398 COOK 



between a sterile hybrid and a fertile combination, one which 

 might have restricted the use of the term hybrid to the former. 

 Sterile hybrids, like false hybrids, are scarcely to be reckoned 

 as forms of conjugation. They are rather to be looked upon 

 as more nearly allied to parthenogenesis, a development through 

 stimulation merely, but without the possibility of forming new 

 relations of heredity or of making new combinations of charac- 

 ters. Sometimes there is not even enough cooperation between 

 the mismated partners of the cell-units to carry the organism 

 through even the normal cycle of individual existence. Hybrids 

 often refuse to grow up, or they may die suddenly and without 

 apparent external cause. 



The building up of each cellular organism involves a contin- 

 ued cooperation between the parental sex-elements, which may 

 be thought of as persisting in all the cells of which the body is 

 composed. Whenever this cooperation breaks down, or proves 

 inadequate, the further development of the conjugate organism 

 becomes impossible. 



