In assuming the existence of comparable types, however, we are not denying the individuality of all historical processes.

in assuming the existence of comparable types为介词+动名词构成介词短语作条件状语。不能用by替换

(1)in assuming sth 假设/假定/如果某事

You were mistaken in assuming it.  

你这样臆断是错了.

Rather previous in assuming I am going to pay?  

你以为我会付款未免太自作聪明了 吧 ?

(2)by doing sth通过做某事的方式，介词短语做方式状语

You will bring disgrace on yourself by doing this.  

你这样做会给自己带来耻辱.

He makes a living by doing odd job.  

他通过做临时的工作来谋生.