Chap. X. SUMMARY. 343 



may overlook how important a part migration must 

 have played, when the formations of any one great 

 region alone, as that of Europe, are considered ; he may 

 urge the apparent, but often falsely apparent, sudden 

 coming in of whole groups of species. He may ask 

 where are the remains of those infinitely numerous 

 organisms which must have existed long before the 

 first bed of the Silurian system was deposited : I can 

 answer this latter question only hypothetically, by say- 

 ing that as far as we can see, where our oceans now 

 extend they have for an enormous period extended, and 

 where our oscillating continents now stand they have 

 stood ever since the Silurian epoch ; but that long- 

 before that period, the world may have presented a 

 wholly different aspect ; and that the older continents, 

 formed of formations older than any known to us, may 

 now all be in a metamorphosed condition, or may lie 

 buried under the ocean. 



Passing from these difficulties, all the other great 

 leading facts in palaeontology seem to me simply to 

 follow on the theory of descent with modification 

 through natural selection. We can thus understand 

 how it is that new sj)ecies come in slowly and succes- 

 sively ; how species of different classes do not neces- 

 sarily 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 pro- 

 duction of new forms. We can understand why when 

 a species has once disappeared 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 of the 

 larger dominant groups tend to leave many modified 



