126 PHYSIOLOGY 



organism. The sum of these activities represents the 

 activity of the independent organism, the particular 

 way in which the many-celled organism as a whole 

 shows its living characteristics, motion, reproduction, 

 assimilation. 



In different organisms the differentiation of the cells, 

 tissues, and organs, has proceeded along different lines, 

 or, proceeding along the same line, has reached different 

 stages of development. According to the resulting pe- 

 culiarities animals are divided into groups. Each 

 group of organisms has its own peculiar way of moving 

 from place to place, of assimilating food, and of repro- 

 ducing its kind. These characteristics can not differ 

 essentially in the various groups, for an organism can 

 not assume characteristics other than those of the living 

 matter which composes its cells ; but they may differ in 

 details, and these details depend on the peculiar ar- 

 rangement of the cells of its body into tissues and 

 organs, and the extent of their differentiation. 



Some organisms have developed special organs for 

 locomotion whose activity is governed by mechanical 

 principles. To know how an animal moves from place 

 to place, then, we must know how the organs of locomo- 

 tion are constructed. Some animals have special means 

 of protecting their young until they can fend for them- 

 selves. The more highly developed the animal, the 

 longer is its period of infancy and the more perfect the 

 arrangements for protection. We must know, then, 

 something about the special organs set apart for repro- 

 duction and something about the habits of the group. 

 As animals increase in complexity, the digestive appa- 



