404 
TROPICAL NATURE 
VI 
by butterflies . 1 But bees are always (in the temperate zone) 
far more abundant than butterflies, and this will be another 
reason why flowers specially adapted to be fertilised by the 
latter should be rendered unusually conspicuous. We find, 
accordingly, the yellow primroses and cowslips of the plains 
replaced by pink and magenta- coloured Alpine species; the 
straggling wild pinks of the lowlands by the masses of large 
flowers in such mountain species as Dianthus alpinus and D. 
glacialis; the saxifrages of the high Alps with bunches of 
flowers a foot long as in Saxifraga longifolia and S. cotyledon, 
or forming spreading masses of flowers as in S. oppositifolia ; 
while the soapworts, silenes, and louseworts are equally superior 
to the allied species of the plains. 
Why Allied Species of Flowers differ in Size and Beauty 
Again, Dr. Muller has discovered that when there are 
showy and inconspicuous species in the same genus of plants, 
there is often a corresponding difference of structure, those 
with large and showy flowers being quite incapable of self- 
fertilisation, and thus depending for their very existence on 
the visits of insects, while the others are able to fertilise 
themselves should insects fail to visit them. We have 
examples of this difference in Malva sylvestris, Epilobium. 
angustifolium, Polygonum bistorta, and Geranium pratense — 
which have all large or showy flowers, and must be fertilised 
by insects — as compared with Malva rotundifolia, Epilobium 
parviflorum, Polygonum aviculare, and Geranium pusillum, 
which have small or inconspicuous flowers, and are so con- 
structed that if insects should not visit them they are able to 
fertilise themselves . 2 
Absence of Colow in Wind-fertilised Flowers 
As supplementing these curious facts, showing the relation 
of colour in flowers to the need of the visits of insects to 
fertilise them, we have the remarkable, and, on any other 
theory, utterly inexplicable circumstance that in all the numer- 
ous cases in which plants are fertilised by the agency of 
the wind they never have specially coloured floral envel- 
opes. Such are our pines, oaks, poplars, willows, beeches, 
1 nature, vol. xi. pp. 32, 110. a IK, vol. ix. p. 164. 
