_Why Allied Species of Flowers Differ in Size and Beauty._--Again, Dr. Müller 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-fertilization, and thus depending for their very existence on the visits of insects; while the others are able to fertilize themselves should insects fail to visit them. We have examples of this difference in _Malva sylvestris_, _Epilobium augustifolium_, _Polygonum bistorta_, and _Geranium pratense_--which have all large or showy flowers, and must be fertilized by insects--as compared with _Malva rotundifolia_, _Epilobium parviflorum_, _Polygonum aviculare_, and _Geranium pusillum_, which have small or inconspicuous flowers, and are so constructed that if insects should not visit them they are able to fertilize themselves.[24] [24] _Nature_, vol. ix. p. 164. _Absence of Colour in Wind-fertilized Flowers._--As supplementing these curious facts showing the relation of colour in flowers to the need of the visits of insects to fertilize them, we have the remarkable, and on any other theory, utterly inexplicable circumstance, that in all the numerous cases in which plants are fertilized by the agency of the wind they never have specially coloured floral envelopes. Such are our pines, oaks, poplars, willows, beeches, and hazel; our nettles, grasses, sedges, and many others. In some of these the male flowers are, it is true, conspicuous, as in the catkins of the willows and the hazel, but this arises incidentally from the masses of pollen necessary to secure fertilization, as shown by the entire absence of a corolla or of those coloured bracts which so often add to the beauty and conspicuousness of true flowers.