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Show [CHAP. X. GEOLOGICAL SUCCESSION. 288 attention either to the puted. For if we. confife ou;he series is far less perf~ct living or to the ~xtinct a .o~~' one eneral system. W lth than if we combine both In h 1 g a es could be filled respect to the. V erte~rata, w m 0 o~rp ~eat palreontolog~st, with striking Illustratio!!s ~ro ·mals fan in between existOwen, showin5 ho.w extnlrc da~he Ruminants and Pachying groups. uvier r~ndi:tinct orders of mammals; but derms, as t~e two mos man fossil links, that he has had Owen has discoveref s~fi t"~n of these two orders; and to alter the who~e c assi ~:r~s in the same sub-order with has placed certain pach) h dissolves by fine gradaruminants : for examv de, d. fference between the pig and tions the aplfurently ;~ eth~ Invertebrata, Barrande, and the camel. · :egar 1; t be named asserts that he is a higher authority co~ ~~ozoic animals though belongevery day taught thd pa families · or g~nera with those ing to the same ort ~·s, ere n~t at this epoch limited living at the presen ay, w d. . t ups as they now are. in such 1str~1c gro b. ted to any extinct species or Some wn~ers h.ave o ;s1~ered as intermediate between group of sp~eCleS bmng co If by this term it is meant that living ~peCies or gro~ps. tly intermediate in all its charan extinct form lS . I~ec ms the ob. ection is probably acters between two ~VI~gt{0~ in' a perf~ctly natural classivalid. B"Q.t I app:f en . , ould have to stand between £.cation ma~y fossl speCies ~·net enera between living living speCies, and some ex~elon~ng to distinct families. genera, even between genera ecially with respect to very The most common cas~ }:P d re tiles seems to be, that distinc~ grou[s, s~ch ~! Jistf:guisled at the present day supposing t em o n characters the ancient Inemfrom each other by a doze uld b~ distinguished by a bers of the same two groups w~ ters so that the two somewhat .lesser numb~r of ·~ ad~~tinct at that period groups though former y qui e hi th ' ' · 11 approach to eac o er. · made ~orne snta b. 1' f th t the more ancient a form ~s, It 1s a common e le a t b some of Its by so much the more it. tends to ~:Jcfro~ each other. characters groups nbow Wldte~ se!s~rioted to those groups This remark no dou t mus -.ve r .· CHAP. X.J AFFINITIES OF EXTINCT SPECIES. 289 which hav·e undergone much change in the course of geological ages ; and it would be difficult 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 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 complex, I must request the reader to turn to the diagram in the fourth chapter. We 1nay 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 for1nations, and all the forms beneath the uppermost line may be considered as extinct. The three existing genera a 14 q14 p 14 will form a small family· b14 and fa, a close'l y a'l lied' fam' ily or ·sub-family; and o14', e14,' m14; a third family. These three families, together with the many extinct genera on the several lines of descent diverging from the parent-forn1 A, will form an order; for all will have inherited something in common from their ancient and common 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. lienee we can understand the rule that the most ancient fossils differ most fron1 existing forms. vV e must not, however, 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 o~ some Silurian for1ns, that a species might go on being S~Ightly modified in relation to its slightly altered conditions of life, and yet retain throughout a vast period the 13 |