For the selection of gamete size necessary to catalyze the transition from isogamy to oogamy, the fitness of an organism would have to increase as egg size increases, and increase as sperm size decreases. The number of gametes within the organism alone would exist as a type of selective pressure that promotes smaller sperm size, but disproportionately selects for less eggs as they grow larger within the organism. This supported Bell's theory (1997) that the selective pressure had to select for growth of egg size more than shrinkage of sperm.