Darwin on the Effects of Cross- and Self- Fertilization. 135 



in the very flowers which are adapted for crossing. In Impa- 

 tit >n, Viola, and the like, it is provided for by separate flowers, 

 the special adaptations of which are unmistakable. 



H. Miiller appears to have shown " that large and conspicu- 

 ous flowers are visited much more frequently and by many 

 more kinds of insects than are small inconspicuous flowers. 

 He further remarks that the flowers which are rarely visited 

 must be capable of self-fertilization, otherwise they would 

 quickly become extinct." Mr. Darwin's list seems to show 

 that, as a rule, they are so ; yet many very small flowers, like 

 those of Irifolium "arvense, and small and dingy ones, like 

 those of Asparagus, are freely visited by bees ; and, conversely, 

 many large and conspicuous flowers which are frequented by 

 insects are none the less self-fertilizable. Throughout we find 

 that such things do not conform to arbitrary or fixed rules : 

 and this favors the idea that the differences have been acquired. 

 Mr. Darwin conjectures that the self-fertilizing capabilities of 

 many small and inconspicuous flowers may be comparatively 

 recent acquisitions, on the ground that, if they were not occa- 

 sionally intercrossed, and did not profit by the process, all their 

 flowers would have become cleistogenous, "as they would 

 thus have been largely benefited by having to produce only a 

 small quantity of safely protected pollen." 



Mr. Darwin's experiments tending to prove that cross-fertili- 

 zation between flowers on the same plant is of little or no use, 

 he is naturally led to consider the means which favor or en- 

 sure their fertilization with pollen from a distinct plant. This 

 must needs take place with dioecious plants, and is likely to 

 occur with the monoecious, and is in some cases secured (as in 

 Walnut and Hazelnut) by some trees being proterano. 

 others proterogynous, so that they will reciprocally fertilize 

 each other. In ordinary hermaphrodite species the expansion 

 of only a few blossoms at a time greatly favors the intercross- 

 ing of disti though, in the case of small 

 flowers it is attended with the disadvantage of rendering the 

 plants less conspicuous to insects. Our common Sundews fur- 

 nish a good illustration of this. They abound wherever they 

 occur, and are for a long while in blossom, but each plant or 

 spike opens but one flower at a time. The fact of bees visit- 

 ing the flowers of the same species as long as they can, instead 

 of promiscuously feeding from the various blossoms nearest 

 within reach, greatly favors such intercrossing. So does the 

 remarkable number of flowers which bees are able to visit in a 

 short time (of which mention will be made), and the fact that 

 tbey are unable to perceive without entering a flower whether 

 other bees have exhausted the nectar. Then dichog 

 maturation of one sex in a hermaphrodite flower earlier than 



