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 ( Viola tri- 

 color) is the only British species of Viola in 

 which the showy flowers produce seeds. The 

 other species are all propagated by their cleisto- 

 gamous flowers. The genus Viola 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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