ix THE DEBT OF SCIENCE TO DARWIN 463 



enormous mass of facts and observations here given, the 

 portion relating to plants is often but an abstract of the 

 results of his own elaborate experiments, carried on for a long 

 series of years, and given at length in three separate volumes 

 on The Fertilisation of Orchids, on Cross and Self-Fertilisation of 

 Plants, and on The Forms of Flowers. These works may be 

 said to have revolutionised the science of botany, since, for 

 the first time, they gave a clear and intelligible reason for 

 the existence of that wonderful diversity in the form, colours, 

 and structure of flowers, on the details of which the system- 

 atic botanist 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 subject 

 of crossing and hybridity had shown that, although hybrids 

 between distinct species usually produced sterile offspring, 

 yet crosses between slightly different varieties led to in- 

 creased fertility ; and, during some experiments on this sub- 

 ject, Darwin found that the produce of these crosses were 

 also remarkable for vigour of growth. This led to a long 

 series of experimental researches, the general result of which 

 was to establish the important proposition that cross-fertilisa- 

 tion is of the greatest importance to the health, vigour, and 

 fertility of plants. The fact that the majority of flowers are 

 hermaphrodite, and appear to be adapted for self-fertilisation, 

 seemed to be opposed to this view, till it was found that, in 

 almost every case, there were special arrangements for ensur- 

 ing, either constantly or occasionally, the transference of pollen 

 from the flowers of one plant to those of another of the same 

 species. In the case of orchids, it was shown that those 

 strange and beautiful flowers owed their singular and often 

 fantastic forms and exceptional structure to special adapta- 

 tions for cross-fertilisation by insects, without the agency of 

 which most of them would be absolutely sterile. Many of 

 the species are so minutely adapted to particular species or 

 groups of insects, that they can be fertilised by no others ; 

 and careful experiment and much thought was often required 

 to find out the exact mode in which this was effected. In 

 some instances the structure of the flowers seemed adapted to 

 prevent fertilisation altogether, till it was at length discovered 

 that a particular insect entering the flower in one particular 



