II. WHAT IS A SPECIES ? 33 



circumstances; — {secondly) by the production of another 

 species from its seeds, eitlier at once, or more gradually 

 in course of several descents ; — {thirdly) by tracing its 

 intimate connexion with another species through inter- 

 mediate varieties, in which any differential characters be- 

 come lost or blended ; — or {fourthly) by the absence of 

 permanent differences, sufficiently clear and strong (" im- 

 portant ") to serve for written diagnosis. 



Botanists appear to be theoretically agreed as to the 

 two first of these four tests of a false species. If they do 

 not always practically concur, it seems not to arise from 

 any diversity of view as to the principle, but from dif- 

 ferent estimates of the evidence adduced in proof of 

 re-conversion or of re-production. The third test is less 

 generally allowed in practice ; several botanists evincing 

 much inclination to look upon such intermediate forms 

 simply as the approximating varieties of one or other of 

 the two alleged species, and not as truly constituting 

 links of connexion between them ; or, otherwise, they 

 groui:) the intermediates together as a third species, dis- 

 tinct from both ; or, if close-driven, they make a guess at 

 hybridity, and call this guess an exj)lanation. On the 

 fourth test there is little unanimity of view, in respect to 

 the kind or to the amount of difference which justifies a 

 separation, or in respect to the closeness of resemblance 

 which should compel combination. Scores, perhaps 

 even hundreds, of examples might be cited from the flora 

 of Britain alone, in illustration of this want of unanimity 

 in respect to the sufficing characters of j)roposed species. 



The fourth test, indeed, is commonly nothing better in 

 practice than the caprice or conjecture of the individual 

 botanist. Between a Joseph Hooker and a Jordan the 

 diversity of view is very wide, on the question as to what 

 kind or quantity of difference should be held sufficient to 



VOL. IV. B' 



