PART 1, | HISTORICAL INTRODUCTION. 13 
to a certain extent possible, but not necessary ; cross- 
fertilisation more probable. Ex. Asclepiadee. 
tt The aid of insects not essential. 
Self-fertilisation possible, but cross-fertilisation also effected 
by insects, 
II. Anthers lying close to the stigma ; self-fertilisation inevitable. 
* No seed produced without cross-fertilisation, which is effected 
by insects. Ex. Corydalis cava. 
** Seed is produced on self-fertilisation, but cross- fertilisation by 
insects is not excluded, 
(d) The flowers never expand (flores cleistogami, Kuhn). 
Only self-fertilisation occurs, and all cross- fertilisation is 
excluded ; but the plants have other flowers which open 
and are liable to cross-fertilisation. Ex. Owalis acetosella, 
Although the statements contained here with reference to the 
possibility of cross- or self-fertilisation have turned out not to be 
universally correct, and although the divisions merge into one 
another by many intermediate forms, as Hildebrand himself 
allowed, yet Hildebrand’s scheme is very well adapted to serve 
as an artificial classification for the consideration of individual 
cases. 
Delpino described the structure of very many. flowers belonging 
to the most distant orders, showing them all to be adapted for 
cross-fertilisation. His descriptions are so clear and minute that 
he dispenses to a large extent with illustrations. In many cases 
he has directly observed the fertilising agents, and he states at 
least the families or genera to which they belong. He has shown 
that water as well as wind, and snails and honey-sucking birds as 
well as insects, play a part in the conveyance of pollen. He has 
shown in the most diverse instances how the form of the flower is 
determined by the special fertilising agent, and in several families 
(Maregraviacee, Marantacece, Artemisiacee, &c.) he has made happy 
use of the gradual stages in adaptation to special fertilising agents 
in order to trace the mutual genetic relations of the members of 
the family. He has also shown strong evidence that in many cases 
the distributional area of a species is determined in the first 
instance by the presence of that animal which the flower has 
become adapted to be cross-fertilised by. And finally, choosing the 
various means of transport for the pollen as his basis of classifica- 
tion, and taking advantage ingeniously of the almost complete want 
of direct observations on the cross-fertilisation of Cryptogams, he 
evolved the following scheme of the whole vegetable kingdom, in 
all of whose members he also is of opinion that cross-fertilisation 
is either certain, or probable, or at least possible. 
