3J0 SUMMARY OF THE Chap. X. 



numerous or<Tanisnis wliicli must have existed long before the 

 Cambrian system ^vas deposited ? We now know that at least 

 one animal did then exist ; but I can answer the above ques- 

 tion only l)y supjiosin<^ that where our oceans now extend tliey 

 have extended for an enormous .period, and wliere our oscillat- 

 ing continents now stand, they have stood since the coinmence- 

 mcnt of the Cambrian system ; but that, long before that 

 epoch, the world presented a Avidcly-diirerent aspect ; and that 

 the older continents, formed of formations older than any 

 known to us, exist now only as remnants in a metamorphosed 

 condition, or he still buried under the ocean. 



Passing from these difficulties, the other great leading 

 facts in paleontology seem to me simply to follow on the the- 

 ory of descent with modification through natural selection. 

 We can thus understand how it is that new species come in 

 slowly and successively ; how species of different classes do 

 not necessarily change together, or at the same rate, or in the 

 same degree ; yet in the long-run that all undergo modification 

 to some extent. The extinction of old forms is the almost 

 inevitable consequence of the production of new fonns. We 

 can imderstand Avhy Avhcn a species has once disappeared it nev- 

 er reappears. Groups of species increase in numbers slowly, 

 and endure for unequal periods of time ; for the process of 

 modification is necessarily slow, and depends on many com- 

 plex contingencies. The dominant species belonging to large 

 and dominant groups tend to leave many modified descend 

 ants, which form new sub-groups and groups. As these are 

 formed, the species of the less vigorous groups, from their infe- 

 riority inherited from a common progenitor, tend to become 

 extinct together, and to leave no modified oQ'spring on the face 

 of the earth. But the utter extinction of a whole group of 

 species has sometimes been a slow process, from the surnval 

 of a few descendants, lingering in protected and isolated situ- 

 ations. When a group has once wholly disappeared, it does 

 not reappear; for the link of generation has been broken. - 



AVe can understand how the dominant fonns which spread 

 widel}' and yield the ;;Teatest number of varieties Avill tend to 

 people the world with allied, but modified, descendants ; and 

 these will generally succeed in displacing the groups which are 

 their inferiors in the struggle for existence. Hence, after long 

 intervals of time, the jiroductions of the world apjicar to have 

 changed sinmltaneously. 



We can understand how it is that all the forms of life 



