RESEMBLE VARIETIES. 55 



how the lesser differences between varieties tend to increase 

 into the greater differences between species. 



There is one other point which is worth notice. Varie- 

 ties generally have much restricted ranges. This state- 

 ment is indeed scarceh^ 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. 0. 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 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 fr.om species, 

 — except, first, by the discovery of intermediate Imking 

 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 countrv, 

 the species of these genera have more than the average 

 number of varieties. In large genera the species are apt 

 to be closely but unequally allied together, forming little 

 plusters round other species. Species very closely allied to 



