is it pretended for
a moment that use and disuse are the sole or even the chief factors
in variation."

As early as 1868 the Lamarckian factor of isolation, due to migration
into new regions, was greatly extended, and shown by Moritz Wagner[234]
to be a most important agent in the limitation and fixation of varieties
and species.

"Darwin's work," he says, "neither satisfactorily explains the
external cause which gives the first impulse to increased individual
variability, and consequently to natural selection, nor that
condition which, in connection with a certain advantage in the
struggle for life, renders the new characteristics indispensable.
The latter is, according to my conviction, solely fulfilled by the
voluntary or passive migration of organisms and colonization, which
depends in a great measure upon the configuration of the country; so
that only under favorable conditions would the home of a new species
be founded."

This was succeeded by Rev. J. T. Gulick's profound essays "On Diversity
of Evolution under One Set of External Conditions"[235] (1872), and on
"Divergent Evolution through Cumulative Segregation"[236] (1887).

These and later papers are based on his studies on the land shells of
the Hawaiian Islands. The cause of their extreme diversity of local
species is, he claims, not due to climatic conditions, food, enemies, or
to natural selection, but to the action of what he calls the "law of
segregation."

Fifteen years later Mr. Romanes published his theory of physiological
selection, which covered much the same ground.

A very strong little book by an ornithologist of wide experience,
Charles Dixon,[237] and refreshing to read, since it is packed with
facts, is Lamarckian throughout. The chief factor in the formation of
local species is, he thinks, isolation; the others are climatic
influences (especially the glacial period), use and disuse, and sexual
selection as well as chemical agency. Dixon insists on the "vast
importance of isolation in the modification of many forms of life,
without the assistance of natural selection." Again he says: "Natural
selection, as has often been remarked, can only preserve a beneficial
variation--it cannot originate it, it is not a cause of variation; on
the other hand, the use or disuse of organs is a direct cause of
variation, and can furnish natural selection with abundance of material
to work upon" (p. 49). The book, like the paper