Lamarck did not have, as we now have, a knowledge of the geological succession of organic forms. The comparatively full and detailed view which we possess of the different vast assemblages of plant and animal life which have successively peopled the surface of our earth is a vision on which his eyes never rested. His slight, piecemeal glimpse of the animal life of the Paris Basin, and of the few other extinct forms then known, was all he had to depend upon or reason from. He was not disposed to believe that the thread of life once begun in the earliest times could be arbitrarily broken by catastrophic means; that there was no relation whatever between the earlier and later faunas. He utterly opposed Cuvier's view that species once formed could ever be lost or become extinct without ancestors or descendants. He on the contrary believed that species underwent a slow modification, and that the fossil forms are the ancestors of the animals now living. Moreover, Lamarck was the inventor of the first genealogical tree; his phylogeny, in the second volume of his _Philosophie zoologique_ (p. 463), proves that he realized that the forms leading up to the existing ones were practically extinct, as we now use the word. Lamarck in theory was throughout, as Houssay well says, at one with us who are now living, but a century behind us in knowledge of the facts needed to support his theory. In this first published expression of his views on palæontology, we find the following truths enumerated on which the science is based: (1) The great length of geological time; (2) The continuous existence of animal life all through the different geological periods without sudden total extinctions and as sudden recreations of new assemblages; (3) The physical environment remaining practically the same throughout in general, but with (4) continual gradual but not catastrophic changes in the relative distribution of land and sea and other modifications in the physical geography, changes which (5) caused corresponding changes in the habitat, and (6) consequently in the habits of the living beings; so that there has been all through geological history a slow modification of life-forms.