Introduction 1 1 



determine the quality of the cell. As it is not con- 

 ceivable that there should be an exact distribution 

 of the constituents of the cell in cell division, it 

 necessarily follows that no two cells can be exactly 

 alike, and these differences may be increased by 

 subsequent changes in the cells during the develop- 

 ment of the organism. Although it is probable that 

 the invisible units that make up the protoplasm 

 are of many kinds, it is also pretty certain that the 

 differences between two protoplasts may be due not 

 so much to diversity in the actual structure of the 

 particles of each, as to a different grouping of the 

 particles, involving a different reaction toward the 

 many stimuli to which the protoplasmic structures 

 are subjected in the course of their development. 



Plants and Animals. While all the higher organ- 

 isms may be readily assigned to either the plant or 

 animal kingdoms, this is not the case with many 

 of the simplest forms of life, and the division of 

 living things into plants and animals is more or less 

 arbitrary. The living cells of all organisms are 

 composed of protoplasm, and all of them perform 

 the same life functions, i.e., they respire, feed, 

 grow, and reproduce, and these functions are very 

 similar in all living things. The differences be- 

 tween plants and animals are largely physiological 

 ones, and are by no means universal. Typical 

 plants i.e., those that possess chlorophyll can live 

 entirely upon inorganic food, and are able to utilize 

 the radiant energy of sunlight for the manufacture 



