Xiv INTRODUCTION. 



be insufficient tests whereby to distinguish plants 

 from animals. And the same must be said of all 

 apparently more valid distinctions, drawn from 

 chemical analysis of their respective tissues. 

 There is, in short, no definable character, or 

 combination of characters, common to all plants 

 and foreign to all animals, or vice versa. 



Some persons, influenced by these considera- 

 tions, have gone so far as to assert that there is 

 no real line of demarcation between the animal 

 and vegetable kingdoms, and that both merge in- 

 sensibly into each other. Others have attempted 

 to prove the existence of three distinct kinds of 

 organised bodies, namely, plants, animals, and in- 

 termediate beings. Neither of these extreme 

 views can be adopted. The popular opinion that 

 there are two and only two great divisions of 

 organic forms, distinct from one another, has been 

 amply confirmed by the combined researches of 

 our most able and trustworthy investigators, who 

 believe that a line of separation between these 

 two divisions does exist, though they may not be 

 able precisely to define its limits. For it should 

 be borne in mind, that the real difficulty now 

 under consideration lies, not so much in our ca- 

 pability to determine whether any organism which 

 may be offered for our inspection be a plant or an 

 animal, since this, to any competent and unpre- 

 judiced observer, is seldom a matter of impossi- 

 bility, but in framing such a definition as will 

 embody our abstract conception of those essential 



