LESSON XVII 



THE DISTINCTIVE CHARACTERS OF ANIMALS AND PLANTS 



Hitherto the words "animal" and "plant" have been 

 either avoided altogether or used incidentally without any 

 attempt at definition. We are now however in a position to 

 consider in some detail the precise meaning of the two words, 

 since in the last half-dozen lessons we have been dealing 

 with several organisms which can be assigned without hesi- 

 tation to one or other of the two great groups of living things. 

 No one would dream of calling Paramoecium and Stylonychia 

 plants, or Mucor and Vaucheria animals, and we may" there- 

 fore use these forms as a starting-point in an attempt to form 

 a clear conception of what the names/>/anf and animal really 

 signify, and how far it is possible to place the lowly organisms 

 described in the earlier lessons in either the vegetable or the 

 animal kingdom. 



Let us consider, first of all, the chief points of resemblance 

 and of difference between the indubitable animal Paramoecium 

 on the one hand, and the two indubitable plants Mucor and 

 Vaucheria on the other. 



In the first place, the essential constituents of all three 

 organisms is protoplasm, in which are contained one or more 

 nuclei. But in Paramoecium the protoplasm is invested 



