72 ABSENCE OF INTEllMEDIATE VAKIETIES [Chap. X. 



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 dis- 

 covering 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 wliich 

 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 gTcatly 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 



