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CHAPTER x. I:f XI. 2 ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS. 3 On the slow and successive appearance of new species-On their different rates of change-Species once lost do not reappearGroups of species follow the same general rules in their appearance and disappearance as do single species-On ExtinctionOn simultaneous changes in the forms of life throughout the world-On the affinities of extinct species to each other and to living species-On the state of development of ancient formsOn the succession of the same types within the same areasSummary of preceding and present chapters. 3:c chapter. 4 LET us now see whether the several facts and rules relating to the geological succession of organic beings, better accord with the common view of the immutability of species, or with that of their slow and gradual modification, through descent and natural selection. 4:e and laws relating 4:f beings accord best with the common/through variation and natural 5 New species have appeared very slowly, one after another, both on the land and in the waters. 6 Lyell has shown that it is hardly possible to resist the evidence on this head in the case of the several tertiary stages; and every year tends to fill up the blanks between them, and to make the percentage system of lost and new forms more gradual. 6:e between the stages, and to 6:/ make the proportion between the lost and existing forms 7 In some of the most recent beds, though undoubtedly of high antiquity if measured by years, only one or two species are lost forms, and only one or two are new forms, having here appeared for the first time, either locally, or, as far as we know, on the face of the earth. 7:e are extinct, and only one or two are new, having appeared there for 8 If we may trust the observations of Philippi in Sicily, the successive changes in the marine inhabitants of that island have been many and most gradual. s.ยท[e] 9 The secondary formations are more broken; but, as Bronn has remarked , neither the appearance nor disappearance of their many now extinct species has been simultaneous in each separate formation. 9:e of the many extinct species embedded in each formation has been simultaneous. 9:1 many species 10 Species of different genera and classes have not changed at the same rate, or in the same degree. rot] Species belonging to different 11 In the oldest tertiary beds a few living shells may still be found in the midst of a multitude of extinct forms. I I:C In the older tertiary 12 Falconer has given a striking instance of a similar fact, in an existing crocodile associated with many strange and lost mammals and reptiles in the sub-Himalayan deposits. I2:C strange lost I2:e fact, for an existing crocodile is associated with many lost 13 The Silurian Lingula differs but little from the living species of this genus; whereas most of the other Silurian Molluscs and all the Crustaceans have changed greatly. 14 The productions of the land seem to change at a quicker rate than those of the sea, of which a striking instance has lately been observed in Switzerland. I4:f seem to have changed 15 There is some reason to believe that organisms, considered high in the scale of nature, change more quickly than those that are low: though there are exceptions to this rule. I5:d organisms considered I5:e organisms high in the scale, change 16 The amount of organic change, as Pictet has remarked, does not strictly correspond with the succession of our geological formations ; so that between each two consecutive formations, the forms of life have seldom changed in exactly the same degree. I6:e remarked, is not the same in each successive so-called formation . 17 Yet if we compare any but the most closely related formations, all the species will be found to have undergone some change. 18 When a species has once disappeared from the face of the earth, we have reason to believe that the same identical form never reappears. I8:c we have no reason/form ever reappears. 19 The strongest apparent exception to this latter rule, is that of the so-called "colonies" of M. Barrande, which intrude for a period in the midst of an older formation, and then allow the 52 2 pre-existing fauna to reappear; but...

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