FLORA OF ST. CROIX AND THE VIRGIN ISLANDS. 15 



is mainly to different periods of immigration under varied physical 

 conditions that we must ascribe the remarkable discrepancies in the 

 flora of those apparently homogeneous islands. Some few species, it is 

 true, are indeed given in my list as having been found only in the Vir- 

 gin Islands, such as a few Cactese, Vernonia Thomw, and the new species 

 described by me on the present occasion. But as long as Porto Eico, 

 Hayti, and even Cuba, are still insufficiently explored, it may very well 

 remain doubtful whether those species do not also occur in one or sev- 

 eral of them, just as several Cuban plants, described as endemical in 

 that island by Prof. Grisebach, have been found by me to occur not at 

 all unfrequently in the Virgin Islands and St. Croix, such as Arthrosty- 

 Helium cwpillifolium, Reynosia latifolia, and R. mucronata. 



It may thus be confidently asserted that both the groups in question 

 have derived their stock of plants from the neighbouring larger island of 

 Porto Eico. The question that remains to be solved is merely why 

 have they not all received the same species, and particularly why is it 

 that St. Croix, although the largest of all, has received a comparatively 

 and absolutely much less number of species than for instance the far 

 smaller St. Thomas? 



For the explanation of these interesting facts we have no doubt to 

 look to the geological history of the islands, as the conditions for immi- 

 gration over sea, even if possible to all the species, are essentially the 

 same in both groups, and therefore give no solution of the problem in 

 question. 



I am thus led to think that at a former period all the West India 

 islands have been connected mutually, and perhaps with a part of the 

 American continent also, during which time the plants in common to 

 all the islands, as well as to the West Indies and the continent, have 

 expanded themselves over their present geographical areas, at least as 

 far as they are not possessed of particular faculties for emigration over 

 the sea. By a subsequent volcanic revolution, St. Croix, as well as 

 many of the other islands, has thereafter been separated from Porto 

 Eico and the Virgin Islands, and put into its present isolated position, 

 which it seems to have retained ever since, whflst the latter group of 

 islands has either still for a long period remained in connection with 

 Porto Eico, or, if separated at the same time from it as St. Croix, has, 

 by another revolution, been again connected with the former. 



The plants now found in the Virgin Group, but not occurring in St. 

 Croix, would thus have immigrated into the former from Porto Eico 



