Osborn 1902

 

The Law of Adaptive Radiation

Henry Fairfield Osborn The American Naturalist ,

Vol. 36, No. 425. (May, 1902), pp. 353-363.

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AMERICAN

T H E LAW O F

NATURALIST

A D A PT IV I;

R-IDIATION .

O S E of of t h e e s s e n t i a l f e a t u r e s of of d i v e r g e n t e v o l u t io io n a s conceived in t h e br an ch in g sy ste m successively successively cleve clevelo lopcd pcd by Lamarck, Darwin, Hu sley, and Cope has been termed by the w r ~ t e r < a d a p ti t i v e a d i at a t i o n ." ." T h i s t e r m s e e m s t o e x p r e s s n l os os t clearlyy th e idea of differentiation of h abit in seve ral directions clearl from p r im i m i t iv i v e t y p e , a s s h o ~ v n n t h e a c c o m p an a n y i n g d ia ia g r a m s . T h c law is a fanliliar fanliliar one it resu lts in in th e fornlation of of analogous radii radii in different group s of anim a ls. T h e first first com preh e n s i v e i l l u s t r a t io io n of o f t h e l aw aw k n o n n t o t h e w r i t e r i s t h a t u n d e r t h e h e a d in in g s Homologous Groups and Heterology," in Cope's paper of 1868 o n t h e O r i g i n of G e n e r a , " r e p r i n t e d i n t h e Ori rf~c of the Fittest (pp. 95-106 . T h i s b r il il l ia ia n t e s s a y i s m a r r e d o n l y b y g r e a t c o n f u s i o n i n t h e u s e of t e r m s b u t t h e parallelism paralle lism s in in un relate d gro ups of am phi bian s ancl ancl of m a m nlals srich as nlarsupials and placentals, as first observed by O w e n , a r e c l ea ea rrll y b r o u g h t o u t . I n t he pres ent pap er citatio ns fro111 earlier essays of m y own m a y b e g i v e n b e a r i n g u p o n ,oz.nci.rzZ nir nir np npti ti;v ;v r nd int io? ~ n d t h e ind epe nde nt production of of an alogou s radii uncle unclerr th e con verg ent 3j3

 

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p r i n c i p l e s of h o n ~ o p l a s y ,parallelism a n d c o n v e r g e n c e , w h i c h , a s s h o ~ v n n t h e l a s t n u m b e r of t h e .i ;r tz ~i ~i- lz li lis f , a r e b y n o m e a n s s y n o n y m o u s t e r m s o r i d e n t iicc a l p r o c e s ssee s . T h e a l to to g e t h e r s i m i l a r l a\ a\ \\-- of Zo~rrZ clajti.iv rrril7rizfz'oi~ 1- i n c i p i e n t d i v e r g e n c e o n a s ~ n a l l e r c a l e i n a s i n g l e l o c a l i ty t y m a y 11 11ow b e I n o re re c l e a r ly ly d e v e l o p e d . T h i s i d e a of r a d i a t i o n b e c o ~ l l e sa s a m e a n s of of i n t e r p r e t a t i o n , a n d a way of i m a g i n i n g t h e r e l a t i o n s ooff e x t i n c t a n d li li v i n g faunze. A s percei percei\. \.ecl ecl b y C o p e , i t a p p l i e s b o t h o n a s m a l l a n d o n a v as t s cale.

I n t h e R i s e of t h e ; I llaa m m a l ia ia 193 p . 3 0 - 3 3 the ancient M e s o z oi oi c ( L I e s e u t h e r i a ) a n d m o d e r n C e n o z o i c ( C e n e u t h e r i a ) d i f f e r e n t i a t i o n of of t h e p l a c e n t a l s ( E u t h e r i a ) w a s s p o l te te n of of a s follows : T h e I 'u 'u e rc rc o i s e s ~ e n t i a l l y n a r c h a i c f a u n a , a n d i s t o h e r e g a r d e d a s t h e c lim a x of of th e f ir st st p e r io io c l of p la c e n ta l d if f e r e n tia tio n : a c u lm in a tio n of of th c f i rs rs t a t t e m p t s o f n a t i l r e t o e s t a b l i sh sh i n s e c t i v o r o u s ~ c a r n i v o r o u s , nd h e r b i vo v o ro r o u s g r o u p s . T h e s e a t t e m p t s b e g a n i n t h e C r e ta t a c e o u s, s, a n d s o m e o f t h e t y pe p e s t h u s p r o d u c e d cliecl o u t i n t h e P u e r c o , s o n le le i n t l le le I Y a s a t c h a ~ i d n r i d g e r o n ly a fexv f le s h -e -e n te r s s u r l- iv e to th e L Z io io c en en e . I t is no st i m p o r i O ia~ll d ir e c tio n s t a n t t o g r a s p c l e a rl r l y t h e i d e a o f t h i s ~ ~ c l i ( ~ f i i ~ ~i ii ~i iz f~i i ~ f ill o f t h i s o l d P u e r c o f a u n a , r e s u l ti ti n g i n f o r n l s l i k e t h e m o d e r n i n ssee c t i\ i\ . or o r e ss,, r o d e n t s , b e a r s, s , d o g s a n d c a t s , m o n k e y s , s l o t h s , h u no n o c lo lo n t a n d s e l e n o d o i ~ t u n g u l a t e s , a n d l o p h o d o n t u n g ~ 1l 1 l a :e :e s . T h i s \ \\- as as a n i n d e p e n d e n t r a d i a t i o n of p l a c e n t a l s , l i k e t h e A ~ i s t r a l i a n a d i n t i o n o f m a r s u p i a l s . S o m e o f t h e l e a s t s p e ci ci a llll z e ti ti s p u r s of of t li li is is r n d i a ti ti o n a p p e a r t o h a v e s u r v iv iv e cl cl a n d b e c o m e t h e c e n te r s o f th e s e c o n d o r n iic iicii-T T eerr tia tiarr . r a d ia tio n . f r o m wh ic h o u r m o c le r ii f a ~ u n ah a h a s e ll.o .oli li-- eecc ll.. Ye t we h a v e n o t in a s in g le c a s e s u c c e e d e d in tl. tl.aa c in in g tlie cli clill-ee cctt c o n n e c tio n .' T o s u n l u p , we f ind ind o n tlie N o r t h ; i ~ n e r i c a n o i l t i n e nt n t e v id id e n c e o f t h e r is is e a n t i d e c l in in e a n d d i s a p p e a r a n c e o f m o n o t r e m e s a n d m a r s ~ i p i a l s . a n d t w o g r e a t p e r i od o d s of p l a ce c e n t al al n/ccit.i~frizrl rizrl iitfioii b e g i t l n i n g i n t h e L I e s o z o i c , r e a c h i n g a r a c lia tio n , th e n/ccit.i~f c liin a s in th e P u e r c o a1lc1 u n k n o lr -n -n p o s t- l'l' iie rc rc o , a nncc l s e n d in g ith ith s p u r s into t he hig he r Terti Tertiar ar ., an d t le //ioiJt.i.n r.nil i izti oi~ e a c l i i n g i t s c l i n l a x i n t h e L l i o ce c e n e , an an ccll s e n d il i l l g d o ~ v n o 11s o u r e s is tin g ty p e s . This sta tz~ ue nt 1x 1x5 b e e n indclified by subsequent discovery.

 

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I n t h e O r i g i n of t h e M a m m a l s developed as follo~vs T o g uuii ccll e o u r

R A D f r l TIOA L: 99,

355

p . 92 tlae idea was

p e c u l a t i o n i n t l i e ~ ~ n l c n o ~pvr en - T e r t i a r y p e ri ri oc o c l, l, w e m a j

g a th e r c e r ta in p o s itiv e p r illc illc ip le s f r o m tlie 1; 1;n oonn .n .n e v o lu tio n o f th e T e r tia r h l a m m a l i a . F i r s t , n e Icnon. that iz ii fl jf iz ~ c~ h aracteristic of all ~~iaiZthfi'o ~iaiZthfi'oiz, iz,haracteristic v e r t e l ~ r a t e s ,a ,a n d b e a u tif u lly illu s tr a te d a n lo llg lie p tili a , is in in a v e r y llig llig ll c le gr gr e e d is tin c tiv e of of 5 I a m m a lia . b e c a u s e of th e i r s u p e r io r p la s tic tic ity . T i l e fbcnZ-types, o r n lo s t p r im itiv e f o r m s o f th e r a c lia tio tio n s , I - IT IT - , V J L , ) . ~ cerfiziii  sii~irZ ~ ~ i ~ ~ ~ ~ ~ Ls .t Zi ~ Z z '~i Lz i,l /L, ~~, ,e ~:,o. f).o /r.s o r oiit iz i7,ot. i7,ot.oo / / s ,fiut ,fiut-i/~. -i/~., I t i s il il o te te l v or o r t llii y t h a t i n t h e e v o l u t i o n of of e a c h r a d i a t i o n , s o f a r a s lv lv e It It no no iv iv a t p r e s e n t , l a n d t y p e s a n d o r g a n s a r e i n v a r i a b l y p ri r i m i ti ti v e , a n d w a t e r t y p e s a n d o r g a n s a r e s e c o n d a r y , e x a c t l y a s w e fi f i n d a m o n g t h e I i e p t i l i a . I11 f a c t , we 1 3a 3a vvee n o t f o u lld a s iilgle iilgle iils ta n c e in \v liic liichh a m a m m a l o r r e p tile s e r ie s i s I < n o ~ v no no b e t r a n s f o r m e d f r o m a w a t e r i n t o a l a n d t y p e : i t i s a l w a y s tl tl i e r e v e r s e . T l i e r e i s c e r t a i n ly ly n o e v i d e n c e f o r a c e t o i d ( X l l ~ r e c h t ) s te in o f t h e ~ T a m m a l s . A g a i n , it it iiss o bv b v i o u s t h a t i l e i t l ~ e rc rc a r i l i v o r o u s n o r h e r b i v o r o u s ty p e s with liig liig llly llly s p e c ia liz e d o r r e d uc uc e c l te e th a n ti f e e t c a n lje s o c e n t r a l a s i i l s e c t i v o r o u s a n d o m n i v o r o u s t y p e s . I n f a c t , ti ti l e I i l s e c ttii v o r e s a m o n g P l a c e ~ l t a l s . n tt11 O p o s s u ~ n s n l o n g R l a r s u p i a l s, s , a r e t i le le on onl - a n i n l a l s w h i c h h a v e p r e b e r v ed ed t h e d e n t a l pprr o ttoo t - ppee c l o se se t o t h a t o f t li li e P r o m a n i n ~ a l .

T h e r a d i a t i o n s s p o k e n of of i n t h i s e s s a y w e r e

:

T h e G e o l o g i c a l ant1 F a u n a l R e l n t i o n s of E u r o p e a n d A m e r i c a d u r i n g t h e T e r t i a r y P eerr iioo tl tl , a r id t h e T h e o r y of t h e S u c c e s s i v e I n v a s i o n s of of a n A f r i c a n F a u n a (loo), t h e s u b j e c t w a s f u r t h e r d e v e l o l ~ e r l s f o l l o ~ v s I

a subsequent paper,

I q o v it is is a we ll-k ll-k n o iv n p r in c ip le o f z o o lo g ic a l e v o lu tio n t h a t a n is o la te d r e g i o n , if if la r g e a n d s u f f ic ic ie n tly v a r ie d in its to ~ o g r a p liy , s o il. c lim a te , a n ti v e g e t a t i o n , w i l l g i v e r i s e t o a d iv e r s if ie d f a u n a a c c o rd rd in,q in,q to t h e la w o f a d a p t i v e r a d i a t i o n f r o m p l. l. im im i ti ti vvee a n d c e n t r a l t > - pe pe s . B r a n c h e s ill s p r i n g o ff i n a l l d i r e c t i o n s t o t a k e a c l v a ~ l t a q e o f e v e r y p o s s i l ~ l c o p p o r t u n i t y o f s e c u r i n q f o o d . T h e i n o di d i f iicc a t io i o n s ~v ~ v l iiii c ll ll a n i m a l s u n d e r g o i n t h i s a d a p t iivv e r a d i a t i o n a r e i a r g e i y o f m e c h a ~ l i c a l a t u r e . t h e y a r e l i nnll i te te t i i n t i u m l ~ e s

 

356

T H E A l f E I I I C A ,Y 11.-AT U R A L I S T .

[ V O I ~XXXV , I,

ancl kind by hereditary, stirp, or germinal influences, and thus result it1 tlie inciependent inciependent erol utio n of of similar types in widely sep ara ted regiolis regiolis under tlie Zn.ii~ Zn.ii~c~' c~' (zi.alZcli.r//i oi iloi/cc~ ing~.~

11. A D A P T I V E A D I A ~ I Of NO R D I L RASS D F U I I L I E S s GEAREOGRAPIIICIL DISTRICU~IOK. ITG O N This lalv causes the inclepe~ide~itrigin r igin not otily otily of of similar gene ra but tlius us repeats hersel herselff of sini ilar faillilies faillilies and even of similar ord ers. Sa tu re tli 011 a vast scal e, but the si~i lilarity is never colnplete or exact . JVlien migrations are favored by over-population or geographic al changes, a new an d seve re tes t of fitness arise s by t he miilgling miilgling a11 a11d competitioa of th e parallel types. Un de r tile oper atio n of tliese laws a 1 11 11ost int er est i~ ig eneralization or hy1)othesis can be made as to tlie tliree [zoological] realms: geographical

FIG. I

Orders of manlmal? placed in their llypotlleticnl chief Centers of aclalitive r.idiatiu11 daring the Tertiary period.

Fro m Orborn.)

isolation isolatio n ha s been been so co~ltilluous nd prolo~lged hat grea t orders of InamInammals liavt liavt:: heen evol evolve vecl cl io each. Th us i-cta~en coil tai~ iin g lie l~roaclestand l~roaclest and most hi high ghly ly-- diversified diversified lalid ar ea, app ear s hypothetical ly as tile center in which fourteen primitive and specialized orders radiated fro111 each other. I n the southern portion of .\i.ogen t least four orders sprang from pri~ilitiv e meinbers of tlie ab ove orders, and t he Hystricomorpli Hystricomorpli rodents enjoyed their chie chieff radiation. In .\fqqeiz two orclers were cut off

by the se a; one of thein a rapidly rapidly declining ty pe, tlie hI o~iotrern es, he othe r, t he Alarsupials, Alarsupials, enjoyenjoy-iiig iiig a very highly diversified diversified radi atio n. T his At this time the distinction between homoplasp and parallelism was not appreciated by tile writer.

 

 

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m e a n s t li li e e s c l u s i ~ c r e a s o distribution, f o r d u r i n g t h e p e r i o c l s of lnntl contact certain n1enl11e1.s of t ~ e s e ortiers fo un d tlieir nn-a - into ad jac e~ l realms.

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i t s m i g r a n t o r d e ri ri vvee c l t y p e s .

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n t l i c o t h e r h a n d , m a y b e d i bb-

t i n g u i s l i e d f r o m r e a l u ~ i s as g e o g r a p h i c a l a i i d 7 o i i l o g ic ic a l a r e a s . v rl rl li li cchh l i a l - c b e e n i i o l a t e d f r o m e n c li li o t h e r f o r s h o r t e r p e l .i .i od o d a , e i t h e r 11 climntic 1xi1.r i er e r s , a s i n t h e c a s e o i t llii e A r c t i c o r ci c i ll.. cm cm m p ol ol ar ar r e g i o ~ i , r l ~ y r e a t p h p s i c a l b a r r i e r s , s u c l ~ s m a s s e s o f w a t e r a nc nc l of c l e se se rrtt sa sa iill ccll x. x.

I n certain cases

t h e s e r e g i o n s . s u c l i a s A f r i c a , a p p e a r t o h a y e l ~ e e n o l ar a r ggii ., ., tl t l i s t i ~ l c tantl tantl i so so la la te te c i, i, a s t o i aall- e l ~ e c o m e r n p o r t a ~ l t e n t e r s o f t h c r a d i a t i o n of of c e r t a i n oriicilr

o i i n a m m a l s a n d a l m o s t a t t a i n t h e r a n k o f r e a lm lm s , b u t r e y i o n s in in

g e n e r a l a r e c h i ef e f llyy a ~ i t i e r m a n e ~ i t l i s ttii n q ui u i s h e d 11y t h e

i z i i i 7 j i i ; ' c i z z ( ii ii 1 r i ii ii i iiii

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c e n t e r of i n t l e p e ll~ ~ t l e t lltt \ ~ o l u t iioo l l nntt i a s t h e s o ~ 1 l . c ~f succesui\.e n o l . t h ~ ~ a r c 1i grat i ons of ani mal s . T h i s h y p o t h eess i s h a s r e c e n t ~ l y been col ~fi rnl ecl b y r em em a rr11 ;;aa bbll e p n l e o n t o l o ~ i c a l d i s c o v e r i e s i n n o r t h e r n A f r i c a . T h i s n d ttll s t o t h e a b o v e llii st s t of f i vvee r a d i a t i o n s a sisth , namely

 

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the familie familiess Antelopidie Antelopidie Gi ~a ff id a Hippopotamidre Hippopotamidre etc.

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~ ~ \ t ~ f ~. i 2 tan l -tc t i c a ~,  1 ~1 ~ ~ 1 0t 0t l iiee t i cc.. ci le n t e r Teltin ly ad.il~tive aiiintioi~, . A in O s b o ~ n . ) vatiorl to the u u s o u n d i n g l i n e , s h ~ \ \n g u l d c o l i t i l ~ c n t a l iill ie ie s . ( I r o in

As s e e n i n operation a m o n g t h e u n g u l a t e s t h e c o m p e t i t i o n

a nc nc l ra n ge for food ori gi na t e 5 t h e l e ng t he ni n g of of l i m b fr fro11 1111 s l o~ o ~ v - ffoo o t ec ec l n t o c u r s o r i n l t y p e s , aauu cl cl t h e l e n g t h e n i n g of t e e t h from short-crowned (brachyodont) into long-croivnecl (hypsod o n t ) t ~ p e s , nd f rree c jjuu e nntt ly ly t h e l e n g t h e n i n g of s k u l l f r o m b l a c hyc e ph a l i c i nt o c l ooll ic ic hoc hocee pha phall ic ic t yp e s (O sb orn , 02a). T h e e s p eecc i aall a p p llii c aatt io i o n t o p a l eeoo n t ooll o g y t o b e n o t e d h e r e is t h a t a s t h e s e t y p e s m a y h a v e l iivv e d t o g e t h e r o r in in p r o x i m i t y and rcsortecl t o t h e s a m e w a t e r c o u r s e s f o r d r i n k , t h e i r ffoo i s i llii z e d

 

rem ains ar e often fo~ int l og eth er.

S T e t if w e c s a llll llii llll e a n a ll-

o g o u s t y p e s l i v i n g ttoo --cc la la yy,, w e s e e t h a t t h e y d o n o t f r e q u e n t e x a c t l y t l i e s a m e f e e d i n g r a n g e s n o r ( lloo t h e y s u b s i s t u p o n e s a c t l p t h e s a m e f oooo d ; t h e y t h u s d o n o t c o m p e t e . G oooo ccll i llll u sst ra t i o ns of t h i s i oocc iri iri rr vlz~  i t t ~ i ~ t ~ il ~e os ei e~n i n t h e d i s t ri ri b u t i o n i n A f r i c a of t h e c l o s e l y a l li li e d s q u a i c - l l p p e c l l i h z i ~ u c t i o s s i ~ i ~ i l si t h 11 p s o c l o i l t t e e tthh , ~ ~ h i c lliv ivl eess u p oonn g r as a s s eess , a n d t h e nnol e poin tetl-lipp ed 1 2 b ll'' c oorr nnii s w i t h h r a ch c h y o c lloo n t t e e t h , ~ v l l i c h l iv iv e s m a i n l y u p o n s h l u b s . Not11 s p e c i e s b e l o n g t o t h e s a m e phy l um ooff rhi n oc e rose s. An1ong fos5 fos5ii l t yp e s c l ose l y 1-el 1-el a ttee d t o t h c a b o v e w e o b ssee ll- I e s i i n i i a r c a s eess , s u c h n s t h e c o e x i s t e n c e i n t h e lo lo r vvee r P l e i s t o c e n e of of t h e h y p s o d o n t ir ~tipuztat~s I?

~ L ~ I I Z ~ ( P C ~ I ~ S .

( ~ v o o l l yr yr h i n o c e r o s ) a n d t h e b r a c h y o d o n t If c a r r i e d f a r t h e r t h a n i n t h e a b o v e i n s t a n c e s , w e m a y b e justified in placing these I'ui.izL /~(t'irpiiw~.trii'ic~tioirs /~(t'irpiiw~.trii'ic~tioirsnn s e p a r a t e s u bf b f n m iill ie i e s , l ~ e c a u s e n m a n y c a s e s t h e y g i v e s iiss e t o d i s t in in c t and long-persistent collateral phyla. E s n m p l e s ooff t h i s 11;;intl a r e n u m e r o u s a m o n g t h e a n c i e n t I ''ee r is is s o d aacc t yl y l a o r u n g u l a t e s r e l a t c ttll t o t h e h o r s e s , t a p i r s , a n d rhinoceroses, as seen below TI

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I t xvill b e o b s e r v e d a t o l l c c t h a t t h e r e i s n o i n h e r e n t c o r r e l a ti t i o n b e t \ ~ e e n ii ~r~hj~oi?c~~ or hj;hiofJoiitis~i~ ~r~hj~oi?c~~iti.siii iti.siii ~ i i t lbi~~r1-hjpo~(j1, a i l t i r J ~ I l ' ~ / i c ~ ~ t1-< l ' ,l,l o i ~ g a t i o no nof t h e f e e t , a s w e m i g h t h a v e a n t i c i p a t e d , a l t h o u g h h y p s o d o n t i s m i s g r a d ~ i a l l yd yd e v e l o p e d i n 1

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long-footed series because subsisteilce upon grasses is as s o ci at ed w i t h s uc h col l di t iiol ol l s ooff l i ffee as ar e aaff ffor ortt lec lecll by es t en sive open plains, long ranges for food, and rapid flight from

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enem ies. A g a i n , a s s h o n - n e l s e \ v h e ~ - e , c l ooll ic i c h o c eepp h a llyy a n t i c l o l i c h o p o d y , b r a c h y c e p h a l y a n d b r a c l ~ y ~ o t l yl e f r e q u e n t l y b u t n o t i n v a r i a b llyy c o r r e l a ttee d . ( S e e O s b o r il il , 0 2 a T h e v a l u e ooff t h i s l aaw w o i lorrri n d a f t i ; ~ t7.irtr l iztziirris especially g r e a t a s a n llee a iiii s of i n t e r p i - e t n ttii o n of t h e f r e q u e n t c o n t e n i p o ~ , l n e o u s e x i s t e i ~ c e r a s s o c i aatt i o n ooff m o r e p r i m i t i v e ( h i - a c h y o t l o n t ) wit11 m o r e s p e ccii a llii z eedd ( h y p s o d o n t ) t y p e s . A m o ng Te rtiary i n a c h x r o c l o n t c a t s it it i s s e e n i n t h e c o n t e m p o r a r y l o n g --ll im im b e t l E-(cp/ip/loi~i.lc~. 11 fact , t l l e as s oci at i o n h a s b e e n s o o f t e n o b s e r v e d t h a t if w e f in in d o n e l , h y l u n ~ ,w e

Drrizc-ti.s ant 1 s hort -l i mbed

m a y a l l n o s t a n t i c i p a t e o r p r e d i c t t h e d i s c o v e r y ooff t h e o t h e r T h e llaa w i s m a d e m o r e c l e ,,~ ~ rb r b y r e f e r r in in g t o t h e a b o v e t a b l e a n d t h e ffoo l lloo w i n g e x p l a n n t iioo n of of i t : I ) A s c o rrnn p aall et et 1 w i t h t h e P n l x o t h e r i i n x , t h e P aa11 aapp lloo tthh eerr iiii n: n: e a r e s o l o n g - f o o t e d t h a t I -I -I ux u x le le y b e li l i e v ed e d t h a t t h e y g a v e r l s e t o t h e llll o llss e , a n d h e a c t u al l y pl aced Ptrloj/c~i /i~~i~iiii ~i~iicis a s t h e a n c e s t o r of t h e h o r s e series. 2 ) T h e T e l m n t o t h e r i i n z a r e l a r g e a n iim m a l s a llss o ttll i s t in i n g u iiss h eedd b y v e r y l oonn g , s l e ~ ~ t l e ri m b s ; t h e y i n t l e p e n t l e n t l y a c q u ir i r e d h o r n s , a n d t h e s p e ccii e s T . c o i . ~ ~ n t ~ c ir ia~s b e l i e v e d b y O s b o r n t o b e a n c e s t r a l t o t h e t i ttaa n o t hhee l --ee s ; b u t a s a m a t t e r o f f a c t , a s ssll lloo rrvv n b y E a r l e a n d I - Ia Ia t cchh e r , t h i s h o r n i s a p u r e c a sstt . of p a r a l l e l i s ~ l ~ ,i n c e t h e t i ttaa n o t h e r e s p r o b a b l y s p r a n g f r o m t h e

s h o r t - s k u l l e d a n d I - el el a ttii v el e l y s t o u t b p e c iiee s , i t r / @ o s j ~ o j ~ilnrlfir C L Y O S a m e m b e r of t h e P c r 1 n - o s y o p i n c ~ ( 3 ) T h e T r i p llo o p o t llii n ,,~ ~ present the most estrenie instance of light-limbed tlevelopi l llee n t a n l o n g t h e p e r i s s o d a c t y l u n g u l a t e s , s i n c e t h e l ii ii li l i b s h aaxx -c -c t h e ~ ~ r o p o i - t i o n sf sf s o n l e of t h e n l o sstt s l e n d e r a n d s w i ft ft f o o t e d r u m i n a n t m a m m a l s , a l tthh o u g h t h e s e a n i m a l s a r e f o uunn tl tl i n t h e s a m e b e d s \ I --ii tthh t h e c o rrll -e - e llaa te t e tl tl s u b f a m i l y H y r a c h y i n x . 4 ) A n l o n g t h e L , o p h iio o t l o n t iitt l ~ c , h c I --II el el al al et et in in r e b r i n g o u t w i t h add i t i ona l fo rce t h e 13 1311-iinc nciipl plee t h at t h i s e l ong at i on ooff l i mb occurrecl at a very earl y geol ogi cal peri ot l ; b e c a u s e t h c f i r s t l i n o w n m e m b e r ooff t h i s p h y l u m a p p e a r s n r a y c lo lo w wnn i n t h e 1 ,o ,o w wee llE o c e n e , \ Y a s a t c h , 01- S u e s s o n i e n b e t l s , i n t h e g e n u s ~fotJoii C o p q a r e m a r k a b l y l i g h tt-- l iinn i b eecc l f o r m , an a n t1 t1 i t r u n s r i g h t t h r o u g ~ ,

 

so fa r as Tve know , int o t h e Oligoce ile g en us Co CoiiorJ orJori, th e thre et o e d t y p e w i t h e x ttrr a o rrdd i n aarr i lq lq - e l o n g a t e d i g i t ss.. S o f a r a s w e k n o w , t h i s l i g h t --ll i m b e d s e r i e s iiss ffoo u iill tl tl b o t h i n A m e r i c a a n d E u r o p e j w h i l e t h e h e a v y - l iim m b e d 1 , o pphh i o d oonn t iinn a : a r e i o u i l d o n l y in E u r o p e . S i n c e t h e a l ~ o v e w a s w r i tttt e n t h e ttii t aann o t h e rree s h a ~ r eb eb e e n n lo l o r e c a r e f u llll y e x a m i n e d b y t h e w r i t e r ( O s b o r n , Qzbj, a n d , l i k e the rhinoceroses, they are found to subdivide into four cont e m p o r a n e o u s p h y l a c l i s t i n g ~ ~ i s h e d h ie i e fl f l y b y c lo l o li l i ch c h o ccee p hhaa ly ly ailcl b r a c h y c e p h a l y a n d b y r e l a ttii v e l y l o n g a n tt11 s h o r t l i m b s , t h u s a f f o r d i n g a n o t h e r c o n s p i c u o u s i l l u s t r a ttii o n ooff t h i s i t r 7 s r f Local Loc al adr fi-; fi-;,i ,i.. ~, n di (r fi o r~ .

I n t h e c a r e f u l c o n s i d e r a t i o i l ooff a d a p t i v e r aatt li li a ti ti o iiii f r o m c e r t a in in s t e m t y p e s i s t o b e f o u nntt 1 t h e t r u e s i ggnn i ffii c aann c e of C u v i e r ' s T Z U o C O I ~ ~ F L ' ~ Z ~ Z OsI I mocl moclif ifie iedd b y th e to him unl;no~vi unl;no~viii pr inciple of evo lution . Re fer rin g to th e diag ram s, 1;ig. 4, t w o i n l p o r t a n t p r i n c il i l ) le le s a r e b r o u g h t o u t F i r s t , p r a c t i c a llll y a llll t h e a d a p t a t i o n s li110~v11 a m o n g m a m m a l s h a v e a r i s e n b y c o m b i n a t i o n s of d i v e r g e n c e independently p u r s u e d i n t h e l i n l b s a n c l t e e t h ; f o r e x a m p l e , a n 1 1e 1e rb rb i1 i1 -o -o ro ro us us t o o t h t y p e m a y c o m b i n e w i t h a t e r r e s t r i a l , a r b o r e a l , o r v o l a n t l i m b t y p e , a c c o r t llii n c a s

t h e s e a r c h f o r p l a n t f o o d iiss o n t h e e a r t h , iill l t h e t r e e s , o r i n t h e a i r . X l t h o u g l i e v e r y i m a g i n a 1 ) l e c o m b i n a t i o n r . ~ . cluatic l i m b ss,, m y r m e c o p h a g o ~ ~ ssee n t i t i o n ) c a n n o t b e r ea e a li l i ze ze d , y e t t h e s e c o m b i n a t i o n s h a v e b e e n m u l t i p l i e d a l m o 5 t tt i i / i ~ ~ i t ~ ancl ~ir/ c o n s t i t u t e t h e f a t a l d e f e c t o f C u v i c r ' s llaa w a s h e c o n ccee i vc v c t l i tt.. A s t e s t e d b y a s i n g l e c a s e , t h e E o c e n e m o n k e y s of t h e f a m iill y N o t h a r c t i t l r e a c q u i r e d t e e t h e x a c t l y l ~ o m o p l a s t i cw cw i t h t h o s e o f E o c e n e h o r s e s , b u t t h e f o r m e r w e r e p rroo vi v i c le le tl tl w i t h a r b o r e a l , t h e l a t t e r \\vv iitt h t e r r e s t r i a l , l i m b t y p e s . S e c o n d , c o r r e l a ttii o n of li ll- t b a i ld ld t o o t h s t r u c t u r e i n a g i v e n g r o u p is f u r t h e r c o n ttll iit io io n eedd b y t h e p a rrtt ic i c u llaa r c o i ~ ~ l ~ i n a t i o~nl d leg legree ree ooff specializat i o n of l in i n l b s a n tt11 t e e t h w h i c h t h e r a d i a t i o i ~ o r i g i n a ttee s ~ v i t h . l io i o r e x aam m p l ee,, t h e p r iim m i t iivv e p l a c e n ttaa l s c o m l ~ i n e t l r i t u b e r c ~ ~ l a r insec tivor ous t ee th \\-it11 a generalizetl or pro bab ly terr estrial

 

t y p e ooff f e e t t h e A u sstt ra ra llii aa11 1 m a r s u p i a l r a d iiaa t i o n , o n t h e o t h e r hand, began \\.it11 a d e n t al a l t y p e s i iill ~ i l a r o t h a t of t h e p l a c e i ~ t ~ ~ l s , LIMBS

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S h o r t - l i n i l ~ e t l , l a ~ l ti ti g l - ,i ,i t llee , ~11113LTl.~A~l~ ~)l<\. pe11r;~tlacrl unyui c u0 ; late s t e i n TLI<I<I:STI<I-~I.,

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but , as D ol l o ant 1 U ens l cy har-c she\\-n, t h e f o o t t j r p c was of hi ghl y s peci al i zct l arbol -cal charact el . . T h e s e ttnn7 o f u n d a m e n t a l e s c e l t i o n s makc clear n-lly it is i m p o s s i b l e , a s i n a n y w r i t e r s lnr-c obs ervet l , t o r e c o n s t r u c t a n

 

e n t i r e a n i m a l f r o i l l e i t h e r a c l a \ ~ o- r a t o o t h . T h u s , w h i llee t h e l a \\ \\ , of c o r r e l a t ~ o n s n o l e ss ss d o m i n a n t t h a n C u v i e r s u p p o s e d , o n l y a v e s t i g e i s l e f t ooff t h e n l o d e ooff a r c h e t y ~ ~ a lp e r a t i o n ooff t h e llaa w a s h e co c o n ccee iv i v e d i t. t.

I t m a y h e n o w r e s t a t e d a s f ol ol -

lows F e e t ( c o r r e l a t e d c h iiee f llyy w i t h l i llnn b a n d h o ttll y s t l u c t u r e ) a n d t e e t h ( c o r r e l a t e d c h iiee f ly ly w i t h s l ttuu l l a n d n e c l i s t r u c t u l c ) diverge independently in adaptation respectively to securing a n d e a t i n g f o o d u n d e r d i f f e rree n t c o n d i t iioo n s . E a c h e v o lv lv e s t l iirr e c t llyy f o r i t s o \iT rn m e c h a n i c a l f u n c t i o n s o r p u r p o s e s , y e t i n such a manncr that each subservcs the other. C o n -e - e la l a ti t i on o n i s tthh e r e f o r e n o t m o r p l ~ o l o g i c a l ,a ,a s C u v i e r s u p ~ o s e c l , u t p h y s i o l oogg i c aall , f u n c t i o n a llnn ..aa y s p r e c e d i n g s t r u c t u r e . I t b e co c o n l eess c l o s eess t n h e l - e t e e t h a n ccll f e e t c o m b i n e i n t h e s a m e f u nc n c ti t i oi o i l a s iinn t h e p r e h eenn s i llee c z n i n e s a n d c l a ~ v s f t h e F e l i d z , and 1110st diverse where the functions are 111ost diverse, as i n t h e t e e t h a n d p a d d l e s ooff t h e P i n n iipp e d i aa..