358 OUTLINES OF EVOLUTIONARY BIOLOGY 



prevent cross-fertilization, for foreign pollen will be prepotent 

 even if deposited on the stigma some time afterwards. 



" To test this belief," Darwin observes, " I placed on several 

 stigmas of a long-styled cowslip plenty of pollen from the same 

 plant, and after twenty-four hours added some from a short- 

 styled dark-red polyanthus, which is a variety of the cowslip. 

 From the flowers thus treated, thirty seedlings were raised, and 

 all these, without exception, bore reddish flowers, so that the 

 effect of pollen from the same form, though placed on the stigmas 



twenty-four hours pre- 

 viously, was quite des- 

 troyed by that of pollen 

 from a plant belonging to 

 the other form." 



The flowers of the 

 common sage, and of 

 other species of the genus 

 Sal via (Fig. 181), afford a 

 no less striking example 

 of profound structural 

 modification in adapta- 

 tion to the visits of insects. 

 These flowers are pro- 

 tandrous, the stamens 

 maturing and shedding 

 their pollen before the 

 stigma is ready to receive 

 it, so that self-fertiliza- 

 tion is absolutely prevented. Moreover, the stigma in young 

 flowers (Fig. 181, c/r') lies in such a position that it will not 

 be touched by visiting insects, which crawl right under it in 

 order to reach the honey at the bottom of the corolla-tube. In 

 older flowers the style elongates and curves downwards, so that 

 the stigma (Fig. 181, cjr") comes to lie in the mouth of the 

 corolla-tube and must be brushed against by insects in search of 

 honey. 



The most remarkable feature of the flower, however, is a 

 special mechanical contrivance for placing the pollen on the back 

 of any insect which attempts to suck honey from it in its young 

 or male condition. There are only two fully developed stamens, 

 and these are most curiously modified in structure. Each anther 



U 



FIG. 181.- -Flower of ftalvia^ratensis. (From 

 Weismanu's " Evolution Theory," after 

 H. Miiller.) 



gr' immature stigma; gr", mature stigma; st', 

 anther-lobe concealed in the " helmet " ; st", 

 anther-lobes lowered ; U, lower lip of corolla. 



