T etracyclidce. 223 
descendants. Reduction in size of the individual, coupled with 
increase in efficiency of detailed structure, seems, in fact, to be a 
general principle of descent, in both animal and vegetable kingdoms. 
In any case none will deny to the Asclepiadacese a place of highest 
specialization among the Contortse. The Gentianaceae are not so 
obviously advanced; but the constancy of floral character throughout 
the group, the characteristic habit—almost invariable glabrousness 
coupled with a generally rigid, stiff appearance easier to recognize 
than to describe—and a definite tendency to the unilocular condition 
of the ovary, all point to this order as being, at any rate, not 
relatively primitive. 
Of the remaining orders the Loganiaceae and Apocynaceae are 
those which, we have seen, shew the greatest amount of variability 
among the Contortae—little though this may be ; and it is to these 
orders that we are led to assign the position of greatest proximity 
to the ancestral archichlamydeous stock. Among these two orders, 
as we have already remarked, we find a few traces of the multi- 
locular ovary; but it is in the number of ovules that they appear to 
display a definite evolutionary tendency—namely, to economy in 
ovule production. This will be a matter of significance when we 
approach the question of the origin of the Tubiflorae. In the 
Apocynaceae, again, we find much more variability in leaf-arrangement 
than in the other orders, in which opposition of the leaves is a 
remarkably constant feature. 
The Apocynaceae and Loganiaceae are not readily separable on 
grounds of broad evolutionary tendencies, and there seems to 
be little doubt that they both arose from a comparatively recent 
common ancestor. Each has progressed upon a special line ; the 
Apocynaceae, with the tendency of their to approximation stamens 
and adhesion to the stigma, and with their more or less 
specialized mechanism for seed-dispersal, seem to be on the whole 
more advanced than the Loganiaceae. The higher Apocynaceae, 
in fact, pass by way of a series of transitional forms into the 
Asclepiadaceae, the two orders being connected by the tribe 
Periploceae of the latter, in which the filaments are free, or almost 
so, and the pollen granular. These higher Apocynaceae, although 
not herbaceous, are mostly slender climbing shrubs, with a general 
habit closely resembling many Asclepiads. 
The recent (sympetalous) common ancestor of the Apocynaceae 
and Loganiaceae we may name the contortal or apocynal stock. 
From this the latter order represents a relatively short side-branch, 
