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282 highly attractive to the proper insects having increased in number. We know that certain Orchids require certain insects for their fertilisation, as in the cases before given of Vanilla and Sarcochilus. In Madagascar Angræcum sesquipedale must depend on some gigantic moth. In Europe Cypripedium calceolus appears to be fertilised only by small bees of the genus Andrena, and Epipactis latifolia only by wasps. In those cases in which only a few flowers are impregnated owing to the proper insects visiting only a few, this may be a great injury to the plant; and many hundred species throughout the world have been thus exterminated; those which survive having been favoured in some other way. On the other hand, the few seeds which are produced in these cases will be the product of cross-fertilisation, and this as we now positively know is an immense advantage to most plants.

I have now nearly finished this volume, which is perhaps too lengthy. It has, I think, been shown that the Orchideæ exhibit an almost endless diversity of beautiful adaptations. When this or that part has been spoken of as adapted for some special purpose, it must not be supposed that it was originally always formed for this sole purpose. The regular course of events seems to be, that a part which originally served for one purpose, becomes adapted by slow changes for widely different purposes. To give an instance: in all the Ophreæ, the long and nearly rigid caudicle manifestly serves for the application of the pollen-grains to the stigma, when the pollinia are transported by insects to another flower; and the anther opens widely in order that the pollinium should be easily withdrawn; but in the Bee Ophrys, the caudicle, by a slight increase in length and decrease in its thickness, and by