Now, however, it seems as if the fundamental proposition of this theory of flowers will have to be rejected. Not only do the cleistogamous flowers just mentioned exhibit a great fertility, not at all less than that of the open flowers of the same species which are adapted for cross-fertilization, but there is a small number of plants which produce seeds by _self-fertilization alone_. Thus in _Myrmecodia_ cross-fertilization is absolutely prevented by the fact that the flowers never open, and according to Charles Darwin _Ophrys apifera_ also reproduces by self-fertilization alone, and is nevertheless a thoroughly vigorous plant. There are several other cases of this sort, and particularly among the orchids, though the whole of the structure of their flowers is specially adapted for pollination by insects. Many of them are only rarely visited by insects, some not at all, we know not why, but it is readily intelligible that in such cases they should have adapted themselves to self-fertilization wherever that was possible. For this _no great_ variation was necessary; it was enough that the pollinia, which formerly only became detached from their attachment at a touch or a push from an insect, should free themselves spontaneously. And this, according to Darwin, is what happens, for instance, in _Ophrys scolopax_, which at Cannes is frequently self-fertilizing. For the development of seed, however, it is not enough that the pollen should reach the stigma; the pollen-grain has to send out its tube and penetrate into the ovary, and in many orchids this does not happen; they are infertile with their own pollen. Various other plants are also non-fertile with their own pollen, for instance the common corydalis, _Corydalis cava_, or the meadow cuckoo-flower, _Cardamine pratensis_ (Hildebrand). How are we to reconcile these apparently absolutely contradictory facts? On the one hand, the innumerable devices for securing crossing lead us to conclude that it is necessary, or at least advantageous, and on the other we find a small number of plants which reproduce continually by self-fertilization and yet remain strong and vigorous. And again there are many plants which yield seed when fertilized with their own pollen, and others which remain absolutely sterile in the same circumstances, yielding no seed or very little, and there is indeed one on which its own pollen has the effect of a poison, for if it reaches the stigma the flower dies. If there is anything injurious in self-fertilization (Darwin), we can understand that it will be avoided, but how can it be continued so long in many cases, and even become in others the exclusive method of fertilization without visible evil results?