THE OUTER TISSUES OF PLANTS. 267 



slowly to fade away. How, then,, is the intensification of 

 them to be explained ? 



If a contrast of the kind described favours the propagation 

 of a race in which it exists, it will be maintained and 

 increased; and if we take into account an agency of which 

 Mr. Darwin has shown the great importance the agency of 

 insects we shall have little difficulty in understanding how 

 such a contrast may facilitate propagation. We cannot, of 

 course, here assume the agency of insects so specialized in 

 their habits as Bees and Butterflies ; for their specialized 

 habits imply the pre-existence of the contrast to be explained. 

 But there is an insect-agency of a more general kind which 

 may be fairly counted upon as coming into action. Various 

 small Flies and Beetles wander over the surfaces of plants in 

 search of food. It is a legitimate assumption that they will 

 frequent most those parts in which they find most food, or 

 food most to their liking especially if at the same time 

 they gain the advantage of concealment. Now the ends of 

 axes, formed of young, soft, and closely-packed folia, are the 

 parts which more than any others offer these several advan 

 tages. They afford shelter from enemies; they frequently 

 contain exuded juices; and when they do not, their tissues 

 are so tender as to be easily pierced in search of the sap. 

 If, then, from the first, as at present, these ends of axes 

 have been favourite haunts of small insects ; and if, where 

 the closely-clustered folia contained the generative organs, 

 the insects frequenting them occasionally carried adherent 

 fructifying cells from one plant to another, and so aided 

 fertilization; it would follow that anything which made 

 such terminal clusters more attractive to such insects, or 

 more conspicuous to them, or both, would further the multi 

 plication of the race, and would so be continually increased 

 by the extra multiplication of individuals in which it was 

 greatest. Here we find the clue. This contrast of colour 

 between the folia next to the fructifying parts and all other 

 folia, must constantly have facilitated insect-agency; sup- 



