m a n if e s t e d ; b u t th e r e is n lira y s a m o rp h o lo g ic a l ch a n g e i t th e ch a n g e o f co n d itio n s b e su d d en , or
w h e n , th r o u g h la p s e o f tim e , i t b e c om e s e x tr em e . T h e n ew fo rm is n e c e s sa r ily th a t b e s t su ite d to
th e c h a n g e d c o n d itio n , a n d a s it s p r o g en y are h en c e fo r th a d d itio n a l e n em ie s to th e o ld , th e y w i
e v e n tu a lly te n d to r ep la c e th e ir p a r e n t fo rm in th e sam e lo c a lity . F u r th e r , a g r ea te r prop o r tion
o f th e se ed s a n d y o u n g o f th e o ld w ill a iiiiiia lly b e d e s tr o y ed tb a n o f th e n ew , an d th e su rv iv o r s o l
t lie o ld , b e in g le s s w e ll a d ap ted t o th e lo c a lity , w ill y ie ld l e s s s e ed , an d h e n c e h a v e few e r d e sc en d a n ts.
I l l th e ab o v e o p e r a tio n s N a tu r e a c ts s low ly o n a ll o r g a n ism s , b u t m an d o e s so r a p id ly o n th e
few h e c u ltiv a te s o r d om e s tic a te s ; lie s e le c ts a n o r g a n ism su ite d to liis ow n lo c a lity , an d b y so mocli-
fv in g it s su r r o u n d in g c o n d itio n s th a t th e fo o d an d sp a c e th a t w e r e th e sh a re o f o th e r s fa lls to i t , ho
m s u r e s a p e r p e tu a tio n o f h is v a r ie ty , an d a m u lt ip lic a tio n o f i t s in d iv id u a ls, b y m e a n s o f th e d e stru c tio
n o f th e p r e v io u s in h a b ita n ts o f th e sam e lo c a lit y ; an d in ev e ry in s ta n c e , wh e r e h o h a s w o rk ed
lo n g en o u g h , h e fin d s th a t c h a n g e s o f fo rm h a v e r e su lte d far g r ea te r th a n w o u ld suffice t o c o n s t itu t e
c o n v e n tio n a l sp e c ie s am o n g s t o rg a n ism s in a s ta te o f n a tu r e , an d h e k e ep s th em d is tin c t b y m a in ta in -
in g th e s e c o n d itio n s . _ _ . . .
M r D a vw in a d d u c e s an o th e r p r in c ip le in a c t io n am o n g s t liv in g o r g a n ism s a s p la y in g an im p o r -
ta n t p a r t in th e o r ig in o f sp e c ie s, viz . th a t th e sam e sp o t w ill su p p o r t m o st lif e w h en p eo p led w ith
v e r y d iv e r se f o rm s, as i s e x em p lified b y th e fa c t th a t in a ll iso la ted ar ea s th e n um b e r o f C la sse s, Orde rs,
a n d G en e r a i s v e r y la r g e in p rop o r tion t o th a t o f S p e c ie s.
§ 3.
0 » th e G en e ra l P h e im in en a o f D is tr ib u tio n in A r e a .
T u r n in g n ow to a n o th e r c la s s o f fa c ts , th o s e th a t r e fe r to th e d is tr ib u tio n o f p la n ts o n th e su r fa
c e o f th e g lo b e , th e fo llow in g ar e th e m o s t o b v io u s ;—
1 4 T h e m o s t p r om in e n t fea tu r e in d is tr ib u tio n is th a t c ir e um so n p tio n o f th e ar ea o f sp e c ie s,
w h ic h so fo r c ib ly s u g g e s t s th e h y p o th e s is th a t a ll th e in d iv id u a ls o f ea ch sp e c ie s h a v e sp ru n g fr om a
c om m o n p a r en t, a n d h a v e sp r ead in v a r io u s d ir e c tio n s fr om it . I t is tr u e th a t th e . a r ea o f som e
(e sp e c ia lly C ry p to g am ic a n d A q u a t ic p lan ts) is so g r e a t th a t w e c a n n o t in d ic a te an y ap parent c en tr e
o f d iffu sion an d th a t o th e r s a r e so sp o rad ic th a t th e y appear to h a v e h a d m a n y su ch c e n t r e s ; bu t
th e s e sp e c ie s , th o u g h m o r e n um e r o u s th a n is u su a lly su p p o sed , ar e few in com p a r iso n w ith th o se
th a t h a v e a d e fin ite o r c ir cum sc r ib ed area.
W i t h r e sp e c t t o th is lim ita t io n in a r ea ,* sp e c ie s d o n o t e s s e n t ia lly differ from v a r ie tie s o n th e
o n e h a n d , o r from g en e r a a n d h ig h e r g ro u p s o n th e o th e r ; an d in d e e d , in r e sp e c t o f d istr ib u tio n ,
th e y h o ld an e x a c tly in te rm e d ia te p o s itio n b e tw e en th em , v a r ie tie s b e in g m o r e r e str ic ted in lo c a lity
th a n sp e c ie s , an d th e s e a g a in m o r e th a n g en e ra .
. It is a 1-em.rkablc fact that there are some striking anomalies in the distribution of plants into provinces as
eomnared with animala. Thus there is no peenliarity in the vegetation of Australia to be compared with the
3 o f p t e n t a l mammals, nor with the fact o f so m an , o f the mammals, birds, and fish of Tasmania ffif^ering from
t o of t e ronrineut of Alrstralia. Nearer home, we find the basin of tbe Medrterianean with a tolerably uniform
Flora on the European and North Aftiean sides, but these ranking as different zoological prmuuces. Ilie
m u l n a lw e r delimitarion in area of animals tban plants, and Bicater restriction o Faunas than Flo,as, should
r f n r r i n M p a t e that plant types are, geologically speaking, more ancient and pernraneut tlian he higher
a n t ia l types are, and so I believe tbem to be, and I would extend the doctrine even to plants of highly complex
structure.
T h e u n iv e r sa lity o f th is fea tu r e (o f g r o u p s h a v in g de fin ed areas) affords to m y m in d a ll b u t
c o n c lu s iv e ev id en c e in fa v ou r o f th e h y p o th e s is o f s im ila r fo rm s h a v in g h ad b u t o n e parent,^ or pair
o f p a r en ts. A n d fu r th e r , th is c ir cum sc r ip tio n o f sp e c ie s a n d o th e r g ro u p s in area, h a rm o n iz e s we)
w ith th a t p r in c ip le o f d iv e r g en c e o f fo rm , w h ich i s op p o sed t o th e v iew th a t th e sam e v a r ie ty or
sp e c ie s m a v h a v e o r ig in a ted a t d iffe r en t sp o ts. I t a lso fo llow s th a t , as a g en e r a l ru le , th e sam e sp e c ie s
w ill n o t g iv e r is e to a se r ie s o f sim ila r v a r ie tie s (an d h e n c e sp e c ie s) a t d iffe r en t e p o c h s ; w h e n c e th e
g e o lo g ic a l e v id en c e o f co n tem p o r a n e ity d e r iv ed from id e n t ity o f f o s s il fo rm s m a y b e r e lied u p on .
T h e m o s t o b v io u s ca u s e o f th is lim ita t io n in area n o d o u b t ex is ts in th e w e ll-k n ow n fa c t th a t
p la n t s d o n o t n e c e s sa r ily in h a b it th o s e ar eas in w h ic h th e y a r e c o n s t itu t io n a ily h o s t fitted t o th r iv e
a n d to p r o p a g a t e ; th a t th e y d o n o t g row w h e r e th e y w o u ld m o s t lik e to , b u t w h e r e th e y can find
sp a c e a n d few-est en em ie s . W e h a v e s e en (1 3 ) th a t m o s t p la n ts are a t warfare w ith o n e o r m o r e
c om p e tito r s for th e ar ea th e y o c cu p y , and th a t b o th th e n um b e r o f in d iv id u a ls o f an y o n e sp e c ie s and
th e ar ea i t co v e r s are c o n tin g e n t o n th e c o n d itio n s w h ich d e te rm in e th e s e r em a in in g so n ic e ly b a lan c ed
th a t ea ch sh a ll b e a b le a t le a s t to h o ld it s ow n , a n d n o t su c cum b t o th e e n e r v a tin g o r e tio la tin g or
sm o th e r in g in flu en c e s o f it s n e ig h b o u r s. T h e e ffe c ts o f th is warfare are to e x tin g u ish som e sp e c ie s
t o sp a r e o n ly th e h a rd ier r a c e s o f o th e r s, a n d e sp e c ia lly to lim it th e r em a in d e r b o th as to area and
ch a ra c te r s. E x c e p t io n s o c cu r in p la n ts su ited t o v e ry lim it e d or a b n o m a l c o n d itio n s , su c h a s d e se r t
p la n ts, th e c h ie f o b s ta c le s to w h o se m u ltip lic a t io n are su ch in o r g a n ic an d p rm cip aU y a tm o sp h e n c
c a u se s a s o th e r p la n ts c a n n o t o v e r com e a t a ll; su ch p la n ts h a v e n o c om p e tito r s , are g e n e r a lly w id e ly
d istr ib u ted , and n o t v e ry v a r ia b le .*
1 5 . T h e th r e e g r e a t c la sse s o f p la n ts, A c o ty le d o n s , M o n o c o ty le d o n s , an d D ic o ty le d o n s (G ym n o -
sp c rm on s an d A n g io sp e rm o n s ), are d istr ib u ted w ith to le r a b le eq u a lity over th e su r fac e o f th e g lo b e ,
in a sm u ch a s w e c a n n o t in d ic a te an y o f th e sLx c o n tin e n t s (E u rop e , A sia , A fr ic a , N o r th an d S o u th
A m e r ic a , a n d A u str a lia ) a s b e in g p e cu lia r ly r ich in o n e to th e e x c lu s io n o f an o th e r . F u r th e r , th e d is tr
ib u tio n o f som e o f th e la rg e r O rd e rs is r em a rk ab ly eq u ab le , as Composita., L e g um in o s e , G rm u m o t,
an d o t h e r s ; fa c ts w h ich (su p p o sin g e x is tin g sp e c ie s to h a v e o r ig in a ted in v a r ia tion ) w o u ld se em to
in d ic a t e th a t th e m e a n s o f d istr ib u tio n h a v e o v e r com e , or b e en in d ep en d en t o f th e e x is t in g ap p a r en t
im p ed im en ts, an d t lia t th e pow e r o f v a r ia tio n i s eq u a lly d istr ib u ted am o n g st th e s e c la s s e s , an d c o n t
in u o u s ly exe r ted u n d e r v e ry d iffe rent c o n d itio n s . I d o n o t m e a n th a t a ll th e c la sse s are eq u a lly
v a r ia b le , b u t th a t ea ch d isp la y s as m u c h v a r ie ty in o n e c o n t in e n t as in an o th e r .
1 6 . T h o se C la sse s and O rd e rs w h ich are th e le a s t c om p le x in o r g a n iz a tio n are th e m o s t w id e ly
d istr ib u ted , th a t is t o sa y , th e y c o n ta in a la r g e r p ro p o r tio n o f w id e ly diffused sp e c ie s. T h u s th e
sp e c ie s o f A c o ty le d o n s are m o r e w id e ly disp e r sed th a n th o s e o f ^Monocotyledons, an d th e s e ag a in
m o r e so th a n th o s e o f D ic o ty le d o n s ; so a lso th e sp e c ie s o f T h a llo p h g te s are am o n g th e m o st w id e ly
d ispe rsed o f A c o ty le d o n s , th e G r am in e x o f M o n o c o ty led o n s , an d th e Chenopodiacea: o f D ic o ty le d o n s .
T h is t e n d e n c y o f th e le a s t com p lex sp e c ie s to h e m o s t w id e ly diffused is m o s t m a rk ed m A c o ty led o n s,
a n d le a s t so in D ic o ty le d o u s .t a fa c t w h ich is a n a lo g o u s to th a t a lread y sta ted (4 ) , th a t th e le a st
c om p le x are a lso th e m o s t vai-iable.
* Tliouo-li invariable forms, tliev may be, and often are, themselves varieties or races of a species that inhabits
more fertile spots, as Poa bulbosa, wiiich is a very well-marked and constant form o f P .p ra ien sis, occurring in drv-
sandy sod, from England to North-wcsteni India, its “ meadow” relative being a veiy variable species in the same
countries, and always struggling for existence amongst other Grasses, etc. _ . . .
t Veiy much, no doubt, because of the difficulty in classif\-ing Dicotyledons by complexity o f organization ; in
other words, o f our inability to estimate in a classificatoiy point o f view the relative value of the presence or absence