THE OUTER TISSUES OF PLANTS. 251 



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 oi' 

 young, soft, and closely-packed folia, are the parts which more 

 than any others offer these several advantages. 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 con- 

 tained 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 multiplication of the race, and would so be con- 

 tinually increased by the extra multiplication of individuals 

 in which it was greatest. Here we find the clue. This con- 

 trast of colour between the folia next to the fructifying parts 

 and all other folia, must constantly have facilitated insect- 

 agency ; supposing the insects to have had the power of dis- 

 tinguishing between colours. That Bees and Butterflies 

 have this power is manifest : they may be watched fly- 

 ing from flower to flower, disregarding all other parts 

 of the plants. And if the less-specialized insects pos- 

 sessed some degree of such discrimination, then the initial 

 contrasts of colour above described would be maintained 

 and increased. Let such a connexion be once established, and 

 it must tend to become more decided. Insects most able 

 to discern the parts of plants which afford what they seek, 

 will be those most likely to survive and leave offspring. 

 Plants presenting most of the desired food, and showing most 

 clearly where it lies, will have their fertilization and multi- 

 plication furthered in the greatest degree. And so the 

 mutual adaptation will become ever closer ; while it is ren- 



