It must be allowed that the colour is certainly not the sole attraction to the flower; the fragrance helps in most cases, and even this is not the object of the insect's visits. The real object is the nectar, to which colour and fragrance only show the way. The development of fragrance and nectar must, like that of the colour, have been carried on and increased by processes of selection, which had their basis in the necessity for securing insect-visits, and as soon as these main qualities of the flower were established greater refinements would begin, and flower-forms would be evolved, which would diverge farther and farther, especially in shape, from the originally simple and regular form of the blossom. The reason for this must have lain chiefly in the fact that, after insect-visits in general were secured by a flower, it would be advantageous to exclude all insects which would pillage the nectar without rendering in return the service of cross-fertilization--all those, therefore, which were unsuited either because of their minute size or because of the inconstancy of their visits. Before the butterflies and the bees existed, the regularly formed flat flower with unconcealed nectar would be visited by a mixed company of caddis-flies, saw-flies, and ichneumon-flies. But as the nectar changed its place to the deeper recesses of the flower it was withdrawn from all but the more intelligent insects, and thus the circle of visitors was already narrowed to some extent. But when in a particular species the petals fused into a short tube, all visitors were excluded whose mouth-parts were too short to reach the nectar; while among those which could reach it the process of proboscis-formation began; the under lip, or the first maxillæ, or both parts together, lengthened step for step with the corolla-tube of the flower, and thus from the caddis-flies came the butterflies, and from the ichneumon-flies the burrowing-wasps (_Sphegidæ_) and the bees. At first sight one might perhaps imagine that it would have been more advantageous to the flowers to attract a great many visitors, but this is obviously not the case. On the contrary, specialized flowers, accessible only to a few visitors, have a much greater certainty of being pollinated by them, because insects which only fly to a few species are more certain to visit these, and above all to visit many flowers of the same species one after another. Hermann Müller observed that, in four minutes, one of the humming-bird hawk-moths (_Macroglossa stellatarum_) visited 108 different flowers of the same species, the beautiful Alpine violet (_Viola calcarata_), one after the other, and it may have effected an equal number of pollinations in that short time.