Chap. XL] SUCCESSION OF OBGANIC BEINGS. 89 



CHAPTEE XL 

 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 ap- 

 pearance and disappearance as 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 tis now see whether tho 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 modi- 

 fication, 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, having 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 3 



