It is mainly to the influence of Cuvier's authority that we owe the general acceptance of the views respecting the physiological character of species, which up till within the last few years have been almost universally prevalent.
In the introduction to the "Règne Animal" (1816), Cuvier writes:
"There is no proof that all the differences which now distinguish organized beings are such as may have been produced by circumstances. All that has been advanced upon this subject is hypothetical; experience seems to show, on the contrary, that, in the actual state of things, varieties are confined within rather narrow limits, and, so far as we can retrace antiquity, we perceive that these limits were the same as at present.
"We are thus obliged to admit of certain forms, which since the origin of things have been perpetuated, without exceeding these limits; and all the beings appertaining to one of these forms constitutes what is termed a species. Varieties are accidental subdivisions of species.
"Generation being the only means of ascertaining the limits to which varieties may extend, species should be defined, the reunion of individuals descended from one another, or from common parents, or from such as resemble them as closely as they resemble each other; but, although this definition is vigorous, it will be seen that its application to particular individuals may be very different when the necessary experiments have been made."
It need hardly be said, however, that in practice Cuvier founded his species upon purely and exclusively morphological characters, just as his predecessors and successors have done. The combination of Cuvier's views on the fixity of species with the discovery of the succession of life on the globe, which was so largely the result of his labors, led his successors into curious difficulties. Developing the fundamental idea of the "Discours sur les Révolutions de la Surface du Globe," naturalists were forced to conclude not only that existing species are the result of creation, but that the creative act by which they were brought into being was only the last repetition of a series of such acts by which the often depopulated world has been as frequently repeopled, and thus orthodox belief respecting the existing flora and fauna led to a terribly heterodox cosmogony.
The contemporary and countryman of Cuvier, Lamarck, must be regarded as the chief founder of the reaction against the doctrines which Cuvier advocated—a reaction which, overpowered and disregarded for many years, has acquired such force since and through the publication of the "Origin of Species," that it has almost swept opposition away. Lamarck's vast acquaintance with the details of invertebrate zoölogy rendered him familiar with the great variability of many species, and led him to see that variation is in some way related to change of conditions; the frequent occurrence of transitional forms between apparently distinct species, when large suites of specimens (especially when they are obtained from different parts of a wide geographical area) are examined, tended to bring into strong
