ABSENCE OF CONNECTING LINKS. 339 



whom we constantly find checked in their special series 

 of investigations by the really insuperable difficulty of 

 sharply distinguishing individual species. In all sys- 

 tematic works, which are in any degree thorough, one 

 meets with endless complaints, that here and there species 

 cannot be distinguished because of the excessive number 

 of transition forms. Hence every naturalist defines the 

 hmit and the number of individual species differently. 

 Some zoologists and botanists, as I mentioned (vol. i. p. 276), 

 assume in one and the same group of organisms ten 

 species, others twenty, others a hundred or more, while 

 other systematic naturalists again look upon these different 

 forms only as varieties of a single "good" species. In most 

 groups of forms there is, in fact, a superabundance of tran- 

 sition forms and intermediate stages between the individual 

 species. 



It is true that in many species the forms of transition 

 are actually wanting, but this is easily explained by the 

 principle of divergence or separation, the importance of 

 which I have already explained. The circumstance that 

 the struggle for existence is the more active between 

 two kindred forms the closer they stand to each other^ 

 must necessarily favour the speedy extinction of the con- 

 necting intermediate forms between the two divergent 

 species. If one and the same species produce diverging 

 varieties in different directions, which become new species, 

 the struggle between these new forms and the common 

 primary form will be the keener the less they differ from 

 one another ; but the stronger the divergence the less dan- 

 gerous the struggle. Naturally therefore, it is principally 

 the connecting intermediate forms which will in most cases 



