320 THE ORIGIN OF SPECIES 



vain where are the numberless transitional links which must 

 formerly have connected the closely allied or representative 

 species, found in the successive stages of the same great forma- 

 tion? He may disbelieve in the immense intervals of time which 

 must have elapsed between our consecutive formations; he may 

 overlook how important a part migration has played, when the 

 formations of any one great region, as those of Europe, are con- 

 sidered; 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 Cambrian system was deposited? 

 We now know that at least one animal did then exist; but I can 

 answer this last question only 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 be- 

 fore 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 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 de- 

 pends on many complex contingencies. The dominant species be- 

 longing to large and dominant groups tend to leave many modi- 

 fied descendants, 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 be- 

 come extinct together, and to leave no modified 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 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. 



