126 SUMMARY OF THE [Chap. XL 



they have extended for an enormous period, and where 

 our oscillating continents now stand they have stood 

 since the commencement of the Cambrian system ; but 

 that, long before that epoch, the world presented a 

 widely different 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 lie still buried under the ocean. 



Passing from these difficulties, the other great leading 

 facts in palaeontology agree admirably with the theory 

 of descent with modification through variation and 

 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 forms. 

 We can understand why, when a species has once dis- 

 appeared, it never 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 complex contingencies. 

 The dominant species belonging to large and dominant 

 groups tend to leave many modified descendants, which 

 form new sub-groups and groups. As these are formed, 

 the species of the less vigorous groups, from their in- 

 feriority inherited from a common progenitor, tend to 

 become extinct together, and to leave no modified off- 

 spring on the face of the earth. But the utter extinc- 

 tion of a whole group of species has sometimes been a 

 slow process, from the survival of a few descendants, 

 lingering in protected and isolated situations. When a 



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