One cause of extermination is easily grasped. Cypripeds have not the power of fertilizing themselves, except a single species, _Cyp. Schlimii_, which--accordingly, as we may say--is most difficult to import and establish; moreover, it flowers so freely that the seedlings are always weak. In all species the sexual apparatus is so constructed that it cannot be impregnated by accident, and few insects can perform the office. Dr. Hermann Muller studied _Cyp. calceolus_ assiduously in this point of view. He observed only five species of insect which fertilize it. _Cyp. calceolus_ has perfume and honey, but none of the tropical species offer those attractions. Their colour is not showy. The labellum proves to be rather a trap than a bait. Large insects which creep into it and duly bear away the pollen masses, are caught and held fast by that sticky substance when they try to escape through the lateral passages, which smaller insects are too weak to force their way through.
Natural hybrids occur so rarely, that their existence is commonly denied. The assertion is not quite exact; but when we consider the habits of the genus, it ceases to be extraordinary that Cypripeds rarely cross in their wild state. Different species of Cattleya, Odontoglots, and the rest live together on the same tree, side by side. But those others dwell apart in the great majority of cases, each species by itself, at a vast distance perhaps from its kindred. The reason for this state of things has been mentioned--natural laws have exterminated them in the spaces between, which are not so well fitted to maintain a doomed race.
Doubtless Cypripeds rarely fertilize--by comparison, that is, of course--in their native homes. The difficulty that insects find in performing that service has been mentioned. Mr. Godseff points out to me a reason far more curious and striking. When a bee displaces the pollen masses of a Cattleya, for instance, they cling to its head or thorax by means of a sticky substance attached to the pollen cases; so, on entering the next flower, it presents the pollen _outwards_ to the stigmatic surface. But in the case of a Cypriped there is no such substance, the adhesive side of the pollen itself is turned outward, and it clings to any intruding substance. But this is the fertilizing part. Therefore, an insect which by chance displaces the pollen mass carries it off, as one may say, the wrong side up. On entering the next flower, it does not commonly present the surface necessary for impregnation, but a sterile globule which is the backing thereof. We may suppose that in the earlier age, when this genus flourished as the later forms of orchid do now, it enjoyed some means of fertilization which have vanished.