The instrument used at Harvard was a " meridian photometer," constructed on the principle of polarization; while the " method of extinctions," by means of a wedge of neutral-tinted glass, served for the Oxford determinations.
Such decline is by no means a universal law of life, however, because among many of the continental vertebrates at least we observe extinctions repeatedly occurring during the expression of maximum variability.
His famous " law of correlation," which by its apparent brilliancy added enormously to his prestige, is not supported by modern philosophical anatomy, and his services to stratigraphy were diminished by his generalizations as to a succession of sudden extinctions and renovations of life.
In Cuvier's defence Charles Deperet maintains that the extreme theory of successive extinctions followed by a succession of creations is attributable to Cuvier's followers rather than to the master himself.
Deperet points also that we owe to Cuvier the first clear expression of the idea of the increasing organic perfection of all forms of life from the lower to the higher horizons, and that, while he believed that extinctions were due to sudden revolutions on the surface of the earth, he also set forth the pregnant ideas that the renewals of animal life were by migration from other regions unknown, and that these migrations were favoured by alternate elevations and depressions which formed various land routes between great continents and islands.