This statement is indeed scarcely more than a truism, for if a variety were found to have a wider range than that of its supposed parent-species, their denominations would be reversed.
But there is reason to believe that the species which are very closely allied to other species, and in so far resemble varieties, often have much restricted ranges.

For instance, Mr.H.C.Watson has marked for me in the well-sifted London catalogue of Plants (4th edition) sixty-three plants which are therein ranked as species, but which he considers as so closely allied to other species as to be of doubtful value: these sixty-three reputed species range on an average over 6.9 of the provinces into which Mr.Watson has divided Great Britain.

Now, in this same catalogue, fifty-three acknowledged varieties are recorded, and these range over 7.7 provinces; whereas, the species to which these varieties belong range over 14.3 provinces.

So that the acknowledged varieties have very nearly the same restricted average range, as have the closely allied forms, marked for me by Mr.Watson as doubtful species, but which are almost universally ranked by British botanists as good and true species.
SUMMARY.
Finally, varieties cannot be distinguished from species--except, first, by the discovery of intermediate linking forms; and, secondly, by a certain indefinite amount of difference between them; for two forms, if differing very little, are generally ranked as varieties, notwithstanding that they cannot be closely connected; but the amount of difference considered necessary to give to any two forms the rank of species cannot be defined.

In genera having more than the average number of species in any country, the species of these genera have more than the average number of varieties.