THE OKIGIN OF '' ZYGOMOKPHISM." 117 



thrust into a circular tube. The corolla then " gives," as 

 it were, and expands along the antero-posterior plane. The 

 calyx follows suit, and often assumes a bilobed funnel-shaped 

 tube as well ; while the lateral lobes of the corolla tend to 

 atrophy, since they do not lie along- the line of the pressure 

 due to the weight of the insect (see Fig. 40Z), p. 126.) 



If the floral organs be imagined to consist of some 

 plastic, extensible, but not elastic substance, and be subjected 

 to various pressures, strains, thrusts, etc., in imitation of the 

 motions of insects, it is readily conceivable how the parts 

 would yield, stretch, or bulge, and become fixed into shapes 

 very closely resembling what has actually taken place in 

 nature. In reality, of course, the ability to grow in response 

 to the forces applied is to be substituted for the theoretical 

 plasticity and extensibility of the imaginary material. 



Compensatory degenerations occur in various directions, 

 as in the atrophy of the lateral petal-lobes of Lamium, the 

 loss of the fifth posterior stamen, the reduction in length of 

 the filaments of the posterior pair of stamens. In this latter 

 respect Nepeta differs from other genera , but as we can 

 readily conceive how all sorts of differences may and do exist 

 in the direction and degree of the forces applied 

 to flowers, some exceptional ones must have 

 occurred in that genus which has favoured the 

 growth of the posterior pair, so that they have 

 become the longer ones ; for there is no rule 

 without an exception. As another illustration, 

 Teucrium may be taken. In this genus the 

 *' hood " is entirely wanting ; but here, again, ^. „„ ^, 



•^ , ° ' 'to 5 Fig. 36.— Flower of 



the interpretation is that, no hypertrophy Teucriuvi (after 



^ , ' J f f J jiot. Mag., 1279). 



having been applied to them, the two petals 



of which it is composed have become reduced in size and 



"cleft," as shown in Fig. o6, of T. (Teucris) orientate. Bees, 



