"686. Tous les individus de cette nature, qui existent, proviennent d'individus semblables qui tous ensemble constituent l'espèce entière. Or, je crois qu'il est aussi impossible à l'homme de connoître la cause physique du premier individu de chaque espèce, que d'assigner aussi physiquement la cause de l'existence de la matière ou de l'univers entier. C'est au moins ce que le résultat de mes connaissances et de mes réflexions me portent à penser. S'il existe beaucoup de variétés produites par l'effet des circonstances, ces variétés ne dénaturent point les espèces; mais on se trompe, sans doute souvent, en indiquant comme espèce, ce qui n'est que variété; et alors je sens que cette erreur peut tirer à conséquence dans les raisonnements que l'on fait sur cette matière."[34]
The first three volumes of Treviranus's Biologie, which contains his general views of evolution, appeared between 1802 and 1805. The _Recherches sur l'organisation des corps vivants_, which sketches out Lamarck's doctrines, was published in 1802; but the full development of his views in the _Philosophie zoologique_ did not take place until 1809.
The _Biologie_ and the _Philosophie zoologique_ are both very remarkable productions, and are still worthy of attentive study, but they fell upon evil times. The vast authority of Cuvier was employed in support of the traditionally respectable hypotheses of special creation and of catastrophism; and the wild speculations of the _Discours sur les révolutions de la surface du globe_ were held to be models of sound scientific thinking, while the really much more sober and philosophical hypotheses of the _Hydrogéologie_ were scouted. For many years it was the fashion to speak of Lamarck with ridicule, while Treviranus was altogether ignored.
Nevertheless, the work had been done. The conception of evolution was henceforward irrepressible, and it incessantly reappears, in one shape or another,[35] up to the year 1858, when Charles Darwin and A.R. Wallace published their _Theory of Natural Selection_. The _Origin of Species_ appeared in 1859; and thenceforward the doctrine of evolution assumed a position and acquired an importance which it never before possessed. In the _Origin of Species_, and in his other numerous and important contributions to the solution of the problem of biological evolution, Darwin confined himself to the discussion of the causes which have brought about the present condition of living matter, assuming such matter to have once come into existence. On the other hand, Spencer[36] and E. Haeckel[37] dealt with the whole problem of evolution. The profound and vigorous writings of Spencer embody the spirit of Descartes in the knowledge of our own day, and may be regarded as the _Principes de la philosophie_ of the 19th century; while, whatever hesitation may not unfrequently be felt by less daring minds in following Haeckel in many of his speculations, his attempt to systematize the doctrine of evolution and to exhibit its influence as the central thought of modern biology, cannot fail to have a far-reaching influence on the progress of science.