44 THE PHYSICAL BASIS OF INHERITANCE 



that vital activity depends upon several complex stuffs which, 

 like the members of a carefully constituted firm, are character- 

 istically powerful only in their inter-relations. In the same 

 way, it must be clearly understood that we cannot demonstrate 

 the germ-plasm, even if we may assume that it has its physical 

 basis in the stainable nuclear bodies or chromosomes. The 

 theory has to be judged, like all conceptual formulae, by its 

 adequacy in fitting facts. 



Let us suppose that the fertilised ovum has certain qualities, 

 a, b, c . . . x, y, z ; it divides and re-divides, and a body is built 

 up ; the cells of this body exhibit division of labour and dif- 

 ferentiation, losing their likeness to the ovum and to the first 

 results of its cleavage. In some of the body-cells the qualities 

 a, b, find predominant expression, in others the qualities y, z, 

 and so on. But if, meanwhile, there be certain germ-cells 

 which do not differentiate, which retain the qualities a, 6, c . . . 

 x, y, z, unaltered, which keep up, as one may say figuratively, 

 " the protoplasmic tradition," these will be in a position by-and- 

 bye to develop into an organism like that which bears them. 

 Similar material to start with, similar conditions in which to 

 develop therefore, like tends to beget like. 



May we think for a moment of a baker who has a very precious 

 kind of leaven ; he uses much of this in baking a large loaf ; 

 but he so arranges matters by a clever contrivance that part of 

 the original leaven is always carried on unaltered, carefully 

 preserved for the next baking. Nature is the baker, the loaf 

 is a body, the leaven is the germ-plasm, and each bakitig is a 

 generation. 



