part ui.}| §= THE MECHANISMS OF FLOWERS. 547 
Those flowers fitted for fertilisation by bees are at an advantage 
in that the bees always cause cross-fertilisation of separate plants ; 
but at a disadvantage from having their pollen exposed to robbery 
by Rhingia. The flowers adapted for fertilisation by Rhingia have 
the disadvantage that self-fertilisation is as frequently or even 
more frequently caused than cross-fertilisation; but possess the 
advantage that their pollen is not liable to be wasted, and that 
they receive more numerous visits. Advantages and disadvantages 
must be about equally balanced in the two varieties, since natural 
selection has not enabled either to outstrip the other. The rarity 
of intermediate forms is explained by the fact that such forms 
combine the disadvantages of both extremes; cross-fertilisation 
is not secured, and the pollen is not protected from robbery. 
Besides the visitors already mentioned one or two useless 
guests frequent the flowers. On the variety in which the perianth 
and styles stand wide apart, J once saw Osmia rufa, 9, engaged in 
sucking honey. This bee was enabled, by the length of its 
proboscis (8 mm.), to enjoy the honey without touching either 
stigma or anther. I once saw a hive-bee trying to obtain honey. 
It crept beneath a style without touching the stigma or anther, 
as far as the honey-passages, and then stretched out its 
proboscis (6 mm.), which, however, was too short to reach the 
honey. After several unsuccessful attempts the bee deserted the 
plant and settled on flowers of Ranunculus acris. 
Visitors : A. Hymenoptera--Apide : (1) Bombus vestalis, Fource. @ ; (2) 
B. agrorum, F. 2 $; (3) B. hortorum, L. 9 §; (4) B. Rajellus, Ill. 2, all four 
sucking ; (5) Osmia rufa, L. 9, sucking without effecting fertilisation ; (6) Apis 
mellifica, L. $, vainly sucking honey. B. Diptera—Syrphide: (7) Rhingia 
rostrata, L.; s. and f.p. 
A species of Cypella, in S. Brazil, according to Fritz Miiller, 
increases its conspicuousness very effectually by not expanding 
its flowers regularly day by day but by concentrating them upon 
certain days (597). 
Crocus vernus, All_—Honey is secreted by the ovary, and rises 
in the narrow tube (almost filled by the style) nearly to the upper, 
somewhat expanded end. It can only be completely extracted by 
long-tongued Lepidoptera, while humble-bees as a rule can only 
skim the surface. By the violet or more frequently white colour 
the flowers seem to be adapted for crepuscular and nocturnal 
Lepidoptera. Ihave found them visited by Plusia gamma, less 
often by Vanessa cardui, and occasionally by humble-bees. At first, 
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