CHAPTER XI 



On The Geological Succession of Organic Beings 



On the Slow and Successive Appearance of New Species — On their Different 

 Rates of Change — Species once lost do not reappear — Groups of Spedes 

 follow the Same General Rules in Their Appearance and Disappearance 

 as do Single Species — On Extinction — On Simultaneous Changes in the 

 Forms of Life throughout the World — On the Affinities of Extinct Spe- 

 des to Each Other and to Living Species — On the State of Development 

 of Ancient Forms — On the Succession of the Same Types within the 

 Same Areas — Summary of Preceding and Present Chapter. 



Let us now see whether the several facts and laws relating to the 

 geological succession of organic beings accord best with the com- 

 mon view of the immutability of species, or with that of their 

 slow and gradual modification through variation and natural 

 selection. 



New species have appeared very slowly, one after another, both 

 on the land and in the waters. 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 the stages, and to make the proportion between the lost 

 and existing forms more gradual. In some of the most recent beds, 

 though undoubtedly of high antiquity if measured by years, only 

 one or two species are extinct, and only one or two are new, hav- 

 ing appeared there for the first time, either locally, or, as far as 

 we know, on the face of the earth. The secondary formations are 

 more broken; but, as Bronn has remarked, neither the appearance 

 nor disappearance of the many species embedded in each forma- 

 tion has been simultaneous. 



Species belonging to different genera and classes have not 

 changed at the same rate, or in the same degree. In the older ter- 

 tiary beds a few living shells may still be found in the midst of 

 a multitude of extinct forms. Falconer has given a striking in- 

 stance of a similar fact, for an existing crocodile is associated with 

 many lost mammals and reptiles in the sub-Himalayan deposits. 

 The Silurian Lingula differs but little from the living species of 

 this genus; whereas most of the other Silurian Molluscs and all 



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