140 THE EVOLUTION TIIEOKY 



are (|uite fi)ivi<;n to the original mode of life of the organism in 

 question, and stand, indeed, in apparent contradiction to its funda- 

 mental ])hvsi()l(\gical processes, it is hardly necessary to enter into 

 a special argument to show that they can only have been brought 

 about in the course of natural selection, since every other interpreta- 

 tion of their occurrence fails. Neither climatic nor any other external 

 direct influence could have effected these modifications of the parts of 

 plants, -which are all so difierent., yet all so well suited to their 

 purpose : they are different even in plants growing quite close 

 toirether, like the sundew and the butterwort. The Lamarckian 

 principle of use and disuse hardly enters into the question at all, since 

 plants do not possess a will, and we can hardly speak of ' chance ' 

 where we have to do with such complex and diversely combined 

 transformations. A process of selection actually operative in each of 

 these cases can easily be thought out, and I shall leave it to my 

 readers themselves to do this, and shall only indicate that we have 

 to do with increasing elaboration in two different directions : ffrst, 

 improvements in the ability to utilize animal substances which 

 happened to stick to the leaves, and second, an increase in the 

 probability of animals sticking to the leaves, and so l)ecoming 

 available. Thus there arose, on the one hand, dissolving and 

 digestive juices, and arrangements for absorption ; and, on the other 

 hand, viscid slime, and traps of various kinds to secure the animals, 

 as well as honey and bright colours to attract them. 



But it is not merely transformations in the form of the stems and 

 leaves which have come about : there are also important physiological 

 changes. The sensitiveness to stimulus of various parts of the leaf is 

 greatly increased, to a certain extent in the butterwort, the edges of 

 whose leaves turn inwards in response to stimulus, still more in the 

 sundew, in which the stimulus is conveyed from the tentacles touched 

 to all the others, but most wonderfully of all in the Venus fly-trap 

 and Aldrovandia , whose sensitive hairs so transmit the stimulus that 

 the whole leaf is affected by it, and is set in motion, in a manner 

 quite comparable to the effects of a nerve-stimulus in animals. 



Thus the case of carnivorous or insectivorous plants shows us that, 

 in the course of natural selection, quite new organs can be produced 

 in a plant by a thoroughgoing transformation of old ones, as, for 

 instance, the pitchers of Nepenthes, and that, furthermore, even the 

 physiological capacities of the plant may be changed in the most far- 

 reaching manner, increasing and varying until they come to resemble 

 the functions of the animal body. 



