32 INTRODUCTION. 
that the present distribution of plants within this country is 
mainly determined by its present physical circumstances. 
4. Grapes oF SPECIES. 
An ever recurring difficulty in treating the distribution of 
plants, and one continually increasing from year to year, is found 
in the uncertainty and inequality of species, as described in Floras 
and other botanical writings. Independently of errors in nomen- 
clature—very numerously in print—the same specific name unfor- 
tunately will too often be found not to include or intend the same 
species. As used by one botanist, it means a gross whole ; and it 
is then applicable indifferently to any part or fraction of that same 
whole ;—while another botanist restricts the same name to express 
some special part or fraction of the whole, exclusively of all other 
parts or fractions ;—and more inconveniently still, the special part 
or fraction is itself occasionally made a varying portion of the 
whole. In one book, the same single name may apply to two or 
more different species treated as a single species ;—in another 
book, it may be strictly limited to one of those two species solely 
and exclusively. In the former case, the name is applicable alike 
to all or to any of the included forms or species which go to make 
up the aggregate union ;—in the latter case, it is applicable only 
to some one of the dissevered forms or species, the other forms 
taking different names. 
These discrepancies in the meaning of applied names cause 
enormous trouble to students in phyto geography. Perhaps some 
clearer idea of the practical working of such discrepant meanings 
may be given by taking letters and numerals to represent the 
species and sub-species in successive grades. Let it be supposed 
that two dozens of the many modern species carved out of the old 
book-species Rubus fruticosus, are cach-represented by one letter of 
the alphabet Keeping to the old idea, the name ‘ Rubus fruti- 
cosus’ means and includes the whole alphabet; and as a name it 
may thus be applied alike and indifferently to any one letter. 
But some modern botanist will declare that it means only, and 
