xi THE SPECIAL COLOURS OF PLANTS 325 



insects, they rapidly diminish in size, lose then bright colour 

 or almost wholly disappear 1 



Difficulties and Contradictions. 



The very bare summary that has now been given of the 

 main facts relating to the fertilisation of flowers, will have 

 served to show the vast extent and complexity of the inquiry, 

 and the extraordinary contradictions and difficulties which it 

 presents. We have direct proof of the beneficial results of 

 intercrossing in a great number of cases ; we have an over- 

 whelming mass of facts as to the varied and complex structure 

 of flowers evidently adapted to secure this intercrossing by 

 insect agency ; yet we see many of the most vigorous plants 

 which spread widely over the globe, with none of these 

 adaptations, and evidently depending on self-fertilisation for 

 their continued existence and success in the battle of life. 

 Yet more extraordinary is it to find numerous cases in which 

 the special arrangements for cross-fertilisation appear to have 

 been a failure, since they have either been supplemented by 

 special means for self-fertilisation, or have reverted back in 

 various degrees to simpler forms in which self-fertilisation 

 becomes the rule. There is also a further difficulty in the 

 highly complex modes by which cross-fertilisation is often 

 brought about ; for we have seen that there are several very 

 effective yet very simple modes of securing intercrossing, 

 involving a minimum of change in the form and structure of 

 the flower; and when we consider that the result attained 

 with so much cost of structural modification is by no means 

 an unmixed good, and is far less certain in securing the per- 

 petuation of the species than is self-fertilisation, it is most 

 puzzling to find such complex methods resorted to, some- 

 times to the extent of special precautions against the possi- 

 bility of self-fertilisation ever taking place. Let us now see 

 whether any light can be thrown on these various anomalies 

 and contradictions. 



Intercrossing not necessarily Advantageous. 



Xo one was more fully impressed than Mr. Darwin with 

 the beneficial effects of intercrossing on the vigour and fertility 

 1 H. Muller gives ample proof of this in his Fertilisation of Floicers. 



