78 THE ORIGIN OF SPECIES 



Stages of the same great formation, we find that the im- 

 bed(ied fossils, though universally ranked as specifically 

 different, yet are far more closely related to each other 

 than are the species found in more widely separated for- 

 mations; so that here again we have undoubted evidence 

 of change in the direction required by the theory; but 

 to this latter subject I shall return in the following 

 chapter. 



With animals and plants that propagate rapidly and 

 do not wander much, there is reason to suspect, as we 

 have formerly seen, that their varieties are generally at 

 first local; and that such local varieties do not spread 

 widely and supplant their parent-forms until they have 

 been modified and perfected in some considerable degree. 

 According to this view, the chance of discovering in a 

 formation in any one country all the early stages of 

 transition between any two forms, is small, for the suc- 

 cessive changes are supposed to have been local or 

 confined to some one spot. Most marine animals have 

 a wide range; and we have seen that with plants it is 

 those which have the widest range that oftenest present 

 varieties; so that, with shells and other marine animals, 

 it is probable that those which had the widest range, far 

 exceeding the limits of the known geological formations 

 in Europe, have oftenest given rise, first to local varieties 

 and ultimately to new species; and this again would 

 greatly lessen the chance of our being able to trace the 

 stages of transition in any one geological formation. 



It is a more important consideration, leading to the 

 same result, as lately insisted on by Dr. Falconer, 

 namely, that the period during which each species under- 

 went modification, though long as measured by years, 



