SPECIES 235 without any notable difference being discern- ible between ancestor and descendant. More- over, in some cases, varieties are found to re- vert to the characters of the species from which they have proceeded. The conclusion has been drawn that species are physiologically fixed; that is to say, that, however long the process of generation may be continued, the individuals either retain the identical morphological pecu- liarities of the oldest ancestor, or, if they vary, the varieties remain fertile with one another. Assuming that species have the physiological fixity thus inditated, certain conclusions re- specting the origin of species are inevitable. It is clear that no existing species can have arisen by the intercrossing of preexisting spe- cies, or by the variation of preexisting species ; but that every species must either have existed from all eternity, or have come into existence suddenly in its present form, which is the ob- jective fact denoted by what is termed " crea- tion." At the dawn of modern biology, a cen- tury ago, no scientific evidence respecting the real history of life on the globe was extant, and, for any proof that existed to the contrary, species might have been of eternal duration. But philosophical speculation combined with theological dogma not only to favor the con- trary opinion, but to lead the most philosophic naturalist of his day to embody the hypothesis of creation in a definition of species. Totidem numeramus species quot in principio format sunt creates, ( kt We reckon as many species as there were forms created in the beginning"), is the well known formula of Linnssus.. In practice, Linnaeus regarded species from a pure- ly morphological point of view ; in theory, he assumed the ancestral creation and the limited variability of species, though he was disposed to allow more freedom in this direction than most of his successors. On the other hand, he seems to have attached comparatively little weight to the assumed sterility of hybrids, and to have held a sort of modified doctrine of evolution, supposing that existing species may have been produced by the interbreeding of comparatively few primordial forms. It is mainly to the influence of Cuvier's authority that we owe the general acceptance of the views respecting the physiological characters of species which till within the last few years have been almost universally prevalent. In the introduction to the Begne animal (1817), Cuvier writes: "There is no proof that all the differences which now distinguish organ- ized beings are such as may have been pro- duced 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 pres- ent. We are thus obliged to admit of cer- tain forms which since the origin of things have been perpetuated, without exceeding these limits ; and all the beings appertaining to one of these forms constitute 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 re- union of individuals descended from one an- other, or from common parents, or from such as resemble them as closely as they resemble each other; but, although this definition is rigorous, 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 charac- ters, 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 fol- lowers into curious difficulties. Developing the fundamental idea of the Discours sur les revolutions de la surface du globe, naturalists were necessarily led to conclude, not only that existing species are the result of creation, but that the creative act which brought them into being was only the last repetition of a series of such acts, by which the often depopula- ted world has been as frequently repeopled. Lamarck, Cuvier's contemporary and country- man, 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 Darwin's " Origin of Species," that it has already almost swept opposition away. La- marck's vast acquaintance with the details of invertebrate zoology 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 ap- parently 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- light the tenuity of the distinction between species and varieties. The facts of embry- ology, the occurrence of rudimentary organs, and the fundamental unity of structure which obtains in vast groups, such as the vertebrata and arthropoda, further tended to suggest the existence of a genetic connection between the members of these groups; so that Lamarck was induced to renounce the doctrine of the fixity of species, and to define a species as " a collection of individuals which resemble each other and produce their like by generation, so long as the surrounding conditions do not alter to such an extent as to cause their habits, characters, and forms to vary." According to this definition, the distinction between species and variety once more becomes conventional. A variety is, in fact, a nascent species; and
