f which belong to one family (or
sub-family), the Achatinellidae, 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.
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." 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 another on the same side
an equal distance apart. In a very lengthy paper, presented to the
Linnean Society last year, on "Divergent Evolution through Cumulative
Segregation," Mr. Gulick endeavours to work out his views into a
complete theory, the main point of which may perhaps be indicated by the
following passage: "No two portions of a species possess exactly the
same average character, and the initial differences are for ever
reacting on the environment and on each other in such a way as to ensure
increasing divergence in each successive generation as long as the
individuals of the two groups are kept from intercrossing."[49]

It need hardly be said that the views of Mr. Darwin and myself are
inconsistent with the notion that, if the environment were absolutely
similar for the two isolated portions of the species, any such necessary
and constant divergence would take place. It is an error to assume tha