AN ILLUSTRATION.
Page 284.
To give a simple illustration: in many orchids the ovarium (but sometimes the foot-stalk) becomes for a period twisted, causing the labellum to assume the position of a lower petal, so that insects can easily visit the flower; but from slow changes in the form or position of the petals, or from new sorts of insects visiting the flowers, it might be advantageous to the plant that the labellum should resume its normal position on the upper side of the flower, as is actually the case with Malaxis paludosa, and some species of Catasetum, etc. This change, it is obvious, might be simply effected by the continued selection of varieties which had their ovaria less and less twisted; but, if the plant only afforded varieties with the ovarium more twisted, the same end could be attained by the selection of such variations, until the flower was turned completely round on its axis. This seems to have actually occurred with Malaxis paludosa, for the labellum has acquired its present upward position by the ovarium being twisted twice as much as is usual.
Again, we have seen that in most Vandeæ there is a plain relation between the depth of the stigmatic chamber and the length of the pedicel, by which the pollen-masses are inserted; now, if the chamber became slightly less deep from any change in the form of the column, or other unknown cause, the mere shortening of the pedicel would be the simplest corresponding change; but, if the pedicel did not happen to vary in shortness, the slightest tendency to its becoming bowed from elasticity, as in Phalænopsis, or to a backward hygrometric movement, as in one of the Maxillarias, would be preserved, and the tendency would be continually augmented by selection; thus the pedicel, as far as its action is concerned, would be modified in the same manner as if it had been shortened. Such processes carried on during many thousand generations in various ways, would create an endless diversity of co-adapted structures in the several parts of the flower for the same general purpose. This view affords, I believe, the key which partly solves the problem of the vast diversity of structure adapted for closely analogous ends in many large groups of organic beings.