390 
H. F. Wernham. 
number of plants have one or more characters in common, may we 
conclude therefore that they have derived those characters by 
descent from a common ancestry? If so, then the plants should he 
grouped together, and we must determine further all the other 
plants which should he included in the same group, as well as the 
nature of the ancestry. 
The ancestry is connected with its progeny in the evolutionary 
tree by a line; that line represents the changes induced in the 
ancestral characters by variation and transmitted to the progeny by 
inheritance. The fundamental point at issue is the nature of the 
characters which are to he deemed valid for the purposes just now 
enunciated—the critical characters. These determined, we have to 
seek for the ancestry, and discover in what way the ancestral 
characters have been modified to produce those of the progeny. 
Constancy of Characters. Tendencies. We may gather from 
our previous investigations that the general relation between the 
significant features of the ancestry and those of the descendants is, 
that in the former the characters in question are not constant 
throughout the group, nor may they be completely evolved. In 
other words, we are dealing with tendencies to characters, and not 
with the critical characters themselves, in the case of the ancestry. 
In the progeny, on the other hand, the characters are constant and 
completely evolved; and the line which unites ancestor and 
descendants represents the transition between tendencies and their 
realisation. The descendants in their turn may reveal tendencies 
to other characters, to be realized in groups still more advanced. 
It is in this connection that the significance of constancy and 
inconstancy of characters emerges; and it will he appreciated that 
a character may he of relatively rare occurrence within any group, 
and yet have considerable phyletic value, in so far as it throws light 
upon the evolutionary history of the group and its allies. Critical 
tendencies are no less important than critical characters. 
Thus, the individual zygomorphy and androecial oligomery seen 
in a relative few of the Solanaceae and Caprifoliacese are certainly 
not critical characters for the respective groups to which these 
families belong; hut they are of great importance, since they reflect 
the tendency to characters which become constant and critical in 
Multiovulatae and Dipsacales, the respective descendants of those 
groups. So also the umbellate inflorescence of Araliaceze, repre¬ 
senting the Umbellifloral stock, becomes, in the descendant 
Umbelliferae, a very specialized biological floral-unit. Similarly, the 
