The abrupt manner in which whole groups of species suddenly appear in certain formations has been urged by several paleontologists--for instance, by Agassiz, Pictet, and Sedgwick--as a fatal objection to the belief in the transmutation of species. If numerous species, belonging to the same genera or families, have really started into life at once, the fact would be fatal to the theory of evolution through natural selection. For the development by this means of a group of forms, all of which are descended from some one progenitor, must have been an extremely slow process; and the progenitors must have lived long before their modified descendants. But we continually overrate the perfection of the geological record, and falsely infer, because certain genera or families have not been found beneath a certain stage, that they did not exist before that stage. In all cases positive paleontological evidence may be implicitly trusted; negative evidence is worthless, as experience has so often shown. We continually forget how large the world is, compared with the area over which our geological formations have been carefully examined; we forget that groups of species may elsewhere have long existed, and have slowly multiplied, before they invaded the ancient archipelagoes of Europe and the United States. We do not make due allowance for the intervals of time which have elapsed between our consecutive formations--longer, perhaps, in many cases than the time required for the accumulation of each formation. These intervals will have given time for the multiplication of species from some one parent-form; and, in the succeeding formation, such groups or species will appear as if suddenly created. HOW LITTLE WE KNOW OF FORMER INHABITANTS OF THE WORLD. [Page 283.]