78 INTRODUCTION. 
only non-detected differences; often so, likely, with general sys- 
tematists whose view seeks to take in the flora of the whole 
globe ; and who thus cannot compete on equal terms with the local 
botanists, engaged in minutely examining the species of very 
limited areas. Curious examples of this non-appreciation of real 
differences might be adduced, by citing vecent combinations and 
severances widely at variance with those which a more careful 
and continued observation of the living plants ought to suggest. 
Would it not be held highly absurd in a gardener, for instance, to 
maintain that the black and red currants are simply varieties of 
the same single species, while the white currant is distinct from 
both? Or, equally injudicious in a botanist to combine the 
cherry and bullace together as one species, keeping the sloe 
apart as a distinct species? And yet, it may be asserted 
confidently that the writings of our first-rate systematists show 
severances and unions quite as outrageous as these, only relating 
to plants of smaller size and less familiarity, when they write 
Local Floras or make out species-lists in support of their phyto- 
geographical theories. Here is one of the many difficulties and 
error-traps for those who endeavour to trace out the geographic 
areas of species ;—the comprehensive knowledge of the general 
botanist is not sufficiently precise ;—the precise knowledge of the 
local botanist is not sufficiently comprehensive. 
