Moreover, in many cases, the position of the anthers in the flower entirely excludes the possibility of any currents of air assisting in the carrying of the pollen-dust to the pistil of the same or of different flowers. Hence there is a necessity for the intervention of insects; and that they may be induced to visit such flowers, and unconsciously effect the essential operation of fertilising them, nectar is secreted near the base of the stamens or the ovary, or in some position which will involve, in the gathering of it, the brushing off and conveying away of some of the pollen-grains. It is, indeed, a remarkable fact that fragrance and honey-bearing are scarcely ever associated with plants which can easily be wind-fertilised. Such flowers are, also, for the most part, inconspicuous; while those which need the agency of insects to aid in their reproduction are bright in colour, sweet in perfume, and more or less prolific in honey. It must not be supposed, however, that even the hermaphrodite, or double-sexed, flowers are independent of the visits of bees and other insects. In all of them cross-fertilisation, as Darwin has abundantly proved,[10] is a most important factor in the continued vitality of any species, and cross-breeding gives an immense advantage in "the struggle for existence," where the conditions of life are not wholly favourable. Indeed, in many instances, special provision has been made by the Creator against self-fertilisation: in some cases, by the anthers and pistil coming to maturity, in the same flower, at different times; in others, by the placing of the stamens in such a position relatively to the stigma (or top of the pistil) that it is not possible for the pollen-grains of the one set of organs to fall on the surface of the other. It cannot but be interesting to give examples of these various facts, and so to show the marvellous and necessary connection between the two kingdoms of nature. [Footnote 10: For full and most interesting information on this point, vide _Cross and Self-Fertilisation of Plants_, by Charles Darwin. Murray.]