sive modifications of either kind.
Amphibious animals, which are enabled to see both in the water and in the
air, require and possess, as M. Plateau has shown,[533] eyes constructed on
the following plan: "the cornea is always flat, or at least much flattened
in front of the crystalline and over a space equal to the diameter of that
lens, whilst the lateral portions may be much curved." The crystalline is
very nearly a sphere, and the humours have nearly the same density as
water. Now, as a terrestrial animal slowly became more and more aquatic in
its habits, very slight changes, first in the curvature of the cornea or
crystalline, and then in the density of the humours, or conversely, might
successively occur, and would be advantageous to the animal whilst under
water, without serious detriment to its power of vision in the air. It is
of course impossible to conjecture by what steps the fundamental structure
of the eye in the Vertebrata was originally acquired, for we know
absolutely nothing about this organ in the first progenitors of the class.
With respect to the lowest animals in the scale, the transitional states
through which the eye at first probably passed, can by the aid of analogy
be indicated, as I have attempted to show in my 'Origin of Species.'[534]

* * * * *

{224}

CHAPTER XXI.

SELECTION, _continued_.

NATURAL SELECTION AS AFFECTING DOMESTIC PRODUCTIONS--CHARACTERS WHICH
APPEAR OF TRIFLING VALUE OFTEN OF REAL IMPORTANCE--CIRCUMSTANCES
FAVOURABLE TO SELECTION BY MAN--FACILITY IN PREVENTING CROSSES, AND THE
NATURE OF THE CONDITIONS--CLOSE ATTENTION AND PERSEVERANCE
INDISPENSABLE--THE PRODUCTION OF A LARGE NUMBER OF INDIVIDUALS
ESPECIALLY FAVOURABLE--WHEN NO SELECTION IS APPLIED, DISTINCT RACES ARE
NOT FORMED--HIGHLY-BRED ANIMALS LIABLE TO DEGENERATION--TENDENCY IN MAN
TO CARRY THE SELECTION OF EACH CHARACTER TO AN EXTREME POINT, LEADING
TO DIVERGENCE OF CHARACTER, RARELY TO CONVERGENCE--CHARACTERS
CONTINUING TO VARY IN THE SAME DIRECTION IN WHICH THEY HAVE ALREADY
VARIED--DIVERGENCE OF CHARACTER, WITH THE EXTINCTION OF INTERMEDIATE
VARIETIES, LEADS TO DISTINCTNESS IN OUR DOMESTIC RACES--LIMIT TO THE
POWER OF SELECTION--LAPSE OF TIME IMPORTANT--MANNER IN WHICH DOMESTIC
RACES HAVE ORIGINATED--SUMMARY.

_Natural Selection, or the Survival of the Fittest, as affecting domestic
productions._--We know little on this