382 What are Animals and Plants ? 



utterly unable to frame any definition which shall at the 

 same time include all kinds of one of these two groups, while 

 excluding all kinds of the other group. 



Nevertheless, it is obvious that there is an immense 

 difference between animals and plants generally — a diiference 

 well expressed by that common-sense assertion we quoted at 

 starting, that ' animals are creatures which get their living 

 by the help of their senses, while plants are senseless.' Now, 

 this common-sense view accords with the distinction drawn 

 so many centuries ago by Aristotle, that animals feel, while 

 plants do not. 



In biology, however, groups are characterised by structure 

 rather than by function, and we know, moreover, that every 

 difference in ' function ' has some difference in ' structure ' as 

 its accompaniment. But what is the structure which is 

 related to the function of feeling ? It is the nervous system. 

 ' Nervous tissue ' is the ' organ of feeling,' and modifications 

 of it, with accessory accompaniments, constitute every organ 

 of special sense, i.e. of sight, hearing, smell, taste, and touch. 



Now, no plant is yet known to possess anything like 

 nervous tissue, and the same may be affirmed of the lowest 

 organisms commonly recognised as animals. We know at 

 present no way of defining a plant save the negative one of 

 saying 'a plant is an organism which is not an animal,' while the 

 essence of animal life seems to us to be the power of ' feeling,' 

 together with its necessary correlate, the 'possession of a 

 nervous system.' If, then, we must draw a hard-and-fast 

 line between the two kingdoms, we see no way left for us but 

 that of transferring to the vegetal kingdom those lower 

 organisms generally reckoned as animals, which possess no 

 nervous systems. To botanists they will perhaps be an un- 

 welcome present, but they can hardly be refused on any 

 valid scientific grounds. The activity and irritability of 

 many of them are, no doubt, very suggestive of animal Hfe, 



