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 
limit 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 
