220 The American Naturalist. [March, 
the great lava beds may have had. As glacial cold had its in- 
fluence, the heat resulting from these out-pourings was not with- 
out its effect. 
The great arid deserts of our south-west, which extend into 
California, and make their influence felt even to the coast, have 
had a most important influence in the determination of the char- 
acter of the fauna and flora of Southern California. This influ- 
ence has exerted itself in much the same way as the cold‘ of the 
glacial epoch. The drying up of great tracts of land necessarily 
led to a crowding of the denizens of the tract thus desiccated 
into regions not so greatly affected. The result of an influx of 
individuals intensifies there a struggle for existence, and leads to 
extermination of less fortunate or weak genera and species. The 
regions most distantly removed from the most arid regions would 
necessarily be least affected by the increased desiccation, and on 
these out-posts of the continents, the islands, where the environ- 
ment is least modified by this climatic change, we can look for 
the survivors of the old faunas and floras. The results arrived at 
by this a priori reasoning are exactly what Mr. Greene finds in 
the distribution of the plants on the Santa Barbara Islands. From 
his study of the plants of these islands, more especially Santa 
Cruz, the following, among other conclusions, are derived: 
I. Forty-eight out of two hundred and ninety-six species of 
plants collected are peculiar to these islands, and twenty-eight are 
peculiar to Santa Cruz. 
2. All the species are distinctly Californian and those species 
which are now found in small numbers in a straggling condition 
on the neighboring mainland are very abundant on the island. 
3. The genus Lavatara, of which eighteen species are known 
in the Mediterranean region, and one from Australia, is repre- 
sented by four species on the island of Santa Cruz. There is not 
another known species from the American continent. 
There is also a significant fact which is believed to bear on 
our discussion, viz.: the distribution of land shells on the islands 
and the neighboring mainland, Indefatigable collectors of West 
oo the question of whether the glacial period was or was not one of great 
cold. 



