The principle announced by Dana is accepted by geologists generally. Angelo Heilprin in _"The Earth and its Story,"_ p. 153 ff. has the following: "There has been a steady and progressive advance in the general type of organization from the oldest to the newest periods; more highly developed or more complicated forms have successively replaced forms of simpler construction; and this advance is still continuing to-day. Once more, the correctness of the evolutionary hypothesis is taken for granted. In the oldest rocks, for example, no trace of backboned animals has yet been detected; when such do appear for the first time, they show themselves in their lowest types, the fishes; these are succeeded later by the amphibians (frogs, newts, salamanders), and these again by reptiles. And if we take the fishes by themselves, we find that they, too, begin with their lower, if not absolutely the lowest types, and progressively develop their higher ones. This history is repeated in the cases of the reptiles and quadrupeds--in fact, with every class of animals that is known to us. _Naturalists_ (evolutionists) are to-day well agreed among themselves that all animal and vegetable forms are derivatives from forms that preceded them..... Hence it is, that, in following the geological record, we speak of progressive evolution, the evolving of higher or more complicated types of organisms from those simpler and more general in structure." Now read carefully the following: _"This fact_ has permitted geologists to mark off distinct eras or periods in the life-history of the planet, each of them determined by certain characteristic animal or vegetable forms, which either do not appear before or after such period, or else are by numbers so distinctive of it as to typify it clearly." Evidently, the Philadelphia professor, too, _assumes_ "progressive evolution" _as an ascertained fact_ and in accordance therewith classifies the layers of the earth's surface. "Almost every species of fossil has a definite position in the geological scale, and would by itself serve to locate a formation; but oftentimes the determination of species, owing to insufficiency of knowledge of the obliteration of characters, is a most difficult task, and then recourse is had to the aspect of the entire group 'of fossils which a given rockmass contains. This generally _gives the age_ or position without difficulty." Edward Clodd, in _"The Story of Creation, a Plain Account of Evolution,"_ says, page 18. "The relative _age and place of each stratum .... are fixed by the fossils."_