NATURAL SELECTION 75 



a long interval of time, vary or present individual 

 differences of the same favourable nature as before." 

 We are to understand tbat the new individual 

 difference which begins its modifying work, perhaps 

 after a long interval of time, though it is of the same 

 favourable nature as the last, is another, and different 

 in kind ; inasmuch as the last has completed its work, 

 and exhausted its modifying power. Then follow 

 uncounted generations, during which the possessors of 

 the favourable difference get the upper hand, and are 

 established as a new variety, but still differing from 

 the variety which it has supplanted by an infinitesimal 

 amount of modification. The same process is gone 

 through by the next evolutional variation, and by the 

 next and the next, and so on, step by step ; each step 

 adding another infinitesimal modification, until, in the 

 long result of time, a form emerges so far modified 

 from the original as to take rank as a new species, 

 when compared with it. 



Thus, taking the original as a starting-point, 

 varieties are first formed, each new variety involving 

 the extinction of its predecessor, and each adding 

 somewhat to the accumulations that have been 

 preserved, made by the series of its predecessors. 

 Next comes the specific form, little altered from 

 the form which it has immediately supplanted, but 

 greatly altered from the original form. Thus a 

 measureless gulf of time has been traversed before one 

 species has succeeded to another, and numberless have 

 been the intermediate varieties that have been extin- 



