194 PLANTS AND ANIMALS COMPARED 



of either. In fact, the only satisfactory criterion as to the 

 inclusion of an organism in the one Kingdom or the other 

 is the method of nutrition. Plants absorb their food in liquid 

 or gaseous form, and, with the exception of those lacking 

 chlorophyll, build up their bodies from simple inorganic com- 

 pounds ; animals feed on complex organic substances, and take 

 up a large part of their food in the solid form. 



The most important distinction between the two Kingdoms 

 thus depends on the presence or absence of chlorophyll ; but 

 even this fails in some cases, as, for example, in certain uni- 

 cellular motile forms which may or may not possess a green 

 pigment according to the conditions under which they live. 

 Just as it is amongst the simple unicellular plants and animals 

 that we find the closest resemblances, so too it is the complex 

 organisms of the highest groups, as exemplified by Flowering 

 Plants and Mammals, that exhibit the greatest distinction 

 and portray the salient plant and animal characters in the most 

 marked degree. 



