THE ORIGIN OF " ZYGOMORPHISM." 119 



and stand intermediate between Solanaceoe and Scrophula- 

 rinece. The former genus has a less zygomorphic corolla than 

 many of the latter order, and also retains the fifth stamen 

 in varying degrees of utility. We might regard both these 

 genera as Solanaceous, and on the road to acquiring zygo- 

 morphism, but to which neither has yet fully attained. 



" The short-tube [of Yerhascum nigrum'] widens out into 

 a flat, five-lobed limb, which takes up an almost vertical 

 position ; tlie inferior lobe is the longest, and the two 

 superior are shorter than the lateral lobes, so that an insect 

 settles most conveniently upon the inferior. The stamens 

 project almost horizontally, but curve slightly upwards from 

 the tube, and diverge slightly from one another ; they 

 alternate with the petals, and again the superior is the 

 shortest, and the two inferior longer than the lateral ones. 

 . . . The style is shorter than the inferior stamens, and 

 bent down slightly below them " 



From this description, taken from Miiller's work,* which, 

 with slight modifications, would describe Petunia as well, 

 the reader will see how these flowers fulfil the requirements 

 of self-adaptation to insect agency ; and in every point of 

 detail are they responding to the forces impinged upon 

 them. The weight of the insect being w^ell to the front, 

 hypertrophy is commencing on the anterior side, while 

 atrophy follows on the others, there being no special thrust 

 as yet on the posterior side of the flowers 



There are many other genera and species which stand in 

 intermediate positions between others, and it has always 

 been a matter of doubt to systematists as to which they 

 should be referred. The interpretation of their existence I 

 take to be as here described, namely, that they are in an 

 actual transitional state, brought about by insect agency, if 

 ♦ Fertilisation, etc., p. 429. 



