A scheme of the classification of the families of Pteridophyta is given, with the addition of the base chromosome numbers which have been recorded in each of them. Various observations and comments which emerge from the examination of the scheme are made. The great difference in the quantity of base numbers among the families, and the remarkable quantity of different base numbers in certain families are pointed out, and a comparison between the various families is made, particularly as regards those which are considered as primitive or advanced. Special attention is drawn to the value of the base numbers recorded in each family. In this connection some interesting features of the Pteridophyta are emphasized: the remarkable prevalence of high base numbers; the fact that the great majority of homosporous families has base numbers higher thanx= 25, while no base number higher than that value occurs in the heterosporous ones; the scarcity of base numbers which correspond to prime numbers, particularly to the small ones; and the presence of some genera in which the base number is tetraploid, or even hexaploid or octoploid, in comparison with the diploid base number of most of the genera of the same family. Details on these observations are given and their interpretation is dealt with, particularly as far as the importance of the polyploidy and the aneuploidy in the genesis of new groups is concerned. Finally, the results of the recent electrophoretic investigations on the enzymes and isozymes of some species with high chromosome numbers are discussed in relation to the evolution of the Pteridophyta.