144 CAUSES OF VARIATION 



In assuming the cell as a unit many structural difficulties 

 were solved to the entire satisfaction of the anatomist, but the 

 physiological and evolutionary problems were complicated rather 

 than simplified, because this entire colony of many different cells 

 and activities heart, lungs, liver, muscles, nerves, etc., with 

 their many and diverse functions sprang originally from a 

 single cell ; moreover, this colony will throw off a succession of 

 single cells, each of which will undergo specific and orderly 

 development and finally produce a colony like the parent. This 

 being true, the cell cannot be regarded as the ultimate unit of 

 living matter, unless we assume some kind of unity between all 

 the cells ; some kind of intercellular force to insure that differ- 

 entiation shall take place at the proper points and stages, 

 otherwise the original cell would develop into a lump of proto- 

 plasm or into a colony of cells all alike. 



The single cell from which a new individual is to develop 

 (the " germ cell" or mother cell) is gifted with potentialities 

 for the entire being, with all its complications of structure and 

 with all its variety of function. Biologists at one time were 

 inclined to regard this germ cell as " totipotent," that is, able 

 to develop into almost any kind of structure depending upon 

 the surroundings. This view could not hold because different 

 germ cells under identical conditions of life develop each into 

 its own species. 



The cause of differentiation, therefore, lies primarily within, 

 and the germ cell is to be regarded as gifted with unlimited 

 powers of development only within the characters that belong to 

 the species. 



Specific protoplasm is, therefore, possessed of specific proper- 

 ties as truly as is any chemical substance, and all the characters 

 of structure or function belonging to the mature individual 

 are to be regarded as in some way " inherent in the germ." 

 The cell is, therefore, like the individual, too large and too 

 complicated to be considered as the ultimate unit 'of living 

 matter. 



This view is upheld not only upon theoretical grounds but 

 also by the known facts of its complicated structure and its re- 

 markable behavior during cell division and growth, a subject 



