GEOLOGICAL SUCCESSION 307 



has not been continuously deposited ; that the duration 

 of each formation is, perhaps, short compared with the 

 average duration of specific forms ; that migration has 

 played an important part in the first appearance of new 

 forms in any one area and formation ; that widely 

 ranging species are those which have varied most, and 

 have oftenest given rise to new species ; and that varie- 

 ties have at first often been local. All these causes 

 taken conjointly, must have tended to make the geo- 

 logical record extremely imperfect, and will to a large 

 extent explain why we do not find interminable varie- 

 ties, connecting together all the extinct and existing 

 forms of life by the finest graduated steps. 



He who rejects these views on the nature of the 

 geological record, will rightly reject my whole theory. 

 For he may ask in vain where are the numberless tran- 

 sitional links which must formerly have connected the 

 closely allied or representative species, found in the 

 several stages of the same great formation. He may dis- 

 believe in the enormous intervals of time which have 

 elapsed between our consecutive formations ; he 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 mav 

 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 he 

 buried under the ocean. 



Passing from these difficulties, all the other great 



