CHAr. X. GEOLOGICAL SUCCESSION. 295 



CHAPTER X. 



ON TUE GEOLOGICAL SUCCESSION OF ORGA^^C BEINGS. 



On the Slow nnd Snccci>9ivc Appearance of New Species— On their Diflfcrcnt Rates 

 of t'haii;,'C— Species once lost do not reappear — Groui)8 of Species follow the 

 game (Jc-ncnil Rules in their Appearance and Uisappearunce as do Sinfrlc Species 

 — On Extinction— On Sininltaneoiis C'hnn;,'es in the Forms of Life throu^rhout 

 the World— On the Affinities of Extinct Species to cacli other and to Li\ in;; Spe- 

 cies—On the State of Develo[)nKiit of Ancient Forms— On the Succession of tho 

 same Typea within the same Areas- Summary of preccdinjj and present Chapter. 



Let us now see whether the several facts and laws relatint^ 

 to the geological succession of organic beings better accord 

 with the common view of the immutability of species, or with 

 that of their slow and gradual modification, through descent 

 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 percentage 

 system of lost and new forms more gradual. In some of the 

 most recent beds, though undoubtedly of high antitjuity if 

 measured by years, only one or two species arc extinct, and 

 only one or two arc new, having appeared there for the first 

 time, either locally, or, as far as we know, on the face of 

 the cartli. The secondary formations are more broken ; but, 

 as Bronn has remarked, neither the appearance nor. disappear- 

 ance of the many extinct species embedded in each formation 

 has been simultaneous. 



Species of different genera iind 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 foimd in the midst of a 

 multitude of extinct fonns. Falconer has given a striking in- 

 stance of a similar fact, for an existing crocodile is associated 

 with many lost mammals and reptiles in the sub-IIimalayan 

 deposits. The Silurian Liiigula dilTcrs but little from the 



