EECAPITULATION OF THE DOMINANT FEATURES OF THE FLORA. 309 



more than they constitute in the known flora from North Mexico to Panama. But 

 here, as in the ferns, the xerophilous genera largely predominate, Bouteloua, Sporobolus, 

 Stipa, and Aristida being numerous, and such monotypic genera as Buchloe and Sclero- 

 pogon are characteristic. We have already fully detailed (page 272) the distribution of 

 the genera Beschorneria, Agave, Furcrcea, Yucca, Nolina, and Dasylirion, which have 

 their centre here, and, if we may judge from present evidence, originated here. The 

 Bromeliacese, which have a similar type of foliage, and are partly xerophilous, like the 

 gigantic Puyas of Chili, are mostly epiphytes, inhabiting more humid regions, and only 

 three are recorded from North Mexico. 



Another comparison shows that in South Mexico the monocotyledons constitute 

 about 22 per cent, of the flowering plants, and in our three provinces collectively 21*5 

 per cent., whereas in North Mexico they only amount to a little over 11 per cent. ; and 

 taking the petaloid monocotyledons alone the contrast is still more striking. Even 

 assuming that our total for North Mexico is proportionately 150 too low (and there is 

 no reason to suppose the discrepancy is so great), the numbers would be as 1 in North 

 Mexico to 8 in South Mexico. 



With regard to extensions into North America, there are a few additional facts. It 

 has been shown (page 223) that the number of Mexican species extending northward 

 west of the Mississippi is almost double that extending northward east of that river. 

 It might be urged that this is just what anybody would have expected and taken for 

 granted, considering the position of the country east of the Mississippi. But this is by 

 no means the matter of course it appears to be, for climatal conditions play the chief 

 part. The number of Mexican and Central-American genera represented respectively 

 in eastern and western North America is as 5 to 6, rather more than less ; but many of 

 these genera do not exist in North Mexico. The extensions of North-Mexican genera 

 and species have not been separately tabulated ; but from comparisons of certain natural 

 orders it is evident that the extensions into eastern North America are largely, perhaps 

 mainly, South Mexican and not North Mexican. Dr. Asa Gray*, comparing the 

 constituents of the Atlantic and Pacific forests of North America, enumerates the 

 following arboreous genera as absent from the Pacific forests: — Magnolia, Asimina, 

 Tilia, Bobinia, Liquidambar, Ilex, Biospyros, Bumelia, Ulmus, Celtis, Morus, Ostrya, 

 Carpinus, and Carya. Now, all these genera are represented in Mexico, yet, so far as 

 we know, only four out of the fourteen in North Mexico. Furthermore, the species in 

 eastern North America and in the mountains of South Mexico are often identical ; thus 

 Liquidambar styraciflua, Ostrya virginica, and Carpinus americana are instances. 

 Whether these elements ever inhabited the intermediate country is beyond the scope 

 of this inquiry, and, one way or the other, would not invalidate data concerning the 

 present condition of things. 



Many other facts having the same bearing might be adduced, but ample evidence 

 * " Forest Geography and Archaeology," American Journal of Science and Art, xvi. 1878, p. 183. 



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