XVIII INTRODUCTION, 
modifications; others, again, have only been found in quite 
limited localities. One reason for this circumstance may be found 
in the regular currents which pass through the various channels 
from northeast to southwest and effectually prevent communica- 
tion by sea. 
As the age of the various islands increases in progression 
from east to west it may be inferred that the richness in endemic 
species will stand in the same ratio. Kauai, although in point 
of extent only fourth in the list, is not only richest in species 
but has them also on the whole more differentiated. In several 
genera, as Schiedea, Raillardia, Dubautia, the Kauai species are 
more individualized than the rest. 
The southwest or Kaala range of Oahu would seem to be of 
the same age as Kauai, and perhaps is not inferior to it in 
number of peculiar species. The character of this range differs 
from that of the main range of the same island, inasmuch as 
it is more disintegrated and broken up, and, lying under the 
lee, has a drier climate and no permanent watercourses of any 
importance. It extends in length about twenty miles; its highest 
mountain, Kaala, has an elevation of 4000 ft., the next in 
height, Puakea, being nearly 1000 ft. lower. The main range 
of Oahu, which runs through the whole length of the island, 
has a rather gentle slope on the leeward side, and is there 
furrowed by broad and deep valleys, each with a stream, except 
those of the rather low eastern extremity. This southern or lee 
side is, or rather was, fairly covered with forest. At the head 
of Nuuanu valley, on each side of it, stand the highest peaks 
of the range, Konahuanui and Waiolani, with elevations of 
about 3000 ft., from which point the range gradually lowers to 
its eastern and western ends. It forms a slight curve, approaching 
close to the northern shore at the extremities while the middle 
portion recedes from it and falls of in an almost perpendicular, 
unbroken rock wall into the low lying district of Koolau, which 
is in part surrounded by it as by an amphitheater, and produces 
the impression of having been a vast crater, the northern wall 
of which had been submerged in the sea. This rock wall 
furrowed by streams issuing from the adjacent ranges. 
The island of Molokai is probably of the same age as the 
main range of Oahu, but of a different character, since the 
