.538 The American Naturalist. [June, 
assume that changed conditions of environment acting on isolated 
species have resulted in their evolution into new species, presum- 
ably better adapted to theif surroundings. But just what condi- 
tions have effected a given result we do not know. In the further 
division of a Bahaman species into two. or more races the case 
‘becomes even more perplexing. We have not, then, different 
physiographic or climatic conditions to the influences of which we 
may ascribe observed changes. On the contrary, we find differ- 
ent forms of the same species inhabiting islands almost within 
sight of each other, where they are apparently subjected to simi- 
lar conditions of existence. In several instances these differences, 
though here constant and characteristic, are not greater than those 
presented by individual variation in a larger series of a given 
species from one locality. Perhaps we can assume, then, that 
through the continued isolation of a comparatively small number 
of individuals certain characters, due originally to purely individual 
variation, have become perpetuated and specific. Amonga smaller 
number of birds the extent of variation would, of course, be less; 
but this would be more than counterbalanced by the fact that 
any new character would be far more likely to be prese 
‘through a forced interbreeding of closely related individuals. 
Of the age of the Bahaman avifauna we can, of course, judge 
only by comparison. But the conditions which govern any given 
areas vary so greatly that even in this way we can form only an 
approximate idea of the relative age of their faune. 
The isolation afforded by insular existence in tending to Pr& 
serve new characters would at the same time hasten the consum- 
mation of permanent forms. The rate of divergence, therefore, 
is, without doubt, more rapid among island-inhabiting ee 
than among those confined to the mainland. 
From the comparative ease with which we have been able 8 
trace the specific relationships of most of the endemic birds, and, i 
with one exception, from the absence of peculiar genera, it is ee 
able that the Bahaman avifauna is of recent origin. Being s0 
largely derived from, it is, of course, more recent than, that of the : 
larger West Indian Islands, where sixteen endemic genera occur; — 
indeed, is not so old as the avifauna of the Windward i 
