Origin of Species Variorum

size and in all other characters to the species still living in South America; and some of these fossils may be the actual progenitors of living species. It must not be forgotten that, on our theory, all the species of the same genus are the descendants of some one species; so that, if six genera, each having eight species, be found in one geological formation, and in a succeeding formation there be six other allied or representative genera each with the same number of species, then we may conclude that generally only one species of each of the .. older genera has left modified descendants, which constitute the several species of the new genera; the other seven species of each old genus having died out and .. left no progeny. Or, and this probably will be a far commoner case, two or three species in two or three alone of the six older genera will be the parents of the .. new genera: the other .. species and the other whole genera having become utterly extinct. In failing orders, with the genera and species decreasing in numbers, as .. is the case with the Edentata of South America, still fewer genera and species will leave modified blood-descendants.

Summary of the preceding and present Chapter .

I have attempted to show that the geological record is extremely imperfect; that only a small portion of the globe has been geologically explored with care; that only certain classes of organic beings have been largely preserved in a fossil state; that the number both of specimens and of species, preserved in our museums, is absolutely as nothing compared with the .. number of generations which must have passed away even during a single formation; that, owing to subsidence being almost necessary for the accumulation of .. deposits