Although it is obvious enough that continued inbreeding in its most extreme form, self-fertilization, does not imply an absolute abandonment of amphimixis, the adherents of the rejuvenescence theory have regarded the unfavourable consequences of pure inbreeding as a confirmation of their assumption, according to which amphimixis is indispensable to the continuance of the life of the species, and it is therefore an important fact, if it can be proved, that continued self-fertilization can occur persistently, among plants at least, and yet not cause any injurious results to the species. But how can this fact be understood from our point of view? How does it happen that crossing is striven after in so many different ways and yet so often given up again, and continued self-fertilization resorted to? To this it may be answered, in the first place, that it is not, as far as we can see, for _internal_ reasons that persistent self-fertilization becomes the rule; there is no peculiar condition of the germ-plasm which makes it disadvantageous or superfluous that the diversity of the id-combinations should be maintained; self-fertilization is due to _external_ influences which bring it about that the plant has only the alternative of producing no seeds at all or of producing them by self-fertilization. In this connexion Darwin's experiments with orchids are particularly noteworthy. In this very diversified order of plants there are numerous species whose flowers are infertile with their own pollen, although it does not reach the stigma in natural conditions, and therefore there was no necessity--as far as we can see--for guarding against self-fertilization by 'self-sterility.' These flowers are thus doubly adapted, so to speak, for crossing by means of insects. But as regards many of these, as well as many other modern orchids, insect-visits are very rare, and in some cases do not occur at all, and therefore these species cannot produce seed or can do so only exceptionally.