ibly admit the
above rule, seeing that the grouse and pheasant (considered by some good
ornithologists as forming two families), the bull-finch and canary-bird
have bred together.

{517} This corresponds to a paragraph in the _Origin_, Ed. i. p.
483, vi. p. 662, where it is assumed that animals have descended
"from at most only four or five progenitors, and plants from an
equal or lesser number." In the _Origin_, however, the author goes
on, Ed. i. p. 484, vi. p. 663: "Analogy would lead me one step
further, namely, to the belief that all animals and plants have
descended from some one prototype."

No doubt the more remote two species are from each other, the weaker the
arguments become in favour of their common descent. In species of two
distinct families the analogy, from the variation of domestic organisms
and from the manner of their intermarrying, fails; and the arguments
from their geographical distribution quite or almost quite fails. But if
we once admit the general principles of this work, as far as a clear
unity of type can be made out in groups of species, adapted to play
diversified parts in the economy of nature, whether shown in the
structure of the embryonic or mature being, and especially if shown by a
community of abortive parts, we are legitimately led to admit their
community of descent. Naturalists dispute how widely this unity of type
extends: most, however, admit that the vertebrata are built on one type;
the articulata on another; the mollusca on a third; and the radiata on
probably more than one. Plants also appear to fall under three or four
great types. On this theory, therefore, all the organisms _yet
discovered_ are descendants of probably less than ten parent-forms.

_Conclusion._

My reasons have now been assigned for believing that specific forms are
not immutable creations{518}. The terms used by naturalists of affinity,
unity of type, adaptive characters, the metamorphosis and abortion of
organs, cease to be metaphorical expressions and become intelligible
facts. We no longer look at an organic being as a savage does at a
ship{519} or other great work of art, as at a thing wholly beyond his
comprehension, but as a production that has a history which we may
search into. How interesting do all instincts become when we speculate
on their origin as hereditary habits, or as slight congenital
modifications of former instincts perpetuated by the individu