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Robinson and Greenman— Galapagos Flora. 137 
ative or complete isolation seems necessary to account for the 
racially divergent floras of the islands; and especially for the 
occurrence of only one form upon each island. It would thus 
appear necessary, in accounting for the present distribution, to 
assume that at one time in the remote past the islands were 
either united, or at least that the channels which separate 
them were less formidable barriers to seed-transportation than 
at present, so that a general distribution of species could have 
been effected ; and that subsequently, as the islands separated, 
or as the channels through some change of currents, or other 
cause, became less easily passed, an era of much greater isola- 
tion of the floras of the different islands came about. The 
divergence of character of the vegetation would then begin at 
once and the otherwise unaccountable existence of a single and 
peculiar form upon each island would be readily intelligible. 
_ While not prepared to make any positive assertion regarding 
the probable origin of the islands, the authors fail to see in the 
hitherto generally accepted theory of elevation any satisfac- 
tory explanation for the harmonic yet divergent floras of the 
different members of the group. Upon Dr. Baur’s assumption 
of a former union between the islands, and subsequent separa- 
tion by subsidence, not only is explanation possible, but the 
existing flora of the archipelago is just that which would most 
naturally result from such an origin. A former union of the 
islands would account at once for the occurrence of identical 
ancestral species upon the different members of the group, and 
the subsequent separation give the needed isolation for varietal 
and racial divergence, while the latter could not have come about 
if a continual interchange of seed were taking place from 
island to island. 
Regarding a former land-connection with the continent, 
which would certainly offer much greater geological difficulties, 
the botanical evidence is still too vague to merit regard. The 
affinities of the vegetation of the upper, moister portions of 
the islands are doubtless, as has been assumed, with the floras 
of Columbia, Central America, Southern Mexico, and the West 
Indies, while much of the desert flora of the lower regions has 
doubtless been derived from the arid regions of Chili and 
Peru. Butso far as botanical data are concerned this could 
have come about either by migration by land or by transporta- 
tion by oceanic currents and, as the latter still exist, it seems 
unnecessary to assume the former. However, upon this point 
the evidence from the vegetation appears to be still wholly 
indecisive. 
The harmonic relation of the floras of the different islands, 
which, as we have seen, appears to have such an interesting 
bearing upon the former possible connection of the islands 
