Plateau has shown (20/92. On the Vision of Fishes and Amphibia, translated in 'Annals and Mag. of Nat. Hist.' volume 18 1866 page 469.), eyes constructed on the following plan: "the cornea is always flat, or at least much flattened in the 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 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 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.' (20/93. Sixth edition 1872 page 144.) CHAPTER 2.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.