CHAP. VI.] OF NATURAL SELECTION. 145 



pure water continually fall from two secreting horns which stand 

 above it ; and when the bucket is half full, the water overflows by 

 a spout on one side. The basal part of the labellum stands over 

 the bucket, and is itself hollowed out into a sort of chamber with 

 two lateral entrances; within this chamber there are curious 

 fleshy ridges. The most ingenious man, if he had not witnessed 

 what takes place, could never have imagined what purpose all 

 these parts serve. But Dr. Criiger saw crowds of large humble- 

 bees visiting the gigantic flowers of this orchid, not in order to 

 suck nectar, but to gnaw off the ridges within the chamber above 

 the bucket ; in doing this they frequently pushed each other into 

 the bucket, and their wings being thus wetted they could not fly 

 away, but were compelled to crawl out through the passage formed 

 by the spout or overflow. Dr. Criiger saw a "continual procession" 

 of bees thus crawling out of their involuntary bath. The passage is 

 narrow, and is roofed over by the column, so that a bee, in forcing 

 its way out, first rubs its back against the viscid stigma and then 

 against the viscid glands of the pollen-masses. The pollen-masses 

 are thus glued to the back of the bee which first happens to crawl 

 out through the passage of a lately expanded flower, and are thus 

 carried away. Dr. Criiger sent me a flower in spirits of wine, 

 with a bee which he had killed before it had quite crawled out 

 with a pollen-mass still fastened to its back. When the bee, thus 

 provided, flies to another flower, or to the same flower a second 

 time, and is pushed by its comrades into the bucket and then 

 crawls out by the passage, the pollen-mass necessarily comes first 

 into contact with the viscid stigma, and adheres to it, and the 

 flower is fertilised. Now at last we see the full use of every part 

 of the flower, of the water-secreting horns, of the bucket half full 

 of water, which prevents the bees from flying away, and forces 

 them to crawl out through the spout, and rub against the properly 

 placed viscid pollen-masses and the viscid stigma. 



The construction of the flower in another closely allied orchid, 

 namely the Catasetum, is widely different, though serving the 

 same end ; and is equally curious. Bees visit these flowers, like 

 those of the Coryanthes, in order to gnaw the labellum ; in doing 

 this they inevitably touch a long, tapering, sensitive projection, 

 or, as I have called it, the antenna. This antenna, when touched, 

 transmits a sensation or 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. The pollen-mass 

 of the male plant (for the sexes are separate in this orchid) is thus 

 carried to the flower of the female plant, where it is brought into 

 contact with the stigma, which is viscid enough to break certain 

 elastic threads, and retaining the pollen, fertilisation is effected. 



