Chap. IV. THELTMITBA. 127 



fall or be transported by minute crawling insects on 

 to the stigma. By this means self-fertilisation is 

 assured, should larger insects fail to visit the flowers. 

 Moreover, the pollen in this state readily adheres to 

 any object ; so that by a slight change in the shape of 

 the flower, which is already less open or more tubular 

 than that of Listera, and by the pollen becoming 

 friable at a still earlier age, its fertilisation would be 

 rendered more and more easy without the aid of the 

 explosive rostellum. Ultimately it would become a 

 superfluity ; and then, on the principle that every part 

 which is not brought into action tends to disappear, 

 from causes which I have elsewhere endeavoured to 

 explain,* this would happen with the rostellum. We 

 should then see a new species, in the condition of 

 Cephalanthera as far as its means of fertilisation were 

 concerned, but in general structure closely allied to 

 Neottia and Listera. 



Mr. Fitzgerald, in the introduction to his ' Austra- 

 lian Orchids,' says that ThelymUra carnea, one of 

 the Neottese, invariably fertilises itself by means of the 

 incoherent pollen falling on the stigma. Nevertheless 

 a viscid rostellum, and other structures adapted for 

 cross-fertilisation are present. The flowers seldom 

 expand, and never until they have fertilised them- 

 selves ; so that they seem tending towards a cleisto- 

 gene condition. Thelymitra longifolia is likewise fer- 

 tilised in the bud, according to Mr. Fitzgerald, but 

 the flowers open for about an hour on fine days, and 

 thus cross-fertilisation is at least possible. On the 

 other hand, the species of the allied genus Diuris are 

 said to be wholly dependent on insects for their 

 fertilisation. 



• ' Variation of Animals and Plants under Domestication,' 2ud edit 

 roi. ii. p. 300. 



