424 THE POPULAR SCIENCE MONTHLY. 



from the undigested stuff mechanically lodged for a while in the body) 

 are of three classes : those preparatory to and culminating in the con- 

 version of the food into protoplasm ; those concerned in the dis- 

 charge of energy ; and those tending to economize the immediate prod- 

 ucts of the second class of changes by rendering them more or less 

 useful for tlie first. 



5. It is respikatoey. Taken as a whole, the metabolic changes 

 are preeminently j^rocesses of oxidation. One article of food i. e., one 

 substance taken into the body, viz., oxygen stands apart from all 

 the rest ; and one product of metabolism peculiarly associated with 

 oxidation viz., carbonic acid stands also somewhat apart from all 

 the rest. Hence, the assumption of oxygen and the excretion of car- 

 bonic acid, together with such of the metabolic processes as are more 

 especially oxidative, are frequently spoken of together as constituting 

 the respiratory processes. 



6. It is reproductive. The individual amoeba represents a unit. 

 This unit, after a longer or shorter life, having increased in size by 

 the addition of new protoplasm in excess of that which it is contin- 

 ually using up, may by fission (or by other means) resolve itself into 

 two (or moi'e) parts, each of which is capable of living as a fresh unit 

 or individual. 



Such are the fundamental vital qualities of the protoplasm of an 

 amoeba ; all the facts of the life of an amoeba are manifestations of 

 these protoplasmic qualities in varied sequence and subordination. 

 The higher animals, we learn from morphological studies, are in real- 

 ity groups of amoebae peculiarly associated together. All the physio- 

 logical phenomena of the higher animals are similarly the results of 

 these fundamental qualities of protoplasm peculiarly associated to- 

 getlier. The dominant principle of this association is the physiologi- 

 cal division of labor corresponding to the morj^hological differentia- 

 tion of structure. Were a larger or " higher " animal to consist sim- 

 ply of a colony of undifferentiated amoeba?, one animal differing from 

 another merely in the number of units making up the mass of its 

 body, without any differences between the individual units, progress 

 of function would be an impossibility. The accumulation of units 

 would be a hindrance to welfare rather than a help. Hence, in the 

 evolution of living beings through past times, it has come about that 

 in the higher animals (and plants) certain groups of the constituent 

 amoebiform units or cells have, in company with a change in structure, 

 been set apart for the manifestation of certain only of the fundamen- 

 tal properties of protoplasm, to the exclusion or at least to the com- 

 plete subordination of the other properties. 



These groups of cells, thus distinguished from each other, at once 

 by the differentiation of structure and by the more or less marked ex- 

 clusiveness of structure, receive the name of "tissues." Thus, the 

 units of one class are characterized by the exaltation of the contrac- 



