xi THE SPECIAL COLOURS OF PLANTS 331 



re -developed, but not unfrequently in a different form and for 

 a distinct purpose. 



The chief types of flowering plants have existed during the 

 millions of ages of the whole tertiary period, and during this 

 enormous lapse of time many of them may have been modified 

 in the direction of insect fertilisation, and again into that of 

 self-fertilisation, not once or twice only, but perhaps scores or 

 even hundreds of times j and at each such modification a 

 difference in the environment may have led to a distinct 

 line of development. At one epoch the highest specialisation 

 of structure in adaptation to a single species or group of insects 

 may have saved a plant from extinction; while, at other times, 

 the simplest mode of self-fertilisation, combined with greater 

 powers of dispersal and a constitution capable of supporting 

 diverse physical conditions, may have led to a similar result. 

 With some groups the tendency seems to have been almost 

 continuously to greater and greater specialisation, while with 

 others a tendency to simplification and degradation has resulted 

 in such plants as the grasses and sedges. 



"We are now enabled dimly to perceive how the curious 

 anomaly of very simple and very complex methods of securing 

 cross-fertilisation — both equally effective — may have been 

 brought about. The simple modes may be the result of a 

 comparatively direct modification from the more primitive 

 types of flowers, which were occasionally, and, as it were, 

 accidentally visited and fertilised by insects ; while the more 

 complex modes, existing for the most part in the highly irregular 

 flowers, may result from those cases in which adaptation to 

 insect-fertilisation, and partial or complete degradation to self- 

 fertilisation or to wind -fertilisation, have again and again 

 recurred, each time producing some additional complexity, 

 arising from the working up of old rudiments for new pur- 

 poses, till there have been reached the marvellous flower 

 structures of the papilionaceous tribes, of the asclepiads, or of 

 the orchids. 



We thus see that the existing diversity of colour and of 

 structure in flowers is probably the ultimate result of the 

 ever-recurring struggle for existence, combined with the ever- 

 changing relations between the vegetable and animal kingdoms 

 during countless ages. The constant variability of every part 



