VI COLOURS OF PLANTS 405 
and hazel, our nettles, grasses, sedges, and many others. In 
some of these the male flowers are very conspicuous, as the 
catkins of the willows, and these secrete honey and attract 
numerous insects at a season when there are few other flowers, 
and thus secure cross-fertilisation. Sedges and grasses are 
also occasionally visited by insects. 
The same Theory of Colour applicable to Animals and Plants 
It may be thought that this absence of colour where it is 
not wanted is opposed to the view maintained in the earlier 
part of the preceding chapter, that colour is normal and is 
constantly tending to appear in natural objects. It must be 
remembered, however, that the green colour of foliage, due to 
chlorophyll, prevails throughout the greater part of the vege- 
table kingdom, and has, almost certainly, persisted through 
long geological periods. It has thus acquired a fixity of 
character which cannot be readily disturbed ; and, as a matter 
of fact, we find that colour rarely appears in plants except in 
association with a considerable modification of leaf-texture, 
such as occurs in the petals and coloured sepals of flowers. 
Wind-fertilised plants never have such specially organised 
floral envelopes, and, in most cases, are entirely without a 
calyx or corolla. The connection between modification of 
leaf-structure and colour is further seen in the greater amount 
and variety of colour in irregular than in regular flowers. 
The latter, which are least modified, have generally uniform 
or but slightly varied colours, while the former, which have 
undergone great modification, present an immense range of 
colour and marking, culminating in the spotted and varie- 
gated flowers of such groups as the Scrophularinee and 
Orchidex. The same laws as to the conditions of a maximum 
production of colour are thus found to obtain both in plants 
and animals. 
Relation of the Colours of Flowers and their Geographical 
Distribution 
The adaptation of flowers to be fertilised by insects— 
often to such an extent that the very existence of the species 
depends upon it—has had an important influence on the dis- 
tribution of plants and the general aspects of vegetation, 
