The
interesting point is the extreme restriction of the species and
varieties. The average range of each species is only five or six miles,
while some are restricted to but one or two square miles, and only a
very few range over a whole island. The forest region that extends over
one of the mountain-ranges of the island of Oahu, is about forty miles
in length and five or six miles in breadth; and this small territory
furnishes about 175 species, represented by 700 or 800 varieties. Mr.
Gulick states, that the vegetation of the different valleys on the same
side of this range is much the same, yet each has a molluscan fauna
differing in some degree from that of any other. "We frequently find a
genus represented in several successive valleys by allied species,
sometimes feeding on the same, sometimes on different plants. In every
such case the valleys that are nearest to each other furnish the most
nearly allied forms; and a full set of the varieties of each species
presents a minute gradation of forms between the more divergent types
found in the more widely separated localities.
Pages:
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274