152 The Unity of the Organism 



theory,' by which it is formulated, is, therefore, no longer 

 of an inferential or hypothetical character, but a generalized 

 statement of observed fact which may be outlined as fol- 

 lows": — [only the baldest essentials of the outline are here 

 given]. (1) "In all higher forais of life, whether plants or 

 animals, the body may be resolved into a vast host of minute 

 structural units known as cells, out of which, directly or 

 indirectly, every part is built." (2) "Essentially the cell is a 

 minute mass of protoplasm," this substance being "uni- 

 versally recognized as the immediate substratum of all vital 

 activity." (3) All the cells of each individual organism are 

 descended by cell division from preceding cells and finally 

 from one single cell, the fertilized egg-cell. (4) This fer- 

 tilized egg-cell, which is the beginning of each individual 

 organism, is produced by the fusion of two cells one of 

 which comes from the mother, the other from the father 

 of the individual in question, so that "the ultimate prob- 

 lems of sex, fertilization, inheritance, and development are 

 shown to be cell- problems.'^'' ^ 



The tout ensemble of meaning and of importance of the 

 theory for Wilson are forced home in the very first sentence 

 of his excellent book: "During the half-century that has 

 elapsed since the enunciation of the cell-theory by Schleiden 

 and Schwann, in 1838-39, it has become ever more clearly 

 apparent that the key to all ultimate biological problems 

 must, in last analysis, be sought in the cell." ^ 



A concise formulation of the theory with the expansive 

 meaning given it by Wilson is furnished by Locy in Biology 

 and its Makers. "A statement of the cell-theory at the 

 present time, then, must include these four conceptions : 

 the cell as a unit of structure, the cell as a unit of physio- 

 logical activity, the cell as embracing all hereditary quali- 

 ties within its substance, and the cell in the historical de- 

 velopment of the organism." ^ 



This expanded form of the theory of cells, held implicitly 



