322 THE GEOLOGICAL SUCCESSION 



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 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 — Sum- 

 mary of Preceding and Present Chapter. 



Let us now see whether the several facts and laws relat- 

 ing 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 pro- 

 portion 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 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 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 



