ORGANIC BMNGS. 343 



CHAPTER XL 



ON THE GEOLOGICAL SUCCESSION OF ORGANIC . BEINGS. 



On tlie slow and successive appearance of new species- — On tlieir 

 different rates of cliange — Species once lost do not reappear — 

 Groups of species follow the same general rules in their appear- 

 ance 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 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 modifica- 

 tion, 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 second- 

 ary formations are more broken; but, as Bronn has 

 remarked, neither the appearance nor disappearance of the 

 many species embedded in each formation has been simul- 

 taneous. 



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 multitude of extinct forms. 



