EVOLUTION OF THE TWO KINGDOMS 181 



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. Where 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, 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. 



