The professor can hardly find sufficient grounds on which （he can ) base his arguments in favor of the new theory.which (he can) to base……定語(yǔ)從句的主語(yǔ)跟主句主語(yǔ)一致的時(shí)候可以省略，省略了主語(yǔ)要用不定式短語(yǔ)。