350 STUDIES IN LIFE AND SENSE. 



viscid hairs, but it exerts not the slightest power of digestion or 

 absorption of the rich food thus captured in a word, it can make no 

 use whatever of the insect-prey, any more than the horse-chestnut 

 can utilise the flies which adhere to the gummy surface of the scales 

 which protect its leaf-buds. 



But there are other plants, not ranked amongst the insect-eaters, 

 and which nevertheless appear to possess potential qualifications for 

 such a life. There are some species of the familiar Saxifrages, for 

 instance, the glands of whose leaves possess powers of absorbing 

 certain matters brought into contact with them ; and a species of 

 Primula has been experimentally proved by Mr. Darwin to be capable 

 of exercising a like action. So that, as Mr. Darwin remarks, it is 

 " probable that the glands of some of the above-named plants obtain 

 animal matter from the insects which are occasionally entangled by 

 the viscid secretion." Thus we are presented with a tolerably close 

 series of links leading us from ordinary plants towards their insect- 

 eating neighbours. Beginning with the plant which, like Mirabilis, 

 preserves merely the power of capturing insects, but which makes no 

 use of the food thus laid at its door, we pass to the saxifrage-stage, 

 in which the insect-material adhering to the leaves is probably 

 absorbed by the glands thereof, and this without any special modi- 

 fication of the plant-structure. Thence we arrive at the butterwort 

 itself, a true insect-eater, but one of simple type, and such as may be 

 held to represent merely a slight advance upon the saxifrage form. 

 Progressive modification, then, cannot be doubted to have occurred 

 in the development of these curious habits of plant-life ; and although 

 the exact lines and pathways of the modification are still hidden or 

 obscure, the possibilities seen in the life and structure of the common 

 plants around us testify plainly enough to the evolution of new structure 

 and habit through the variation of familiar types. 



The butterwort's method of insect-capture is in itself simple, and 

 readily understood. When any object is placed near the incurved 

 edge of the leaf, the leaf margin curls inwards, and then after a 

 varying interval expands. This movement may be excited by 

 various causes. Thus, pieces of glass, insects, drops of beef- infusion, 

 and of carbonate of ammonia solution, produced the incurvation of 

 the leaves ; but drops of water, as well as drops of sugar or gum 

 solution, had no such effect. The leaf will not incurve upon pieces 

 of glass to the same extent as upon nutritive matters ; nor does 

 scratching the leaf produce any movement ; such an observation 

 appearing to indicate the existence of some amount of co-ordinated 

 habit. But an important observation regarding this plant is found 

 in the fact, that the period during which the leaf is incurved is 

 remarkably short, as compared with that during which the leaves of 

 other carnivorous plants remain closed. Thus twenty-four hours may 



