272 THE GEOLOGICAL SUCCESSION [CHAP. XL 



CHAPTER 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 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 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 pre- 

 ceding 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 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 ; but, as Bronn has remarked, neither the appear- 

 ance 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 multitude 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 seem to have changed at a quicker rate than those of 

 the sea, of which a striking instance has been observed in Switzer- 

 land. There is some reason to believe that organisms high in the 

 scale, change more quickly than those that are low : though there 

 are exceptions to this rule. The amount of organic change, as 



