294 SUMMARY OF THE [CHAP. XI. 



is probably 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 fre- 

 quently, and have oftenest given rise to new species ; that 

 varieties have at first been local ; and lastly, although each 

 species must have passed through numerous transitional stages, 

 it is probable that the periods, during which each underwent 

 modification, though many and long as measured by years, have 

 been short in comparison with the periods during which each 

 remained in an unchanged condition. These causes, taken con- 

 jointly, will to a large extent explain why though we do find 

 many links we do not find interminable varieties, connecting 

 together all extinct and existing forms by the finest graduated 

 steps. It should also be constantly borne in mind that any 

 linking variety between two forms, which might be found, would 

 be ranked, unless the whole chain could be perfectly restored, as 

 a new and distinct species ; for it is not pretended that we have 

 any sure criterion by which species and varieties can be dis- 

 criminated. 



He who rejects this view of the imperfection of the geological 

 record, will rightly reject the whole theory. For he may ask in 

 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 de- 

 posited ? 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 before that epoch, the world presented a widely 

 different aspect ; and that the older continents, formed of forma- 

 tions 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 



