CEiO-. XL] PRECEDING AND PRESENT CHAPTERS. 313 



that widely ranging species are those which have varied mcst 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 com- 

 parison with the periods during which each remained in an un- 

 changed condition. These causes, taken conjointly, 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 discriminated. 



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 formation ? 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 considered ; 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 con- 

 tinents now stand they have stood since the commencement of the 

 Cambrian system ; but that, long before that epoch, the world pre- 

 sented 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 succes- 

 sively; how species of different classes do not necessarily change 



