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404 TROPICAL NATUREby butterflies. 1 But bees are always (in the temperate zone)far more abundant than butterflies, and this will be anotherreason why flowers specially adapted to be fertilised by thelatter should be rendered unusually conspicuous. We find,accordingly, the yellow primroses and cowslips of the plainsreplaced by pink and magenta -coloured Alpine species; thestraggling wild pinks of the lowlands by the masses of largeflowers in such mountain species as Dianthus alpinus and D.glacialis; the saxifrages of the high Alps with bunches offlowers 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 superiorto the allied speciesof the plains.Why Allied Species of Flowers differ in Size and BeautyAgain, Dr. Miiller has discovered that when there areshowy and inconspicuous species in the same genus of plants,there is often a corresponding difference of structure, thosewith large and showy flowers being quite incapable of selffertilisation,and thus depending for their very existence onthe visits of insects, while the others are able to fertilisethemselves should insects fail to visit them. We haveexamples of this difference in Malva sylvestris, Epilobiumangustifolium, Polygonum bistorta, and Geranium pratensewhich have all large or showy flowers, and must be fertilisedby insects as compared with Malva rotundifolia, Epilobiumparviflorum, Polygonum aviculare, and Geranium pusillum,which have small or inconspicuous flowers, and are so constructedthat if insects should not visit them they are able tofertilise themselves. 2Absence of Colour in Wind-fertilised FlowersAs supplementing these curious facts, showing the relationof colour in flowers to the need of the visits of insects tofertilise them, we have the remarkable, and, on any othertheory, utterly inexplicable circumstance that in all the numerouscases in which plants are fertilised by the agency ofthe wind they never have specially coloured floral envelopes.Such are our pines, oaks, poplars, willows, beeches,1 Nature, vol. xi. pp. 32, 110. Ib., vol. ix. p. 164.