J2i ABSENCE OP INTERMEDIATE VARIETIES [Chap. X, 



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 de- 

 gree. According to this view, the chance of discov- 

 ering 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 ani- 

 mals 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 spe- 

 cies; 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, name- 

 ly, that the period during which each species under- 

 went 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 



