24 THE FERTILISATION OF FLOWERS. [PART I. 
But while we must, on the one hand, strive ‘to limit our 
assumptions as much as possible, in order to keep a basis.fully 
demonstrable by experiment for our conclusions, we must, on the 
other hand, that these conclusions may rest firm,-demand the 
securest possible establishment of all accessible facts which can 
influence the mutual relations between flowers and the agents 
which fertilise them. 
Of the various fertilising agents, wind and water are so simple 
and regular in their action that it is easy to review the circum- 
stances of the case, and to recognise how the comparatively simple 
contrivances in anemophilous and hydrophilous plants are deter- 
mined by the nature of the medium of transport. But the insects } 
which act as carriers of pollen are so various in size, form, length 
of proboscis, food-material, mode of movement, colour-sense, &c., 
their abundance depends on so many conditions, and their visits to 
particular flowers are so influenced by weather, the competition of 
other insects, the attractions of other neighbouring flowers, &c., 
that the mere investigation of floral contrivances, and the mere 
general establishment of the fact that certain flowers are actually 
visited and fertilised by insects, can never lead to a perception of 
the primitive conditions which determine any specific peculiarity 
in a flower. Only when it is accurately ascertained for many 
and various flowers of the same neighbourhood by what insects and 
how abundantly by each each species is visited, what in each case 
each visitor comes in search of, how each acts in its visits, how the 
dimensions of their bodies correspond to the dimensions of the 
flowers (for instance, the length of their proboscis to the length of 
the tube and spur), what parts of their bodies come in contact with 
the pollen and the stigma, and so forth ; when further, for each of 
these flowers, not only the form, arrangement, and order of deve- 
lopment of the separate parts, but also the area of conspicuous 
surfaces, the diameter of the entrance, the depth of the tube in 
which the honey lies, in short every point which can affect the 
insect-visitor, has been accurately ascertained—then only may we 
hope, by comparing the characters of flowers and their insect- 
visitors, to attain to accurate knowledge instead of the mere 
surmises concerning the conditions that primitively determine any 
floral character. Ae 
It is plain that in such facts all previous investigations have 
left wide gaps. Even Delpino, who paid more attention than any 
* Snails and birds may be left out of the question, as they do not play an important 
part in fertilising any native plants, 
