XI THE SPECIAL COLOURS OF PLANTS 329 



form. "With these modifications the species might extend its 

 range into new districts, thereby obtaining increased vigour 

 by the change of conditions, as appears to have been the case 

 with so many of the small flowered self-fertilised plants. Thus 

 it might continue to exist for a long series of ages, till under 

 other changes — geographical or biological — it might again 

 suffer from competition or from other adverse circumstances, 

 and be at length again confined to a limited area, or reduced 

 to very scanty numbers. 



But when this cycle of change had taken place, the species 

 would be very diff'erent from the original form. The flower 

 would have been at one time modified to favour the visits 

 of insects and to secure cross -fertilisation by their aid, and 

 when the need for this passed away, some portions of these 

 structures would remain, though in a reduced or rudi- 

 mentary condition. But when insect agency became of 

 importance a second time, the new modifications would 

 start from a diff'erent or more advanced basis, and thus a 

 more complex result might be produced. Owing to the 

 unequal rates at which the reduction of the various parts 

 might occur, some amount of iiTegularity in the flower might 

 arise, and on a second development towards insect cross- 

 fertilisation this irregularity, if useful, might be increased by 

 variation and selection. 



The rapidity and comparative certainty with which such 

 changes as are here supposed do really take place, are well 

 shown by the great differences in fforal structure, as regards 

 the mode of fertilisation, in allied genera and species, and even 

 in some cases in varieties of the same species. Thus in the 

 Ranunculacese we find the conspicuous part of the flower to be 

 the petals in Eanunculus, the sepals in Helleborus, Anemone, 

 etc., and the stamens in most species of Thalictrum. In all 

 these we have a simple regular flower, but in Aquilegia it is 

 mad-e complex by the spurred petals, and in Delphinium and 

 Aconitum it becomes cpiite irregular. In the more simple class 

 self-fertilisation occurs freely, but it is prevented in the more 

 complex flowers by the stamens maturing before the pistil. 

 In the Caprifoliaceae we have small and regular greenish 

 flowers, as in the moschatel (Adoxa) ; more conspicuous regular 

 open flowers without honey, as in the elder (Sambucus) ; and 



