44:2 WILLIS : MORPHOLOGY OP THE PODOSTHMACEÆ 
selection in reference to the performance of the functior. 
the flowers, but to the direct action of other causes, and thai 
directly or indirectly it is mainly the result of dorsiven- 
trality of the vegetative organs and horizontal position in 
development. If we accept this view, it renders much more 
easy the task of explaining the stfuctural peculiarities of 
many zygomorphic flowers, as we no longer need attempt to 
explain every detail of structure in reference to the visits of 
insects or to the advantages of cross-fertilization, and can 
easily explain many such cases as those in the Labiatæ, where, 
though (following the rule above indicated) the upper 
stamen is absent, the working of the mechanism of the 
flower is effected by having the stamens on the upper side, 
and so on. 
If we do not accept this explanation, we have to find 
another to deal with the very remarkable facts. We can 
hardly suppose that the process of evolution in the order has 
been from the markedly dorsiventral to the less dorsi ventral 
forms, for the former exhibit all the peculiar features of the 
order together, and are perhaps the most aberrant types 
among the flowering plants. If we do, however, accept it, 
we are driven then to suppose that the floral organs have 
become radial and entomophilous, and that the vegetative 
organs have followed by losing their dorsiventrality, though 
we know that the more dorsiventral they are the better they 
are suited to their peculiar mode of life, and the less 
competition the species meets with. This, then, would be a 
case of characters of great importance being forced upon the 
vegetative organs in spite of their disadvantage to those 
organs, or at least non-advantage. Probability, analogy, and 
the general evidence are all in favour of the former view, 
that it is the floral organs whose advantages have been 
disregarded in consequence of the imperative claims of the 
vegetative organs. 
If we suppose the dorsiventrality of the flowers to have 
been the subject of natural selection, independently of th e 
vegetative organs, we have to explain the very remarkable 
