392 ORIGIN OF SPECIES 



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 geo- 

 logical record, will rightly reject the whole theory. For he 

 may ask in vain where are the nmnberless 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 con- 

 secutive 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 ex- 

 isted 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 before 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 spe- 

 cies come in slowly and successively; how species of dif- 

 ferent 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 in- 

 crease in numbers slowly, and endure for unequal periods 

 of time; for the process of modification is necessarily slow, 



