

INTRODUCTION 125 



daily have developed definite and very different char- 

 acteristics ; but it is not easy to formulate a distinction 

 that applies equally to the lower forms, for in the lowest 

 animals and the lowest plants the living matter is so 

 slightly differentiated, that in both its physiological quali- 

 ties are manifested in the same way. They move readily 

 from place to place, they make new protoplasm out of 

 the non-living matter which they assimilate, and they 

 reproduce their kind after the same methods. The only 

 distinction which holds universally for both lower and 

 higher forms concerns assimilation. Animals must have 

 solid food. Plants can take in only liquids and gases, 

 but out of these they manufacture within their bodies 

 the complex solids that they too need. 



Organisms may be again divided into two classes, 

 those which are composed of a single cell and those 

 which are composed of many cells. In one-celled 

 forms there is no distinction between the cells of any 

 particular species, they are all alike. Within the cell 

 the protoplasm moves, assimilates food and manufac- 

 tures new protoplasm; and the organism as a whole 

 moves, assimilates food and reproduces new organisms. 

 Thus the characteristics of living matter characterize 

 the cell as an independent organism. 



In many-celled organisms the cells differ from each 

 other in appearance and work. The protoplasm in 

 each cell has the qualities of living matter; it moves, 

 assimilates food, and reproduces itself; but the cells are 

 not independent organisms. They are associated with 

 each other in groups called tissues and organs, and each 

 group does its work toward maintaining the life of the 



