126 THE GENESIS OF SPECIES. [CHAP. 



not ; as also does the distinctness of forms formerly sup- 

 posed to constitute connecting links. For, it must not 

 be forgotten that, if species have manifested themselves 

 generally by gradual and minute modifications, then the 

 absence, not in one, but in all cases, of such connecting 

 links, is a phenomenon which remains to be accounted for. 

 It appears then that, apart from fortuitous changes, 

 there are certain difficulties in the way of accepting ex- 

 tremely minute modifications of any kind, although these 

 difficulties may not be insuperable. Something, at all 

 events, is to be said in favor of the opinion that sudden 

 and appreciable changes have, from time to time, occurred, 

 however they may have been induced. Marked races have 

 undoubtedly so arisen (some striking instances having been 

 here recorded), and it is at least conceivable that such may 

 be the mode of specific manifestation generally, the pos- 

 sible conditions as to which will be considered in a later 

 chapter. 



