258 ANIMALS AND PLANTS CHAP. 



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 

 organisms. 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 recognise 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 becomes a tadpole or the tadpole a frog : so in the 

 case under discussion. The distinction between the higher 

 plants and animals is perfectly sharp and obvious, but when 

 the two groups are traced downwards they are found 

 gradually to merge, as it were, into an assemblage of organ- 

 isms which partake of the characters of both kingdoms, and 

 cannot without a certain violence be either included in or 

 excluded from either. When any given "protist" has to 

 be classified the case must be decided on its individual 

 merits : the organism must be compared in detail with all 

 those which resemble it closely in structure, physiology, and 



