i8o CHARACTERS OF ANIMALS AND PLANTS 



nutrition takes place by absorption and proteids are not essen- 

 tial, there is no contractile vacuole, and no motile phase. 



Lastly, in the Bacteria the cell-wall is composed of cellu- 

 lose, nutrition is saprophytic, there is no contractile vacuole, 

 and the movements are ciliary. So that in all the characters 

 named, save in the presence of cellulose and the absence of 

 a contractile vacuole, the Bacteria agree with Heteromita, 

 yet they are universally except by Prof. Glaus placed 

 among plants, while Heteromita is as constantly included 

 among animals. 



We see then that while it is quite easy to divide the higher 

 organisms into the two distinct groups of plants and animals, 

 any such separation is by no means easy in the case of the 

 lowest forms of life. It was in recognition of this fact that 

 Haeckel proposed many years ago to institute a third 

 "kingdom," called Protista, to include all unicellular organ- 

 isms. Although open to many objections in practice, there 

 is a great deal to be said for the proposal. From the strictly 

 scientific point of view it is quite as justifiable to make three 

 subdivisions of living things as two : the line between animals 

 and plants is quite as arbitrary as that between protists and 

 plants or between protists and animals, and no more so : the 

 chief objection to the change is that it doubles the difficulties 

 by making two artificial boundaries instead of one. 



The important point for the student to recognize is that 

 these boundaries are artificial, and that there are no scientific 

 frontiers in Nature. As in the liquefaction of gases there is 

 a " critical point " at which the substance under experiment 

 is neither gaseous nor liquid : as in a mountainous country 

 it is impossible to say where mountain ends and valley 

 begins : as in the development of an animal it is futile to 

 argue about the exact period when, for instance, the egg 



