256 THE EVOLUTION THEORY 



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-fertiliza- 

 tion b}'' ' 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. 



This is true of most of the Epidendra of South America, and 

 of CoryantJiMS triloba of New Zealand, two hundred blossoms of which 

 only yielded five seed-capsules, and also of our Ophrys muscifera and 

 0. aranifera, the latter of which yielded only a single seed-capsule 

 from 3,000 flowers gathered in Liguria. We might expect that the 

 species in question must have become very rare, but this is not always 

 the case, since each of these capsules contains an enormous number 

 of seeds, sometimes many thousands. As soon as the visits of insects 

 cease altogether, the species must die out in the particular locality 

 concerned, unless it can revert to self-pollination and self-fertility. 

 There is a whole series of species in which the stigma of the flower 

 is sensitive to its own pollen, and in many of these an adaptation 

 to self-fertilization has actually been efl'ected, for the pollinia detach 

 themselves from their anthers at maturity and fall upon the stigma. 

 I have already mentioned Ophrys apifera, which, according to Charles 

 Darwin, is no longer visited by insects, although its flowers still possess 

 the structure required for insect-fertilization. This species has saved 

 itself from extinction by the normal occurrence of self-fertilization. 



This seems to me noteworthy in two respects. In the first place, 

 it shows that pure self-fertilization need not necessarily result in 

 a weakening of the species, and secondly, it aflbrds a clear instance 

 of a species being transformed in one minute character only, all the 

 other characters remaining unaltered. In this case it was only the 

 pollinia that required to vary a little in their mode of attachment and 

 maturation, in order to eflect the transformation of the flower for 

 self-fertilization, and in point of fact that is all that has varied. The 

 case is not relevant to our investigation at this moment, but cases 



