I90 LIFE : OUTLINES OF GENERAL BIOLOGY 



(2) that cross- pollination is the rule, not the exception, there being 

 many arrangements which effectively hinder self-pollination, or 

 make it unlikely; (3) that one of these is dichogamy, or the ripening 

 of the stamens and carjx'ls of a particular flower at different times. 

 Subsequent exixriments by Andrew Knight, William Herbert, and 

 esjxcially K. E. Gartner disclosed the fact or probability, which 

 Sprengfl had missed, that cross-jx)llination gives better results than 

 self-jx)llination as regards the number and the vigour of the seeds 

 -a conclusion which Darwin was not slow to use in support of his 

 theory that the adaptations ensuring cross-fertilisation are the 

 outcome of a long-continued process of Natural Selection. 



Patient and precise observations, well illustrated by the work of 

 Hermann Miiller, have shown that many kinds of flowers are visited 

 by particular insects, which unconsciously effect cross- pollination. 

 The most imjxjrtant of these pollinating insects belong to the 

 following orders: — Hymenoptera (e.g. bees), Lepidoptera (butter- 

 flies and moths), Diptera (two-winged flies), and Coleoptera (beetles); 

 and the relation between the length of the insect's "tongue" (a 

 suctorial sj)ecialis;ition of diverse mouth-parts) and the depth of the 

 nectary is the most decisive fact in determining the success of the 

 visits, lx)th to the insects and to the flowers. The insects usually 

 s<'ek out the flowers for the sake of the nectar and the pollen, but in 

 most cases for one or the other. Thus hive-bees, which use both as 

 food, are usually jwllen-collectors at one time and nectar-collectors 

 at another. It is an interesting detail that when a successful worker- 

 bee has hlled her honey-.sac and returned to the hive and emptied it, 

 she executes on the comb a jxculiar excited dance, which is different 

 from that in which she similarly indulges when her treasure-trove 

 has consisted of ixjJlen only. 



Of much ecological importance is the general fact, often verified 

 since first stated by .\ristotle, that a hive-bee is for a time constant 

 to one kind of flower. If she has found white clover profitable, she 

 will keep to white clover; and, what is more, some of her sister- 

 workers, taking their olfactory cue from her, will also search for 

 white clover as long as the supplies last. But next day it may be 

 another kind of plant that is visited. The same holds for some other 

 insects, and in many cases a particular insect visits only a few kinds 

 of flowers. To the ])lant this is advantageous, since effective pollina- 

 tion is more s<Ture when the instxt visitor is adapted and habituated 

 to a particular tyix- of blossom. It is ])robably economical to the 

 in.s<ct; and in the ca,s<' of hive-lxes it obviates unnecessary mixing, 

 of different kinds of honey in the .same cell of the comb. At the same 

 time it must Ix* noted that the so-called "constancy" of bees' visits 

 to particular sjx'cies is not more than a general rule. To those who 

 know bees it is now a jxistulate that there is nothing that bees 

 "alwavs do'. I*'or thev are individualities! 



