16 FLORA OP ST. CROIX AND THE VIRGIN ISLANDS. 



after the separation of St. Croix from the latter, and immigration would 

 finally have eeased by the separation between them, as it exists at the 

 present period. Thus, the plants found in the Virgin Islands, but not 

 in St. Croix, would seem to have been more recently created in the 

 probable centre of vegetation, Porto Eico, or some other of the larger An- 

 tilles ; the endemic ones, as in the other islands also, being the youngest 

 of all, not having been formed till after the complete separation between 

 the islands had been effected. This latter suggestion, which perhaps 

 seems contradictory to the general accepted theory of considering the 

 endemic forms on oceanic isles as the remnants of the oldest original 

 vegetation,* appears to be confirmed by the fact that even on such 

 recent formations as the Bahamas, which have as yet been but imper- 

 fectly explored, already no less than eighteen endemic species have been 

 discovered.! 



The supposition that the islands may have been separated from the 

 beginning, and have received their floras through immigration over the 

 sea, is sufficiently confuted, partly by the great number of species com- 

 mon to them all, which clearly indicates the connection in former times 

 with a larger country, partly by the circumstance that most of the spe- 

 cies common to the islands are in no way better adapted for migration 

 over the water than those peculiar to the Virgin Islands only ; in fact, 

 but few of them apparently possess the faculty of crossing salt-water 

 even for a limited distance. 



Supposing the theory of a prolonged or oftener repeated connection 

 between Porto Eico and the Virgin Islands to be correct, it remains 

 still to explain how St. Croix can have obtained a number of species 

 which do not occur in the latter group. A few of these species, viz, 

 Castela erecta, Maytenus elcvodenclroides, Zizyphus reticulatus, Anthacan- 

 thus jamaicensiSj and Buxus Yalilii, occur in St. Croix on the tertiary 

 limestone only, and seem thus to have avoided the Virgin Islands as 

 not finding there the substratum suited to their organisation. The 

 greater part, however, might, for all apparent reasons, as well occur in 

 the Virgin group as in St. Croix, and their absence in the former cannot 

 be explained in this way. It must, however, be understood that whilst 

 my investigation of St. Croix has been thorough, and carried on for 

 several years, my exploration of the Virgin Islands has been so for only 

 a part of them, especially the Danish ones, my collections from the 



* Hooker : On Insular Floras. 



t Griseb. : Geogr. Verbr. tier Pfl. Westindiens, p. 55. 



