On examining the exotic forms of orchids, which are so conspicuous in our conservatories, still more striking facts presented themselves. In the great group of the Vandeæ, relative position of parts, friction, viscidity, elastic and hygrometric movements were all found to be nicely related to one end--the aid of insects in fertilisation. Without their aid not a plant in the various species of twenty-nine genera which Darwin examined would set a seed. In the majority of cases insects withdraw the pollen masses only when retreating from the flower, and, continuing their flower visits, effect a union between two flowers, generally on distinct plants. In many cases the pollen masses slowly change their position while adhering to the insects, and so assume a proper direction for striking the stigma of another flower, and the insects during this interval will almost certainly have flown from one plant to another. The family to which Catasetum belongs furnished the most remarkable examples. This plant possesses a special sensitiveness in certain parts, and when definite points of the flower are touched by an insect the pollen masses are shot forth like an arrow, the point being blunt and adhesive. The insect, disturbed by so sharp a blow, or having eaten its fill, flies sooner or later to a female plant, and whilst standing in the same position as before, the pollen-bearing end of the arrow is inserted into the stigmatic cavity, and a mass of pollen is left on its viscid surface. The strange structures of Cypripedium, or the Lady's Slipper, were then analysed, and the mode of fertilisation by small bees was discovered. The whole structure of orchids, as modified to secure insects' visits and cross fertilisation, was now expounded, and the benefits shown by cases where insects' visits were prevented, and no seed was set. The number of seeds in a capsule was reckoned, and thence it was found that the progeny of a single plant of the common orchis would suffice to cover the globe in the fourth generation. A single plant of another orchid might bear seventy-four millions of seeds: surely an ample provision for a struggle for existence, and selection and survival of the fittest. But, as Darwin remarks, profuse expenditure is nothing unusual in nature, and it appears to be more profitable for a plant to yield a few cross-fertilised than many self-fertilised seeds.