The Making of Species 
that cross- fertilisation is advantageous. The 
amount of seed produced is probably not always 
a criterion as to the advantages of the crossing 
to the plant. Some flowers yield most seed 
when fertilised by the pollen from flowers 
belonging to a different species! 
It is significant that some plants produce 
cleistogamous flowers, that is to say, flowers 
which invariably fertilise themselves. Such 
flowers never open ; so that the visits of insects 
are precluded. 
According to Bentham, the Pansy (V2ola tre- 
color) is the only British species of Vzola in 
which the showy flowers produce seeds. The 
other species are all propagated by their cleisto- 
gamous flowers. The genus Vzola is an ad- 
vanced species: it would therefore seem that 
the production of cleistogamous flowers is an 
advance on the production of entomophilous 
flowers. Cleistogamous blossoms are obviously 
more economical. 
In the case of the malvas, epilobias and 
geraniums, where we see, side by side, races 
of which the individuals produce insect-fertilised 
flowers and those that are characterised by self- 
fertilised flowers, the latter are quite as thriving 
as the former. { 
The common groundsel, which, according to 
Lord Avebury, is ‘rarely visited by insects,” 
flourishes like the green bay tree, as many 
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