Chap. x. in any Single Formation. 279 



is admitted to be very slight ; so that here, unless we believe that 

 these eminent naturalists have been misled by their imaginations, 

 and that these late tertiary species really present no difference what- 

 ever from their living representatives, or unless we admit, in oppo- 

 sition to the judgment of most naturalists, that these tertiary species 

 are all truly distinct from the recent, we have evidence of the fre- 

 quent occurrence of slight modifications of the kind required. If 

 we look to rather wider intervals of time, namely, to distinct but 

 consecutive stages of the same great formation, we find that the 

 embedded 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 formations ; 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 successive 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 geo- 

 logical formations of 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 underwent modification, though long as measured 

 by years, was probably short in comparison with that during which 

 it remained without undergoing any change. 



It should not be forgotten, that at the present day, with perfect 

 specimens for examination, two forms can seldom be connected by 

 intermediate varieties, and thus proved to be the same species, 

 until many specimens are collected from many places; and with 

 fossil species this can rarely be done. We shall, perhaps, best per- 

 ceive the improbability of our being enabled to connect species 



