29C THE GEOLOGICAL SUCCESSION [Chap. XI. 



CEAPTEE 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 species 

 follow the same general rules in their appearance and disappearance a; 

 do single species — On extinction — On 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 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 common 

 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 exist- 

 ing 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, having 

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

 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 formation 

 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 tertiary 

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

 titude of extinct forms. Falconer has given a striking instance 

 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 the 

 Crustaceans have changed greatly. The productions of the land 



