30 



EVOLUTION AND ANIMAL LIFE 



to be able to find in this physical organization of protoplasm 

 any satisfactory explanation of its wonderful properties. 



We have said that it should always be held clearly in mind 

 that the full life capacity of protoplasm is realized only when it 

 is in that differentiated and organized condition typical of the 

 structural unit or cell. The essential thing about the cell is 

 not that it has a definite shape or size or that it is truly cell- or 

 saclike, but that it is a tiny mass of protoplasm with various 



FIG. 16. Amoeba, showing different shapes assumed by it when crawling. 



Verworn.) 



(After 



substances secreted by or held in it. The protoplasm itself is 

 differentiated into at least two parts, an inner, denser, smaller 

 part called the nucleus, and an outer surrounding, usually larger, 

 portion called the cytoplasm. Such a differentiated or organized 

 protoplasmic unit can perform all of the essential functions of life 

 and persist in this performance indefinitely unless destroyed by 

 extrinsic causes. The cell itself may not have any indefinite 

 existence as a unit, but it will be the progenitor of an indefi- 

 nitely prolonged series of cells. A single part of this cell, that is, 

 a bit of protoplasm either of the nucleus or the cytoplasm, or the 

 whole of either can perform for a while most of the activities of 

 life; but such a part always lacks the capacity for reproduction, 

 that is, for persistence as living matter. Thus it is obvious that 



