232 HOMOLOGIES OF THE Chap. VIII 



insect which enters the labellum is thus compelled to 

 crawl out by one of the two narrow passages, on the 

 sides of which the pollen-masses and stigma are placed. 

 We have seen that exactly the same end is gained in 

 the case of Coryanthes by the labellum being half- 

 filled with secreted fluid ; and in the case of Pterostylis 

 and some other Australian Orchids by the labellum 

 being irritable, so that when touched by an entering 

 insect it shuts up the flower, with the exception of a 

 single narrow passage.* 



Homological Nature of the sevei-al Parts of the Flowers oj 

 the Orchidese. 



The theoretical structure of few flowers has been so 

 largely discussed as that of the Orchideas ; nor is this 

 surprising, seeing how unlike they are to common 

 flowers ; and here will be a convenient place for con- 

 sidering this subject. No group of organic beings can 

 be well understood until their homologies are made 

 out ; that is, until the general pattern, or, as it has 

 often been called, the ideal type, of the several mem- 

 bers of the group is intelligible. No one member 

 may now exist exhibiting the full pattern ; but this 

 does not make the subject less important to the natu- 

 ralist, — probably makes it more important for the 

 full understanding of the group. 



The homologies of any being, or group of beings, 



* Selenipedium palmifoUwm is the fish-pot system, i. e. a funnel- 

 one of the CypripedesB, and shaped opening conducts into it, 

 according to Dr. Oriiger (' journ. and insects find it difficult to 

 I.inn. Soo. Bot.' vol. viii. 1864, p. escape through the same. Tbe 

 134) bears very fragrant flowers, only other opening near the base 

 which "in all probability are of tlie labellum is partly closed 

 always impregnated by insects. by the sexual apparatus, and the 

 The labellum is, like some Aris- insect has to force its way out 

 tolochia-flowers, construct d after there." 



