II. This problem has been approached very frequently and by many methods. But here we can only take into account the attempts which have been made in the name of science. Nor can these possess any interest for us until the time when it was at least possible to make a clear statement of the question, which was impossible as long as no clear definition had been given of _organic species_. In an historic account of the attempts which have been made to solve the question, it is useless, therefore, to go further back than Ray and Tournefort. The publication of Maillet in 1748 is the first attempt which deserves passing attention. I do not intend to repeat here the account which I have given elsewhere of the different theories proposed by that talented author, by Buffon, Lamarck, Et. Geoffroy St.-Hilaire, Bory de St.-Vincent, and by Naudin, Gaudry, Wallace, Owen, Gubler, Kölliker, Haeckel, Filippi, Vogt, Huxley, and Mme. Royer. They all have this point in common; they connect the origin of the more highly developed species with transmutations undergone by inferior species. But there the resemblance ceases, and their theories frequently differ entirely on all other points. In short, their ideas may be arranged in two principal groups according as their authors favour a _rapid_ or a _gradual transmutation_. The former admit the sudden appearance of a new type produced by a being entirely different: according to them the first bird came from the egg of a reptile. The latter maintain that the modifications are always gradual, that between one species and another a number of links have intervened which unite the two extremes. They consider that types are only multiplied slowly, and by a progressive differentiation. In reality the first of these two theories has never been stated in such a manner as to form a real doctrine; it has never formed a school. The philosophers who promoted it confined themselves most frequently to pointing out, in a general manner, the _possibility_ of the phenomenon, while they attributed it to some accident. At most they invoke in aid of this possibility, some analogies borrowed from the history of ordinary individual development, from that of alternate generation, or of hyper-metamorphosis; they produce no definite fact in justification of their assertions. With the exception perhaps of the hypothesis of M. Naudin, which we shall presently discuss, all these theories which favour a rapid transmutation deserve a still graver reproach, that, namely, of neglecting the great general facts exhibited by the organic kingdom. An explanation of the multiplication and the succession of principal or secondary types by some hypothesis is not sufficient. Special account must be taken of the relations which connect these types, of the order which rules the whole and which has been maintained from remote geological periods through all the revolutions of the globe, and in spite of changes in fauna and flora.