528 MR. @. BENTHAM ON COMPOSITA. 
the intervention of tropical ungenial regions, far too wide to be 
crossed by the ordinary gradual progress of plant-races. The 
reappearance of identical races, generic or specific, in the north and 
south can therefore only be attributed either to a preexistence of 
different physical or other extraneous conditions allowing of 
gradual migrations, or to ancient colonizations through channels 
which now appear no longer to exist. 
These remarks may be illustrated by the following list of genera, 
identical or representative, in extratropical North and South 
America, without any, or rarely with only a single, species in the 
intermediate tropical regions :— 
Species in || Species in 
extratropical | extratropical 
Genera. A > A A Genera. ————— 
North | South | North | South 
Amer. Amer. | Amer. | Amer. 
Gymnocoronis ...... | ld d D MEA | 8 E 
Grutierrezia ............ BM 6! Jaumea ni 3 1 | 
Qrimda da ET PON | DABIS n. osse 2 1 
Chrysopsis ............ It Habia: soas cunis 19 1 
Hysterionica ......... -.. | 5 | Blennosperma......... 1 1 
Haplopappus ......... 38 26. Ií Vallanoya. a-st 4 
Sobdago sss 80 2 le Closia: atado pad E 5 
Ericameria ............) 4 | Hymenatherum ...... ms 
| Nardophyllum g  Gaillardia ............ | z 
Lepidophyllum } || Hymenoxys............ 9-4 2 
IIE Gs |... Bo | Artemisia ..........-. 30 3 
Antennaria ............ 8^ 1 | Centaura 1 5 
Adenocaulon ......... 1o 2 i| rs n.. e ies oe, 
Eransetia es sese 6 3 | Troximon ........ ... 10 i1 6 
NCAA o 15 3 | J Scorzonella ............ 3 | 
Thelesperma ......... 4 L| Pisos... odii le | 1 
il 
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