258 ANIMALS AND PLANTS chap. 



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 

 life-history : and then a balance must be struck and the 

 doubtful form placed in the kingdom with which it has, on 

 the whole, most points in common. 



It will no doubt occur to the reader that, on the theory 

 of evolution (p. 222), we may account for the fact of the 

 animal and vegetable kingdoms being related to one another 

 like two trees united at the root, by the hypothesis that the 

 earliest organisms were protists, and that from them animals 

 and plants were evolved along divergent lines of descent. 



