Page:Carroll Lane Fenton - Darwin and the Theory of Evolution.djvu/35

 32 to anyone but himself. Plainly, therefore, it was his duty to publish this material, so that it could be examined by other naturalists and used by them in making still further researches.

Thus it was that in 1862 appeared the Fertilization of Orchids, which two years later was described as the most masterly treatise in any branch of vegetable physiology that had appeared. The group of plants which it treated are remarkable for the devices by which they provide for cross-fertilization by means of the insects which visit them. These provisions were skilful enough in the English forms, but in foreign ones, and particularly those from tropical and semi-tropical countries, the mechanisms were still more highly developed. Thus Darwin found that among one group the various parts of the flower were so accurately developed for cross-fertilization that without the aid of insects not a single plant in the whole group of twenty-nine genera could produce seed. In most cases the insects which come to the flowers for food withdraw the pollen masses only when retreating from the blossom, and, by going to another plant or flower effect a union between the two. Commonly the pollen masses slowly change position while attached to the insects, so as to assume the position proper to contact with the stigma of another flower. During the time necessary for this change the insect almost certainly flies to another plant, so that the necessary cross is accomplished.