an acute insight that it revealedto them intuitively, one might almostsay instinctively, this or that systematicrelation among related forms without the necessityof laboriousinvestigation." At the other extremethere have been sonhepoor devils who did wrong whatever they did and who were completelylost without methodology. Survivalof the fittest will decidewhich of the many competing theorieswill prevail. Only one can finally survive. Eachrevisor at- temptsto shortenthe struggleby actingas a selectivefactor. When * This paper was delivered in essenceat the Seventy-fifthAnniversary Meeting of the American Ornithologists'Union on October 1õ, 1958, in New York, during the Symposium on Contemporary Problems in . Participation of Dr. Stresemannwas made possibleby a grant from the National ScienceFoundation, which the American Ornithologists'Union gratefully acknowledges. [" Auk 270 STRESEMANN,Status o[ .4vian Systematics I.Vol. 76

he hasto synthesizea modern system of birdshe is forcedto choosea singleone among many conflicting theories, often without having the opportunityof examiningthoroughly the argumentsof the different authors. I myselfhave painfully experienced the feelingsof a tax- onomiccompiler, for I developed(1934) an eclecticsystem of classificationsome 25 yearsago. While doingso, I made a few mis- takes,as I now realize. Has not the experienceof othersbeen the same?On the wholeall the arian systemspresented in the standard worksin thiscentury are similar to eachother, since they are all based on Fiirbringerand Gadow. My systemof 1934 doesnot differ in essencefrom thosewhich Wetmore (1951) and Mayr and Amadon (1951)have recommended. The most obvious differences between the several recent classifi- cationsare in the delimitation of the higher categories. Wetmore arrangesthem in 27 orders,Mayr and Amadonin 28 orders,while I recognized49 orders(1933-34: 738-853). In otherwords: I preferred to leavethe questionof phylogeneticrelationship open in many more casesthan the authorsof the other two systems.-Today I would recooo-nize51 orders (seeAppendix). An answerhas tentativelybeen given now to someof the questions of relationshipwhich I had consideredas unsolved;others which seemedto be solvedat that time have meanwhilebeen reopenedfor discussion. The outsider who reads some of the recent critical dis- cussionsmay easilyget the impressionthat our conventionalsystem is full of errors. It seemsto me, however,that one must apply to many of the newer proposalsthe sameevaluation which wasmade by Alfred Newton in 1893,who wrote: "Someof the later attemptsto systematicarrangement are in my opinion amongthe most fallacious, and a gooddeal worsethan thosethey are intentedto supersede." A considerablepart of theseobjections are due to effortsto create an avian systemwhich specifieswith great precisionthe degreeof phylogeneticrelationship of all groups. The constructionof phylo- genetictrees has openedthe door to a wave of uninhibitedspecu- lation. Everybodymay form his own opinion on the phylogenyof the higher categoriesof birds, because,as far as birds are concerned, thereis virtually no paleontologicaldocumentation which has revealed suchimportant information on the phylogenyas has been the casewith the otherclasses of .The investigatorof arian phylogeny mustrely on indirectclues, which are nearlyalways ambiguous. It is for this reasonthat Seebohm(1890) recommendedto the systematists to ignore phylogeneticendeavors by the following sentence:"The classificationof existingbirds is the studyof a horizontalsection of July•xo•oa Sx•aA•, Statusof AvianSystematics 271 the greatbird massof the world,and oughtto form a differentand distinctsystem confined to the horizonof the presenttime." To be honest, we must admit that we mix the horizontal and the vertical systemto this very day. It is a concessionto the "horizontalsection" if, for instance,in a much usedcontemporary system the "true birds" are divided into two superorders-theImpennes or Penguinsand the Neognathaeor Typical birds--becausethe author has surelynot doubtedthe closerelationship between and and has surelyknown Simpson's(1946) important conclusions.Yet, in the samesystem, an attempt is made to group the "typical birds" accordingto the vertical principle; that is, not accordingto their apparent similarity or differencebut accordingto their presumable phylogeneticrelationship. For this reasonthe author follows Mc- Dowell (1948)by droppingthe former distinctionbetween Palaeogna- thae and . Max Fiirbringer, with his incomparablepractical experience,has frankly admittedthat the decisionin questionsof relationshipis very often basedon rather subjectiveconsiderations. Some of the more recent revisorsdo not display a similarly wise modesty. Verheyen (1958b)makes the proud assertion:"in contradistinctionto the con- ventional classifications we will introduce rational classification," which is basedon the totality of as many individual charactersas possible. This total sum of charactersVerheyen calls the "morphological potential." After equatingthe morphologicalpotential for a given group of birds with I00, he comparesit with the morphological potentialof other groupsof birds, then calculatesthe percentageof agreement,and drawsfrom this his phylogeneticconclusions. In this methodarithmetic replaces the role of intuition and of a judgment trained in functionalinterpretation. ElsewhereVerheyen (1955) states:"A clear,precise and phylogeneticallysound classification must be basedon charactersthat are practicallyinvariable and which are essentiallyimmune to the adaptationand modificationimposed by thehabitat." To thisI answer: would be indeeda simple matter if such "practicallyinvariable characters"existed. However, the relentlessmodifying power of evolutiondoes not sparea single structuralelement; any changeof the well integratedmorphological total resultseither at onceor graduallyin correlatedchanges from which neither the ,nor the muscles,nor the external features, northe behavior remain excepted. Let megive one example: the loss of the powerof flightcauses not only extensivechanges of muscles andbones of the anteriorlimb andof theshoulder girdle, but it also has effectson the and the posteriorlimb, and even on the [- Auk 272 SXRESEMANN,Status of Avian Systematics [Vol.76 feathersof the entire body, which becomegreatly simplifiedas shown by the struthionsbirds, by Apteryx,by the ,and by someof the flightlessrails. Verheyenis not an isolatedexample. Other recent studentshave believedthey have found invariablecharacters that would showthem the way throughthe labyrinth of avian diversity. Lowe relied on the structureof the in his studiesof the ,but got badlyoff the track (Bock,1958). Beecher(1953) based a new classifica- tion of the largely on the muscles. His methodsand paradoxicalconclusions have likewise receivedwell-merited criticism (Mayr, 1955). The easiestsolution was tried by certainauthors who comparedthe acceptedsystem of birds with the systemof their parasites,and who in all casesof conflictconsidered the parasitologicalevidence as more decisivethan the findingsof comparativeanatomy. This was done by Timmermann(1957). On the basisof their Mallophaga,he con- cludesthat Rostratuladoes not belongto the Limicolae (Charadrii) but with the rails,Phaethon not to the Steganopodes() but with the Laro-Limicolae(Charadriiformes), and so forth. Such an exaggeratedevaluation of the parasitologicalevidence has had the effectthat ornithologistswill utilize parasitologicalinformation only with thegreatest caution. In thisconclusion I am in entireagreement with Ernst Mayr (1957). For instance,the fact that the (Phoenicopterus)are parasitizedby two generaof Mallophagawhich otherwiseoccur only on Anatidaecan by no meansbe consideredas proof for an origin of the flamingosfrom the rather than from the Ciconiiformes.It appearsby far moreprobable, that the Mallophagahave been transferred rather recently from the waterfowl to the flamingos.This is not onlythe viewof the ornithologistMayr (1957),but alsoof the mallophaganspecialist, Dr. von Killer (1957). May I refer to still anothercase. Sometaxonomists have recently consideredit possiblethat Struthioand might form a phylo- geneticgroup because a genusof bird-lice,Struthiolipeurus, has been found on both. Von Kdler, however, informs me that close relation- shipof the mallophaganspecies found on Rhea with that foundon Struthio has not yet been proved, for their anatomicalinvestigation is still lacking. He is inclined to believethat the superficialsimilarity betweenMallophaga on thesebirds is due to convergencecaused by the similarity of structure. To give you an idea of the kind of "reforms"of our systemwhich havebeen proposed for anatomicalreasons during the pasttwenty •9] S•F_S•EMANN,Statuso)• Arian Systematics 27• (someeven earlier), I shalllist only a few, makingcomparison with the classification of •retmore. Miss Cottam (1957) has concludedfrom a detailedstudy of the skull of Balaenicepsthat the Whale-headedStork belongsto the Pelecaniformes and not to the Ciconiiformes. Some 19th century authorssuspected, or claimed,relationship between Sagittarius and Carlama. In the currentsystems one findsSagittarius with the , Cariama with the cranesand their relatives. Verheyen (1957) has again combinedthese two generain a specialorder , basinghis conclusionson their "morphologicalpotential," which in- cludesthe relative length of . Wetmore placesthe (Palamedea) as a suborderof the ,with which, accordingto Verheyen (1953), they have nothing to do. Wetmore has Opisthocomusin the , Musophagain the order Cuculiformes. Verheyen (1956) on the other hand combinesOpisthocomus with the Musophagidaein the order Musiphagiformes. Barnikol (1953) considersOpisthocomus a very isolatedspecies, which can be placed neither with the Galli nor with the Musophagidaeand representsa separateorder Opisthocomae. Wetmore and all previousauthors have placed the Dodo, Raphus, togetherwith Pezophaps,in a highlyspecialized family of the Columbi- formes. Verheyen (1957a),on the contrary,includes both, together with Goura and Caloenas,in the Caloenadidae,another fanally of the pigeons. Quite recently Liittschwager (1958) has contended that Raphus and Pezophapsdo not belong to the pigeonsat all, but rather to the rails, or to a specialorder related to the rails. Fiirbringer'scategory of ,which Wetmore had accepted essentiallywithout change,has been severelyattacked by Verheyen (1957b). He removesfrom the Gruiformesthe (Eurypyga) and the ()and combinesthem with the Jacanidae in the order Jacaniformes.Wetmore placed the Jacanidaein the Charadriiformes,but Lowe (1931)declared the Jacanidaeto be Grui- form. Thinocoryshas been placedby Verheyen(1958a) as a family with the Pterocletes ().The Pterocletes,combined with the buttonquails(Turnices) and the Mesoenatidae,form Verheyen's new order Turniciformes. Accordingto Lowe (1923, 1924),however, Thinocorysis "undoubtedlycharadriiform," while the Mesoenatidae representan entirelyisolated group of birdswith gruiformsimilarities, and theTurnices, together w•th thesandgrouse, belong near'.the pigeons. The swifts (Apodes) are accordingto Verheyen (1956a) near to the Caprimulgi,while accordingto Lowe (1939)they are not at all related [' Auk 274 S•EMANN, Statuso! zlvianSystematics I.Vol. 76 to them. Theseexamples indicate how utterly differentthe taxonomic conclusions of these two authors are. Even thoughthis is only a smallselection from recentproposals, I fear that the readersare already confused. But they need not feel ashamedof their discomfort.In my opiniononly few, if any, of these taxonomicvariants will survivethe strugglefor existence. Most of themwill be forgottenwithin a fewyears, even though they have been to a largepart the resultof laboriousand conscientiousinvestigations. They have howevercontributed to one importantrealization. tkey havemade it apparentthat therelationship of certainspecies or groups of speciesis far lessunequivocably established than one would conclude from a study of currently adopted systems,the authors of which attempt to present a simplifiedphylogenetic of birds. This "attemptto reducethe number of thebranches of the phylogenetictree, to make the ornithic tree simpler,more a noble tree with fewer but more generousbranches," as Friedmann(1955) has put it, may have didacticadvantages, but doesnot give a realisticrepresentation of the actualpattern of development. Fiirbringer's (1888) attempt at phylo- genetictree constructionconveys a more realisticview of actuality. It showsa tall trunk which after sendingout a few sidebranches (the or protocarinates),splits up completelyinto a densebush of individual branches. Thesebranches either divergewidely from each other, or elseremain closelyparallel for longeror shorterstretches. The main branchescorrespond to the 73 familiesor family groups acceptedby Fiirbringer. He attemptedto combinethese into Gentes, Subordines,Ordines and Subclasses, but emphasizedthat the difficulties and uncertaintiesgrow with eachhigher category. Let me quotehis ownwords: "At the presenttime onlyvery little is completelycertain, someis highlyprobable, the majorityof the groupingsare however probableonly to a mediumdegree." The degreeof uncertaintyhas decreased remarkably little sincethe time when the greatanatomist wrote thesewords-in spiteof all the effortsof subsequentauthors. The currently adoptedsystems have eliminatedwith goodreason many of Fiirbringer'shypothetical group- ings,particularly his Subordinesand Ordines,and retainedonly the Gentesof his system,for which they use the name orders. Wetmore hasnot beenentirely consistent in this,because his orderscorrespond sometimesto Fiirbringer'sSubordines, sometimes to his Gentes,which leadsto unharmoniousresults. No doubtWetmore has been guided bydidactic considerations--an endeavor to proposea systemthat would be convenientfor teachingpurposes. Lowe (1939) seemedto have had the sameobjective when he wasloath to place the Apodesand July1959•-• $T•ESEMANN,Statuso! ArianSystematics 275 the Trochili into two separateorders. This, he said,"is an easyway of gettingout of a difficultsituation, but it tells us nothingof their affinities.In effect,it merelytells us that the swiftsare swiftsand the hummingbirdsare hummingbirdlikebirds." For this reasonLowe decided to redefine the order Passeriformes and include in it as sub- ordersthe Passeres,Cypseli, Trochili and Pici. Howeverthis is a rather hypotheticalgrouping, which has not fully pleasedanyone. PersonallyI prefer a systemthat is as realisticas possible,a system in whichno room is given to phylogeneticspeculations, and in which the gapsin our knowledgeare frankly admitted. If onefollows these guidingprinciples one is forcedto recognizea greaternumber of the highestcategories, that is orders,than acceptedby Wetmore-indeed even more than I admitted in 1934. Combining swiftsand humming- birds in the order Macrochires and tufacos and in the order Cuculi, as ! had done following Fiirbringer, was insufficientlysup- portedby the evidenceand hassince been heavily attacked. The new attacksagainst Wetmore's system are particularly directed against those placeswhere the author adopted as an "order" one of Fiir- bringer'sSubordines, that is a categoryof "medium probability" and gaveit the samerank asone of Fiirbringer'sGentes, that is a category of "high probability." It seemsto me that the critics would have donea more usefuljob if they had been satisfiedto leaveisolated, as groups"incertae sedis," those elements from Wetmore'sstructure which theyremoved as incongruous.However, like Lowe, they thoughtthat that would "tell us nothing of their affinities,"and thus they have insertedthese building stones in a differentplace, where they fit even less. To strikea more positivenote, I shouldlike to mentionsome suc- cessfulattempts to improve the arian system. Many authors have recentlystudied the questionof the evolutionof the so-calledRatitae. This was done through ontogeneticalinvestigations by Lutz (1942), McDowell (1948), de Beer (1956), and Charlotte Lang (1956) and throughcomparative studies of the bonypalate by Hofer (1945). All agreein considering,as did Fiirbringer, the Ratitae to have originated not from flightlessProcarinatae but from flying Protocarinatae. How- ever, while Hofer postulatesfor all Palaeognathaea commonorigin from a type not unlike the Tinamids, this is not the caseaccording to McDowell. This author considersthe palaeognathicpalate to have developedfrom the neognathicpalate, and thereforenot to represent a groupcharacter of any taxonomicvalue. He thinksthat Rhea might havedescended from the Tinamidsand have acquiredby neotenya simplifiedpalate. The closeaffinity of Dinornisto Apteryxon the [' Auk 276 STRESEMANN,Status of ,4vianSystematics I.Vol. ,/6 other hand, and their great phylogeneticdistance from the other -likebirds, has recentlybeen confirmedby Starck (1955) and CharlotteLang (1956),who studiedthe endocranium.In mostother casesthe result of successfulattempts has been a clarificationon the level of the lower categoriesof the rank of family, subfamily,or . Owing to convergentevolution, similarities may develop between unrelatedlines which can deceivesytematists into assuminga phylo- geneticrelationship. Sucherrors can only be uncoveredby thorough anatomicalinvestigation. By this methodit hasbeen shown by Stolpe (1935) that the similarity betweengrebes and loonsis due to conver- gence,a similaritywhich had been interpretedby Fiirbringeras a manifestationof phylogeneticrelationship. Areadon (1951) succeeded, througha studyof the , in umnaskingthe Madagascargenus Neodrepanis (in spite of its -likefeatures) as a relative of Philepitta--that is a Mesomyodianpasserine. By the same reliable techniqueit has been shownby Wetmore (1943:306)that the wren- like SouthAmerican genus Ramphocaenus is an Oscininebird and not a Mesomyodian,as had been believed formerly for zoogeographic reasons. Mayr (1931) showedthat the Pitta-like Papuan genus Melampittadoes not havean oligomyodianbut a polymyodiansyrinx and that it thereforedoes not belongto the Pittidae. Syrinxstructure is unfortunatelyof no help in the taskof a subdivisionof the Oscines with their immenselygreat number of specieswhich have enteredthe mostdiverse ecological niches. In order to separatewithin this great mass of forms those which are related from those which have become similar through convergence,one must searchfor combinationsof novel clues,including such as revealedby ethologicalstudies. A combinationof charactershas quite recentlybeen used by Deignan (1958)for clarifyingthe relationship of the curiousgenus Apalopteron fromthe BoninIslands, hitherto regarded as an aberrantPycnonotid or Timaliid. He foundnot onlythat thetongue of Apalopteronsuited the generaltype of the Meliphagidae,but also other structures,like the naresperviae, and he refersbesides to the type of ,which supportsthe morphologicalarguments. Beecher (1951) has, on the basisof the conformationof the jaw muscles,broken up the family Coerebidaeand assigned its genera to the Thraupidaeand the Paruli- dae. His conclusionsappear to be valid. However,it has become apparentthat the taxonomicusefulness of this structuralcharacter, utilizedby Beecher,is in generalrather questionable. In manycases thesystematist who wants to placean aberrantspecies of is essentiallyforced to rely on intuition and courage. I am amazedat thecourage which is apparentin someof themost recent attempts to July1õ5õ..]I STRESEMANN,Statuso[ AvianSystematics 277 classifythe Oscines,for I am one of thosetimid soulsin whosevocabu- lary the word "perhaps" occursvery frequently. My own system containstherefore many monotypicgenera and monotypicfamilies of Oscines. The searchfor casesin which systematistshave so far been deceived by convergencewill surelyproduce further surprisingdiscoveries. We can thereforelook forwardto much usefulactivity by thosewho plan to devote their energy to the field of bird . I recommend that he confinehis comparativestudies to representativesof the same order, family, or genus,the closerelationship of which facilitates separationof relatively recent[unctional modificationsfrom the more stable and taxonomicallymore important structures. He will be fascinatedby this topic, for every true naturalisthas been uplifted by the discoveryof interrelationsbetween form and function. In this fieldhe is sureto moveon firm groundand doesnot needto bridge the gapsin our knowledgeby flimsyspeculations. Professionalzoologists tended in formerdays to look downat occupa- tion with systematiccategories below the levelorders. They founda greaterchallenge to their ingenuityin the searchfor the major lines of . Within recent decades however there has been a com- pleterevaluation of the scientificsignificance of the systematicsof the lowercategories. It is now recognizedthat it is as importantas is a knowledgeof histologyand cytologyfor an understandingof the integralstructure of the body. But asfar asthe problem of therelationship of theorders of birdsis concerned,so manydistinguished investigators have labored in this fieldin vain,that little hopeis left for spectacularbreak-throughs. Onlylucky discoveries of can help us, but the chances of making suchfinds are verysmall. Simpson(1946) has recently pointed out thatthe evolution of birdshas made virtually no progresssince early Tertiarytimes, quite in contrastto thesituation among the . The separationof the existingorders of birdsfrom eachother had alreadytaken place in theCretaceous, if not even in theJurassic. It is thereforenot surprising,that no light has beenshed on avian phylogenyby the fewwell preservedfossil remains from the upper .Hesperornis aswell as were already highly specializedand entirelydifferent from eachother, at leastas muchas are todaya penguinand a gull. The bird life of the Tertiarywas richer in differenttypes than the avifaunaof today. Many branchesof the avian tree died before or during the Pleistocene. In viewof the continuingabsence of trustworthyinformation on the 278 SmaSh.MANN, Status o[ AvianSystematics LVol.[ Auk76 relationshipof the highestcategories of birds to eachother it becomes strictlya matterof conventionhow to groupthem into orders. Science endswhere comparativemorphology, comparative , com- parativeethology have failed us after neary200 yearsof efforts. The rest is silence.

APPENDIX

SUGGESTEDAVlAN ORDERS In parenthesesare indicated the correspondingtaxa and nomenclatureof the Wetmore (1951) classification(names ending in -formes representingorders). Colymbiformesof that (1951)classification was changed to Podicipediformesin the A.O.U. Check-list (1957). Struthiones (Struthioniformes) Rheae (Rheiformes) Casuarii () Aepyornithes (Aepyornithiformes) Apteryges (Dinornithiformesq- Apteryglformes) Crypturi (Tina•niformes) Galli (Galliformes) Opisthocomi (Suborderof Galliformes) Turnices (Suborderof Gruiformes) Columbae (Suborderof Columbiformes) Pterocletes (Suborderof Columbiformes) Ralli (SuperfamilyRalloidea of SuborderGrues of Gruiformes) Heliornithes (Suborderof Gruiformes) Mesoenades (Suborder of Gruiformes) Jacanae (SuperfamilyJacanoidea of SuborderCharadrii of Charadriiformes) Thinocori (SuperfamilyThinocoroidea of SuborderCharadrii of Charadrii- formes) Rhynocheti Suborderof Gruiformes) Eurypygae Suborderof Gruiformes) Cariamae Suborderof Gruiformes) Psophiae Fam. Psophiidaeof SuperfamilyGruoidea of SuborderGrues of Grniformes) Grues Fam. Gruidae q- Aramidaeof SuperfamilyGruoidea of Suborder Gruesof Gruiformes) Otides Suborderof Gruiformes) Laro-Limicolae SubordersCharadrii q- Lari of Charadriiformes) Alcae Suborderof Charadriiformes) Gaviae ) Podicipedes (Colymbiformes,now Podicipediformes) Sphenisci (Sphenisciformes) Tubinares (Procellariiformes) Anseres (Suborderof Anseriformes) Anhimae (Suborderof Anseriformes) Steganopodes (Pelecaniformes) Phoenicopteri (Suborderof Ciconiiformes) Gressores (SubordersArdeae q- Balaenicipitesq- Ciconiae of Ciconiiformes) Accipitres () Musophagae (Suborderof Cuculiformes) Cuculi (Suborderof Cuculiformes) Psittaci (Psittaciformes) Striges (Strigiformes) Caprimulgi () Coraciae (FamiliesCoraciidae q- Leptosomatidaeq- Brachypteraciidaeof SuborderCoracii of ) Halcyones (SuperfamilyAlcedinoidea of SuborderAlcedines of Coraciiformes) Meropes (Suborderof Coraciiformes) Momoti (SuperfamilyMomotoidea of SuborderAlcedines of Coraciiformes) July] 279 •959a STRESEMANN,Status of Arian Systematics

Todi (SuperfamilyTodoidea of SuborderAloedines of Coraciiformes) Upupae (FamiliesUpupidae + Phoeniculidaeof Suborder Coraeli + Sub- order Bucerotesof Coraciiformes) Trogone• (Trogoniformes) Colii (Coliiformes) Apodes (Suborderof ) Trochili (Suborderof Apodiformes) Pici () Passeres (Passeriformes)

LITERATURE CITED

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(Above)Nestling Groove-billed Ani aboutsix daysold, its feathersjust beginning to unsheath. Near Tela, Honduras,August 15, 1930. (Below) The samenestling 24 hourslater, showingrapid advancein leathering. Near Tela, Honduras,August 16, 1930. (Photos. hy A. F. Skutch.)