Transactions. 23 



tho door. In the Thyme lie is supposed to stand on the other 

 flowers, and only liis liead enters the corolla. In Veronica the 

 lower lip has been suppressed, and the insect is supposed to 

 alight on the style and two spreading stamens. 



In all three types, howevei-, if one examines the flower, as 

 compared with the insect, one sees that the shape of the corolla 

 is an almost exact generalised outline of its average visitor's 

 head and mouth parts, affording, of course, play to the ovdinaiy 

 motions on entering or lenving of the latter. Natural selection 

 seems to have ruthlessly pared away any exuberance of shape, 

 until tlie correspondence is sometimes astonishingly correct. 



Anotiier striking effect of insect ^-isitors is shown in the 

 development of nectaries to secrete the honey. In this case the 

 origin of nectaries may very likely be found in insect visits. 

 Priman-al insects probably bit and gnawed the flower paits, and 

 possibly the places most affected by them would be the juicy 

 succulent tissues at the base of the stamens and petals. That is 

 the most usual position of nectaries, and a part often attacked 

 by the gnawing beetles now living. A stimulus of this kind 

 would produce a flow of sugar to the part attacked (one can see 

 a similar flow giving rise to abnormal development in the galls 

 produced by insect injury at present). At first this abnormal 

 supply must have been purely irregular, but gradually the flower 

 took to developing regular spots, where a constant exudation 

 took place, and which the insect could readily tind. Once this 

 took place one can see both how insects began to develop a 

 sucking mouth, instead of strong biting mandibles, and how the 

 nectaries became gradually more and more definite and constant. 

 In fact, every stage of transition can be almost traced from our 

 present forms. Thus Miiller has shown in his " Fertilisation of 

 Plants " a regular series of transitions, from the biting mouths 

 of the sand-wa=p and Frosapis to the complicated purely sucking 

 mouths of Bombus and the hive-bee. On the flower side, I was 

 interested this summer by seeing a very small Erupis fly, which 

 was obviously sucking on the petals of HypericTim. peiforafnm, 

 whose flowers show no trace of nectaries, and which are not 

 known to secrete honey. I was compelled to believe that exuda- 

 tion of honey did take place, though in an irregular and unlocal- 

 ised way, probably as m.ay have been the case with the eai'liest 

 flowers. Before leaving nectaries, I must point out that their 



