10 MR. F. N. WILLIAMS ON THE GENUS SILENE. 
Dianthex, connecting links between Tunica and Dianthus are 
Tunica Pamphylica, Boiss. et Bal., in which each segment of 
the calyx has six or seven nerves (instead of 1-3), and on the 
other hand Dianthus prolifer and Dianthus leptopetalus. Generally 
speaking, as may be seen from this review of differential 
characters, the characters which separate the genera Saponaria, 
Gypsophila, Tunica, Dianthus, and Velezia are much less dis- 
tinctive than those which separate other genera in the suborder 
Sileninew. Some German botanists have proposed to establish 
intergradient genera (‘“‘ Mittelgattungen ”) for definitely charac- 
terized species of an evidently intermediate type: but this device 
would certainly force upon systematists the fact that many genera 
are unnatural entities. 
We find it frequently stated that the production of hybrid 
forms is an important matter, in so far as minute variations in 
the form of the floral organs determine the position of certain 
genera. Herbert enunciated the principle that successful 
crossings can only take place between two individuals of the 
same natural genus, and that, therefore, species which possess 
this faculty ought to be included in the same genus. Nageli 
also seems to join issue on this principle. On the other hand, 
the exceptions to this rule are not many, and the question 
whether in these cases the two genera should not be united, 
presents itself. The capacity for producing hybrids is inherent 
in the productive elements of the species concerned, a kind 
of natural affinity which, apparently homogeneous with the 
similarity in external structure of the reproductive organs, ou 
which, as explained above, the systematic position of genera is 
based, need not necessarily be associated with it. This sexual 
affinity or selective relationship then is peculiar to the species, 
sometimes only to the individual under favoured conditions. 
This sexual affinity, moreover, is apparent only in certain 
species of the same genus, and further in a group of closely 
allied genera it is found that the species of one may hybridize 
easily, as in Dianthus, while the species of another hybridize 
with difficulty, as in this particular genus Silene. So that it 
may happen, though not necessarily, that sexual affinity and 
morphological relationship may go hand in hand. Crossing of 
species then, when successful, produces hybrids. In these the 
characteristics of the two species are combined sometimes in 
equal proportions, sometimes with great preponderance of one 
