PRECEDING AND PRESENT CHAPTERS. 371 



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 w'idely 

 different aspect; and that the older continents, formed o'f 

 formations older than any known to iis, 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 ex- 

 tinction of old forms is the almost inevitable consequence 

 of the production 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 cf modifica- 

 tion is necessarily slow, and depends on many complex 

 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 inferiority 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 extinction 

 of a whole grou]) of species has sometimes been a slow pro- 

 cess, from the survival of a few descendants, lingering in 

 protected and isolated situations. When a group has once 

 wholly disappeared, it does not reappear; for the link of 

 generation has been broken. 



We can understand how it is that dominant forms which 

 spread widely and yield the greatest number of varieties 

 tend to people the world with allied, but modified, de- 

 scendants; and these will generally succeed in dis})hicing 

 the groups which are their inferiors in the struggle for 

 existence." Hence, after long intervals of time, the pro- 

 ductions of the world appear to have changed simultane- 

 ouslv. 



We can understand how it is that all the forms of hfe, 



