CHANGE IN SHAPE OF FLOWER gi 
come guest to deny it to the unwelcome. The first 
stage in this specialization is that the flower, instead 
of having its axis vertical, and facing the sky, is turned 
on its side by the curving of its stalk, and looks out 
horizontally. The effect of this is to cause a flying 
insect on approaching the flower to alight in a par- 
ticular position—namely, on the lowest petal. Follow- 
ing on the adoption of this attitude the next stage in 
development is seen in the parts of the flower begin- 
ning to alter their shape and position relative to each 
other and often also their colour. Thus, beginning 
with a quite regular flower, we can arrange a series 
showing more and more asymmetry. The tendency 
is generally for the lowest petal to become enlarged 
and often conspicuously marked, providing a broad, 
convenient platform on which insects may alight, while 
the remainder form walls and roof, protecting the 
important parts within and by their shape, which is 
often narrowed and tubular behind, barring access to 
all but chosen visitors. To find a full series illus- 
trating these transformations we do not need to go 
to plants widely separated in their affinities. In the 
Buttercup order (Ranunculacee) alone every grada- 
tion may be found. The flowers of the Buttercups 
themselves are upright and quite regular. In the 
Larkspur (Delphinium) the flower is turned on its 
side, and a puzzling combination of coloured sepals 
and petals—five bright blue unequal sepals and a 
single large purplish petal of peculiar shape with a 
long hollow spur behind—produces a quite irregular 
blossom. The process is carried farther again in the 
Monkshood (Aconitum), in whose well-known blue 
flower the sepals and petals combine to produce a 
