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Show 330 GEOLOGICAL SUCCESSION. CHAP. X. wide difference between the pig and the camel. In regard to the Invertebrata, Barrande, and a. higher authority could not be named, asserts that he rs .every day taught that palreozoic animals, thou?h belongu?·~ to the same orders, families, or genera wrth those hvrng at the present day, were not at this early epoch limited in such distinct groups as they now are. Some writers have objected to any extinct species or group of species being considered as inter:mediate. b~tween living species or groups. If by thrs term rt rs meant that an extinct form is directly intermediate in all its characters between two living forms, the objection is probably valid. But I apprehend that in a perfectly natural classification many fossil species would have to stand between living species, and some extinct genera between living genera, even between genera belonging to distinct fa1nilies. The most common case, especially with respect to very distinct groups, such as fish and reptiles, seems to be, that supposing them to be distinguished at the present day from each other by a dozen characters, the ancient members of the same two groups would be distinguished by a somewhat lesser number of characters, so that the two groups, though formerly quite distinct, at that period made some small approach to each other. It is a common belief that the more ancient a form is, by so much the more it tends to connect by some of its characters groups now widely separated from each other. This remark no doubt must be restricted to those groups which have undergone much change in the course of geological ages ; and it would be dif-ficult to prove the truth of the proposition, for every now and then even a living animal, as the Lepidosiren, is discovered having affinities directed towards very distinct groups. Yet if we compare the older Reptiles and CIIAP.X. AFFINITIES OF EXTINCT SPECIES. 331 Batrachians, the older Fish, the older Cephalopods, and the eocene Mammals, with ·the more recent members of the same classes, we must admit that there is some truth in the remark. Let us see how far these several facts and inferences accord with the theory of descent with modification. As the subject is somewhat con1plex, I must request the reader to turn to the diagram in the fourth chapter. We may suppose that the numbered letters represent genera, and the dotted lines diverging from them the species in each genus. The diagram is much too simple, too few genera and too few species being given, but this is unimportant for us. The horizontal lines may represent successive geological formations, and all the forms beneath the uppermost line may be considered as extinct. The three existing genera, a14 , q1 \ p 14, will form a small family; b 14 and f 14 a closely allied family or sub-family; and o 14 e14, m 14, a third family. These three families, together with the many extinct genera on the several lines of descent diverging from the parent-form A, will form an order ; for all will have inherited something in common from their ancient and con1mon progenitor. On the principle of the continued tendency to divergence of character, which was formerly illustrated by this diagram, the more recent any form is, the more it will generally differ from its ancient progenitor. Hence we · o Cf.ln understand the rule that the most ancient fossils · • t: differ most from existing forms. We must not, how-ever, assume that divergence of character is a necessary contingency ; it depends solely on the descendants from a species being thus enabled to seize on many and different places in the economy of nature. Therefore it is quite possible, as we have seen in the case of some Silurian forms, that a species might go on being slightly |