404 TROPICAL NATURE 



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 longif olia 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. Miiller 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 Colour 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. Ib., vol. ix. p. 164. 



