PDF - Wallace Online

ix THE DEBT OF SCIENCE TO DARWIN 463enormous mass of facts and observations here given, theportion relating to plants is often but an abstract of theresults of his own elaborate experiments, carried on for a longseries of years, and given at length in three separate volumeson The Fertilisation of Orchids, on Cross and Self-Fertilisation ofPlants, and on The Forms of Flowers. These works may besaid to have revolutionised the science of botany, since, forthe first time, they gave a clear and intelligiblereason forthe existence of that wonderful diversity in the form, colours,and structure of flowers, on the details of which the systematicbotanist had founded his generic and specific distinctions,but as to whose meaning or use he was, for the most part,profoundly ignorant. The investigation of the whole subjectof crossing and hybridity had shown that, although hybridsbetween distinct species usually produced sterile offspring,yet crosses between slightly different varieties led to increasedfertility ; and, during some experiments on this subject,Darwin found that the produce of these crosses werealso remarkable for vigour of growth. This led to a longseries of experimental researches, the general result of whichwas to establish the important proposition that cross-fertilisationis of the greatest importance to the health, vigour, andfertility of plants. The fact that the majority of flowers arehermaphrodite, and appear to be adapted for self-fertilisation,seemed to be opposed to this view, till it was found that, inalmost every case, there were special arrangements for ensuring,either constantly or occasionally, the transference of pollenfrom the flowers of one plant to those of another of the samespecies. In the case of orchids, it was shown that thosestrange and beautiful flowers owed their singular and oftenfantastic forms and exceptional structure to special adaptationsfor cross-fertilisation by insects, without the agency ofwhich most of them would be absolutely sterile.Many ofthe species are so minutely adapted to particular species orgroups of insects, that they can be fertilised by no others ;and careful experiment and much thought was often requiredto find out the exact mode in which this was effected. Insome instances the structure of the flowers seemed adapted toprevent fertilisation altogether, till it was at length discoveredthat a particular insect entering the flower in one particular