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GEOLOGICAL SUCCESSION. 
Chap. X. 
CHAPTEE X. 
On the Geological Succession of Okganic 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 — 
Summary of preceding and present chapters. 
Let us now see whether the several facts and rules 
relating 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 modi¬ 
fication, 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 them, 
and to make the percentage system of lost and new 
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 lost forms, and only 
one or two are new forms, having here appeared for 
the first time, either locally, or, as far as we know, on 
the face of the earth. If we may trust the observations 
of Philippi in Sicily, the successive changes in the marine 
inhabitants of that island have been many and most 
gradual. The secondary formations are more broken; 
but, as Bronn has remarked, neither the appearance 
