OTHER THEORIES OF SPECIES-FORMING. 195 



ing of our microscopes. This nearly unanimous demand 

 on the part of biologists for a complex physical structure 

 of protoplasm, depends largely on the fact that our present 

 knowledge of the chemical constitution of protoplasm offers 

 absolutely no explanation of its capacities. We know that 

 protoplasm is composed of certain familiar elements, pres- 

 ent in certain proportions. But beyond that nothing; the 

 actual chemical relations of these component elements are 

 too complex for analysis. Besides, certain observations of 

 the processes of protoplasmic behaviour suggest strongly 

 the workings of a machine whose effectiveness lies in its 

 physical make-up. Finally, the phenomena of heredity 

 seem to admit of no other explanation than the assumption 

 of a composition of the germinal protoplasm out of myriads 

 of structural units actually representing the myriads of cells, 

 or groups of cells, of the fully developed body. 



Ever since protoplasm has been recognised as the physical 

 basis of life, therefore, and ever since the germ-cells have 

 been recognised not to be miniature men and women, but, 

 as far as the eye and microscope go, masses of primitive 

 protoplasm differentiated only into cell-plasm, nucleus, and 

 nuclear parts (chromosomes, centrosomes, nucleoli, etc.), 

 there have been "atomic" theories of protoplasmic struc- 

 ture. Unfortunately for the standing of any one of these 

 theories, each working biologist seems to have made one 

 for himself, so that instead of one universally accepted, 

 hence usable and useful, atomic or unit theory such as the 

 chemists have and the modern physical chemists seem 

 to be rebelling even against that, biology has had a host 

 of protoplasmic unit theories of which the one we have here 

 specially to refer to is known as Weismann's theory of 

 biophors and determinants. Several of the better known or 

 more ingenious of these theories are outlined in very sum- 

 mary fashion in the appendix * of this chapter. What we 

 need now to know of biophors and determinants in order to 



