XI 
THE SPECIAL COLOURS OF PLANTS 
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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; 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 residt 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 
