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Show 382 DARWINISM CHAP. $pecies), which are named and described in this monog:aph,. and between which, as the authors show, so many connectmg lmks, clearly illustrating the derivation of th~ newer from the older t es have been detected. On the mmds of those who carc-fuylply ' examine the admirably engraved f.i gures .g.i ven m. t h c lates accompanyino- this valuable mem01r, or still better, the ~:ry large series of ~pecimens fro~ among whic~ the subjects of these figures are selected, a~d whiCh are .now m the ~useum .0f the Reichsanstalt of VIenna, but httle doubt will, '~c ·suspect, remain that the authors have fully made out their case and have demonstrated that, beyond all controversy, the scri~s with highly complicated ornam?ntation ~ere variously derived by descent-the lines of which .are m most case." perfectly clear and obvious-from the Slii_lple an.d unornamented Vivipara achatinoides of the Congene.n-~chiChte? (the lower division of the series of strata). It IS mterestmg to notice that a large portion of these unquestionably ~~ri ve< l forms depart so widely from the type. of the genus. VIvJpara, that they have been separated on so h1gh n,n authonty as that, of Sand berger, as a new genus, under. the name of Tulotoma. And hence we are led to the conclusiOn that a vast number of forms, certainly exhibiting specific disti~ctions, and a.ccordino- to some naturalists, differences even entitled to be regardetl b n of o-eneric value have all a common ancestry. It is as Prof~ssor Judd remarks, owing to the exceptionally favourable circumstances of a long-continued and unbroken series of deposits being formed under physical conditions either identical or very slowly changing, that we owe so complete a record of -~he process of organic change. U sua~ l y, some disturbing elements, such as a sudden change of physH·n l conditions, or the immigration of new sets of forms from other areas and the consequent retreat or partial extinction of the olderfauna, interferes with thecontinuity.of organic developmen t, and produces those puzzling dlscordanccs so generally met with in geological formations of marine origin. \Vhile. a. case of the kind now described affords evidence of the ongm of species complete and conclusive, though on a necessarily vc:'Y limited scn,le, the very rarity of the conditions which are essentml to such completeness serves to explain why it is that in most cases the direct evidence of evolution is not to be obtained. XIII THE GEOLOGICAL EVIDENCES OF EVOLUTION 383 Another illustration of the filling up of gaps between exi. ting groups is afforded by Professor Huxley's researches on fossil crocodiles. The gap between the existing crocodiles and the lizards is very wide, but as we go back in geological time we meet with fossil forms which are to some extent intermediate and form a connected series. The three living genera- Crocodilus, Alligator, and Gavialis-are found in the ~ocene formation, and allied forms of another genus, Holops, m the Chalk From the Chalk backward to the Lias another group of genera occurs, having anatomical characteristics intermediate between the living crocodiles- and the most ancient forms. These, forming two genom Belodon and Stagonolepis, are found in a still older formation, the Trias. They have characters ·resembling some lizards, especially the remarkable Hatteria of N cw Zealand, and have also some resemblances to the Dinosaurians-reptilcs which in some respects approach birds. Considering how comparatively few are the remains of this group of animals, the evidence which it affords of progressive development is remarkably clear. I Among the higher animals the rhinoceros, the horse, 'and the deer afford good evidence of advance in oro-anisation n,nd of the filling up of the gaps which separate th~ living forms from their nearest allies. The earliest ancestral forms of the rhinoceroses occur in the Middle Eocene of the United States and were to some extent intermediate between the rhinoccro~ and tapir families, having like the latter four toes to the front feet, and three to those behind. These n,re followed in the Upper Eocene by the genus Amynodon, in which the skull ~ssumes more d~stinctly the rhinocerotic type. Following this 111 the Lower Mwcene we have the Aceratherium like the last in its feet, but still ~ore decidedly a rhinoceros in its general structure. From this there are two divero-ino- lines-one in the Old \Vorld, the other in theN ew. In the f~rmer, to which th~ Acera.therium is supposed to have migrated in early Mwceue times, when a mild climate and luxuriant veo-etation prevaile~ far within the arctic circle, it gave rise0 to the Ceratorhmus and the various horned rhinoceroses of late Tertiary times and of those uow living. In America a 1 On '' Stagonolepis Robertsoni and on the Evolution of the Crocodilia.." in Q. J. of Geological ociety, l8iiJ ; and ab~tract iu KatuTe, vol. xii. p. 38·. |