ition, displayed in it, I can scarcely
understand how there has come from the same pen a passage in which none
of these traits are exhibited. Even one wholly unacquainted with the
subject may see in the last two sentences of the above extract, how
strangely its propositions are strung together. While in the first of
them I am represented as bringing forward a "new factor," I am in the
second represented as saying that I mentioned it twenty years ago! In
the same breath I am described as claiming it as new and asserting it as
old! So, again, the uninstructed reader, on comparing the first words of
the extract with the last, will be surprised on seeing in a scientific
article statements so manifestly wanting in precision. If "natural
selection is a mere phrase," how can Mr. Darwin, who thought it
explained the origin of species, be regarded as wise? Surely it must be
more than a mere phrase if it is the key to so many otherwise
inexplicable facts. These examples of incongruous thoughts I give to
prepare the way; and will now go on to examine the chief propositions
which the quoted passage contains.

The Duke of Argyll says that "heredity is the central idea of natural
selection." Now it would, I think, be concluded that those who possess
the central idea of a thing have some consciousness of the thing. Yet
men have possessed the idea of heredity for any number of generations
and have been quite unconscious of natural selection. Clearly the
statement is misleading. It might just as truly be said that the
occurrence of structural variations in organisms is the central idea of
natural selection. And it might just as truly be said that the action of
external agencies in killing some individuals and fostering others is
the central idea of natural selection. No such assertions are correct.
The process has three factors--heredity, variation, and external
action--any one of which being absent, the process ceases. The
conception contains three corresponding ideas, and if any one be struck
out, the conception cannot be framed. No one of them is the central
idea, but they are co-essential ideas.

From the erroneous belief that "heredity is the central idea of natural
selection" the Duke of Argyll draws the conclusion, consequently
erroneous, that "natural selection includes and covers all the causes
which can possibly operate through inheritance." Had he considered the
cases which, in the _Principles of Biology_, I have cited to il