Chap. X.] IN ANY SINGLE FOKMATION. 73 



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 perceive the improbability of our being 

 enabled to connect species by numerous, fine, inter- 

 mediate, fossil links, by asking ourselves whether, for 

 instance, geologists at some future period will be able 

 to prove that our different breeds of cattle, sheep, 

 horses, and dogs are descended from a single stock or 

 from several aboriginal stocks ; or, again, whether 

 certain sea -shells inhabiting the shores of North 

 America, which are ranked by some conchologists as 

 distinct species from their European representatives, 

 and by other conchologists as only varieties, are really 

 varieties, or are, as it is called, specifically distinct. 

 This could be effected by the future geologist only by 

 his discovering in a fossil state numerous intermediate 

 gradations; and such success is improbable in the 

 highest degree. 



It has been asserted over and over again, by writers 

 who believe in the immutability of species, that geology 

 yields no linking forms. This assertion, as we shall 

 see in the next chapter, is certainly erroneous. As Sir 

 J. Lubbock has remarked, "Every species is a link 

 "between other allied forms." If we take a genus 

 having a score of species, recent and extinct, and 

 destroy four-fifths of them, no one doubts that the 

 remainder will stand much more distinct from each 

 other. If the extreme forms in the genus happen to 

 have been thus destroyed, the genus itself will stand 

 more distinct from other allied genera. What o-eo- 

 logical research has not revealed, is the former existence 



