64 TKOPICAL NATURE, AND OTHER ESSAYS. 



life. Yet even on tlie upper surface of the forest, fully 

 exposed to the light and heat of the tropical sun, 

 there is no special development of coloured flowers. 

 When from some elevated point you can gaze down upon 

 an unbroken expanse of woody vegetation, it often 

 happens that not a single patch of bright colour can be 

 discerned. At other times, and especially at the 

 beginning of the dry season, you may behold scattered 

 at wide intervals over the mottled-green surface a few 

 masses of yellow, white, pink, or more rarely of blue 

 colour, indicating the position of handsome flowering 

 trees. 



The well-established relation between coloured flowers 

 and the need of insects to fertilize them, may perhaps be 

 connected with the comparative scarcity of the former 

 in the equatorial forests. The various forms of life are 

 linked together in such mutual dependence that no one 

 can inordinately increase without bringing about a 

 corresponding increase or diminution of other forms. 

 The insects which are best adapted to fertilize flowers 

 cannot probably increase much beyond definite limits, 

 because in doing so they would lead to a corresponding 

 increase of insectivorous birds and other animals which 

 would keep them down. The chief fertilizers — bees and 

 butterflies — have enemies at every stage of their growth, 

 from the egg to the perfect insect, and their numbers are, 

 therefore, limited by causes quite independent of the 

 supply of vegetable food. It may, therefore, be the case 

 that the numbers of suitable insects are totally inade- 

 quate to the fertilization of the countless millions of 

 forest-trees over such vast areas as the equatorial zone 

 presents, and that, in consequence, a large proportion of 



