126 SUMMARY OF THE [Chap. XL 



by supposing that where our oceans now extend 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 lead- 

 ing facts in palseontology 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 in- 

 crease in numbers slowly, and endure for unequal 

 periods of time; for the process of modification is neces- 

 sarily slow, and depends on many complex contingen- 

 cies. 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 inferiority inherited from a common progenitor, 

 tend to become extinct together, and to leave no modi- 

 fied offspring on the face of the earth. But the utter 

 extinction of a whole group of species has sometimes 

 been a slow process, from the survival of a few descend- 



