VI 
DIFFICULTIES AND OBJECTIONS 
147 
new species. This was the keynote of Mr. Vernon Wollaston’s 
essay on “Variation of Species,” published in 1856, and it is 
adopted by the Rev. J. G. Guliek in his paper on “ Diversity 
of Evolution under one Set of External Conditions ” (Journ. 
Linn. Soc. Zool., vol. xi. p. 496). The idea seems to be 
that there is an inherent tendency to variation in certain 
divergent lines, and that when one portion of a species is 
isolated, even though under identical conditions, that tendency 
sets up a divergence which carries that portion farther and 
farther away from the original species. This view is held to 
be supported by the case of the land shells of the Sandwich 
Islands, which certainly present some very remarkable 
phenomena. In this comparatively small area there are 
about 300 species of land shells, almost all of which belong 
to one family (or sub-family), the Achatinellidse, found 
nowhere else in the world. 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. 
Guliek 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.’’ He urges, that these constant differences 
cannot be attributed to natural selection, because they occur 
in different valleys on the same side of the mountain, where 
food, climate, and enemies are the same; and also, because 
there is no greater difference in passing from the rainy to the 
dry side of the mountains than in passing from one valley to 
