7o Flowers and their Unbidden Guests. 



to exclude such creeping insects as have a tolerably 

 hard investment of chitin ; and amongst these the 

 wingless ants play the most important part. Creeping 

 animals, whose skin is soft, are kept back by these 

 viscid secretions with much less certainty. Snails and 

 slugs especially show no great wish to avoid such sub- 

 stances ; for they have learnt an easy method of 

 passing over the sticky places when they come on 

 them, without the least risk ; namely, by covering the 

 stickiness with their own slimy secretions. On the 

 other hand, like all animals with soft outsides, they 

 are peculiarly sensitive to thorns, prickles, and sharp 

 points (see p. 15); so that while ants pass without 

 harm hot only over prickly leaves but over most thistle- 

 heads, these softer animals stop short when they come 

 to such obstacles, and avoid aU contact with them. It 

 is impossible, therefore, for flowers to have a better 

 protection against such animals than is given by 

 prickles, sharp points, or hard bristles, set on the parts 

 which must be traversed on the way to them. It 

 matters not whether these structures be mere out- 

 growths of the epiderm, or modifications of the stipules 

 or of the stem-leaves ; provided only that they are so 

 placed as to make the path, which crawling animals 

 must follow in order to reach the flowers, a dangerous 

 one, they have in the vast majority of instances the 

 same significance, namely, that of defences to these 

 flowers. The cases which come under this heading are 



