species within the same genus which regularly fertilise themselves. Considering such cases as those of Ophrys, Disa, and Epidendrum, in which one species alone in the genus is capable of complete self-fertilisation, whilst the other species are rarely fertilised in any manner owing to the rarity of the visits of the proper insects;—bearing also in mind the large number of species in many parts of the world which from this same cause are seldom impregnated, we are led to believe that the above-named self-fertile plants formerly depended on the visits of insects for their fertilisation, and that from such visits failing they did not yield a sufficiency of seed and were verging towards extinction. Under these circumstances it is probable that they were gradually modified, so as to become more or less completely self-fertile; for it would manifestly be more advantageous to a plant to produce self-fertilised seeds rather than none at all or extremely few seeds. Whether any species which is now never cross-fertilised will be able to resist the evil effects of long-continued self-fertilisation, so as to survive for as long an average period as the other species of the same genera which are habitually cross-fertilised, cannot of course be told. But Ophrys apifera is still a highly vigorous plant, and Gymnadenia tridentata and Platanthera hyperborea are said by Asa Gray to be common plants in North America. It is indeed possible that these self-fertile species may revert in the course of time to what was undoubtedly their pristine condition, and in this case their various adaptations for cross-fertilisation would be again brought into action. We may believe that such reversion is possible, when we hear from Mr. Moggridge that Ophrys scolopax fertilises itself freely in one district of Southern France without the aid of insects, and
