424 BOTANY. 
floras of Mexico and the Antilles, the oaks being more numerous _ 
in Mexico than any other portion of the world, whilst they are | 
wholly wanting in the Antilles, although the latter afford climatic 
conditions favorable to their growth in many places. This con- 
trast, can be partially explained, as regards the oaks, by the fact 
that the seeds of the latter soon lose their power of germination, 
and are not easily transported by currents; and besides the oaks 
occur in the mountainous regions of Mexico, remote from the sea, 
and even if the seeds were transported by the aid of rivers, they 
would not find conditions favorable to their development on the 
coast of the Antilles, a fact in harmony with the general rule, that 
the larger number of the plants common to the Antilles and the 
continent belong to the lowlands of the tropics, whilst the plants 2 
of the mountains are generally endemic. The distribution of the 
_ Cupuliferæ also substantiates, in a remarkable manner, the general 
rule that the floras richest in endemic species are those where the 
physical obstructions to diffusion of plants are greatest, the ocean, 
high snow-covered mountain ranges, especially those with their 
axes perpendicular to the direction of the wind, forming sharply 
defined limits of floras. Thus whilst the white oak occurs all over 
Europe, the species of cupuliferse in Sumatra and Java are entirely = 
different from -each other. In like manner the characteristic i | 
cupuliferæ of California are restricted to the western slope of the — 
Nevada chain, and the beeches of Chili are entirely excluded from - 
the east side of the snow-covered Cordilleras. This family also a 
manifests the usual anomalies to the general rule, that the zone of - 
vegetation becomes more elevated near the equator, caused by P% 
culiarities in the form of the mountains, and the influence To 
clouds, a high plateau, with stronger insolation, producing & pe ; 
siderable elevation of the zone and the snow line, as in Bor 
and Thibet, whilst abrupt, isolated peaks have a reverse pe 
Thus Europe exhibits the influence of plateaus upon the cupulifer® é 
in two points, namely in the central part of the Alps, @ = 
1000 feet higher than in the Bavarian Alps. Again the chest 
zone, which reaches 5,000 feet on the Sierra Nevadas, which ‘he 
on the plateau of Grenada, does not rise above 3,200 ep . to 
same latitude, in Portugal. This is owing, it is true, in patt 
