THE ORIGIN OF FLOWERS 197 



to some extent. But when in a particular species the petals fused 

 into a short tube, all visitors were excluded whose mouth-parts were 

 too short to reach the nectar ; while among those which could reach it 

 the process of proboscis-formation began; the under lip, or the first 

 maxillae, or both parts together, lengthened step for step with the 

 corolla-tube of the flower, and thus from the caddis-flies came the 

 butterflies, and from the ichneumon - flies the burrowing-wasps 

 {Sphegidce) and the bees. 



At first sight one might perhaps imagine that it would have been 

 more advantageous to the flowers to attract a great many visitors, 

 but this is obviously not the case. On the contrary, specialized 

 flowers, accessible only to a few visitors, have a much greater cer- 

 tainty of being pollinated by them, because insects which only fly to a 

 few species are more certain to visit these, and above all to visit many 

 flowers of the same species one after another. Hermann Muller 

 observed that, in four minutes, one of the humming-bird hawk-moths 

 (Macroglossa stellatarum) visited 108 different flowers of the same 

 species, the beautiful Alpine violet (Viola calcarata), one after the 

 other, and it may have effected an equal number of pollinations in 

 that short time. 



It was, therefore, a real advantage to the flowers to narrow their 

 circle of visitors more and more by varying so that only the useful 

 visitors could gain access to their nectar, and that the rest should be 

 excluded. Thus there arose 'bee-flowers,' 'butterfly-flowers,' f hawk- 

 moth flowers,' and, indeed, in many cases, a species of flower has become 

 so highly specialized that its fertilization can only be brought about by 

 a single species of insect. This explains the remarkable adaptations 

 of the orchids and the enormous length of the proboscis in certain 

 butterflies. Even our own hawk-moths Macroglossa stellatarum and 

 Sphinx convolvuli show an astonishing length of proboscis, which 

 measures 8 cm. in the latter species. In Macrosilia cluentius, in 

 Brazil, the proboscis is 20 cm. in length ; and in Madagascar there 

 grows an orchid with nectaries 30 cm. in length, filled with nectar to 

 a depth of 2 cm., but the fertilizing hawk-moth is not yet known. 



Thus we may say that the flowers, by varying in one direction or 

 another, have selected a definite circle of visitors, and, conversely, that 

 particular insect-groups have selected particular flowers for them- 

 selves, for those transformations of the flowers were always most 

 advantageous which secured to them the exclusive visits of their best 

 crossing agents, and these transformations were, on the one hand, such 

 as kept off unwelcome visitors, and, on the other hand, such as 

 attracted the most suitable ones. 



