These latter replenish the bucket from which, when half-filled, the water overflows by a spout on one side.
Bees visiting the flower fall into the bucket and crawl out at the spout.
By the peculiar arrangement of the parts of the flower, the first bee which does so carries away the pollen-mass glued to his back, and then when he has his next involuntary bath in another flower, as he crawls out the pollen-mass attached to him comes in contact with the stigma of that second flower and fertilizes it.

In the other example (_Catasetum_), when a bee gnaws a certain part of the flower, he inevitably touches a long delicate projection, which Mr.Darwin calls the antenna.

"This antenna transmits a vibration to a certain membrane, which is instantly ruptured; this sets free a spring by which the pollen-mass is shot forth like an arrow in the right direction, and adheres by its viscid extremity to the back of the bee!" Another difficulty, and one of some importance, is presented by those communities of ants which have not only a population of sterile females, or workers, but two distinct and very different castes of such.

Mr.Darwin believes that he has got over this difficulty by having found individuals intermediate in form and structure[48] between the two working castes; others may think that we have in this belief of Mr.Darwin, an example {57} of the unconscious action of volition upon credence.

A vast number of difficulties similar to those which have been mentioned might easily be cited--those given, however, may suffice.
There remains, however, to be noticed a very important consideration, which was brought forward in the _North British Review_ for June 1867, p.