THE CONTINUITY OF LIFE 3 



celled organisms, or Protozoa, which are the simplest in struc- 

 ture of all living things. The essential body substance con- 

 sists of a minute mass of semi-fluid protoplasm, in the interior 

 of which lies a denser portion which constitutes its most im- 

 portant organ, the nucleus. This latter is the physiological 

 center for the control of all the vital functions of the animal, 

 and is undoubtedly extremely complex in structure, even in 

 the simplest members of the group. In some protozoans the 

 protoplasm is enclosed by a thin but fine cell-membrane, which 

 preserves for the animal a more or less definite shape ; in other 

 cases there is no such membrane, and the protoplasm is free 

 to assume an irregular and constantly changing outline, each 

 species, however, still preserving a certain characteristic range 

 of form. 



Through the intaking of other organisms, either alive or in 

 a state of disintegration, the protoplasm of all Protozoa has 

 the power of adding to its bulk, through assimilation ; a pro- 

 cess perhaps more than all others characteristic of life and 

 not imitated in any way by lifeless matter. For this process 

 a nucleus is absolutely essential, for it has been experimentally 

 proven that non-nucleated fragments of the simpler Protozoa 

 are capable of continuing their existence for some time, and 

 can even receive foreign materials, yet have no power of 

 assimilation. A fragment containing a nucleus, on the other 

 hand, will continue to grow and will^ukimately completely 

 restore the lost part. 



This process of growth is limited, however, not by any 

 failure in the vital process, but by the mathematical law of the 

 ratio of surface to mass.* The intaking of both food and 

 oxygen, and also the expulsion of all waste products, take place 

 on the external surface, or, in the case of those covered by a 

 cell-membrane, over a restricted portion of that area, but on 



* This law is that the surfaces of homologous solids are to each other 

 as the squares, and their masses as the cubes, of their homologous dimen- 

 sions. A protozoan which has increased to twice its normal size, i. e., twice 

 its original diameter, has increased its surface four times and its mass eight 

 times. It has therefore reduced its proportionate surface by one half, and 

 its supply of food and oxygen in the same degree. 



