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 
