256 Dr. J. D. Hooker on the Vegetation 



of the coaimon West Indian forms of vegetation from the east to the west 

 coast of South America. So powerful a check to migration are the Andes, 

 that almost throughout their whole length there is no mingling of the Floras 

 of the east and west sides of America. The narrow Isthmus of Panama offers 

 the only exception to this chain being continuous in the tropics ; there the 

 land is described as low and flat; and Mr. Hinds, in particularly alluding to 

 this circumstance, states, that consequently the mahogany crosses from the 

 Atlantic to the Pacific side. By this means, many species which prevail over 

 the Mexican and Caribbean Seas from the Bahamas to Honduras, are further 

 extended, first, for a short distance west to the Pacific, and then along its 

 shores to Guayaquil on the south, and probably to California on the north, 

 their further extension being probably checked by the dry plains of Peru on 

 the one hand, and the changes in the climate of California on the other. 

 Seeds of these are always liable to be exposed to the influence of that current, 

 which I have noticed as flowing to the Galapagos from the Gulf of Panama. 



I shall conclude this part of my essay with pointing out the adaptation of 

 the several plants for transportation, under the natural orders to which they 

 severally belong. 



Menispermea. — Cissampelos presents a hard inner coat of the pericarp. Albumen scanty, 

 fleshy. 



Cruciferx. — Senebiera didyma, the only Galapageian species, forms an exception to the gene- 

 ral rule, that the plants of this order are impatient of transport from the oily nature of 

 their cotyledons; it is, as DeCandolle remarks, probably a native of Buenos Ayres, 

 whence it has been difiiised over nearly all the globe, and is continuing to spread. 



CurvembryoscE. — An artificial group, sufficiently natural however for the present purpose. 

 Seeds very minute in some, as Drymaria and Mollugo. The Chenopodecp, Phytolaccea 

 and Portulacece have a constitutional predilection to salt water. Albumen farinaceous 

 in the greater part of the Galapageian genera. 



Malvacea. — Indurated pericarp of many. The floral envelopes oiMalachra are well-adapted 

 to stick to various means of transport. 



Sapindacea. — Crustaceous testa and exalbuminous seed of Cardiospermum. 



ZygophyllecB. — Tribulus cistoides offers singular advantages for transportation in its woody 

 seed-vessels, their spines beset with reversed prickles, and exalbuminous seeds. 



Xanthoxyle<E. — Osseous testa of Xanthoxylum. 



Simarubea. — Castela has a crustaceous endocarp and scanty albumen. 



