均变论（英语：Uniformitarianism，又称齐一论）是由查理斯·莱尔（Charles Lyell）的《地质学原理》（Principles of Geology）一书所提出，其理论是以英国人詹姆斯·哈顿（James Hutton）在1785年和1789年所提出的渐变论（Actualism）所衍伸而来。其思想和渐变论大致相同，且提出了地质时间的概念，也否定了达尔文的天择说。其中精髓的一句话就是：「现在是通往过去的一把钥匙」（The present is the key to the past），表示一切过去所发生的地质作用都和现在正在进行的作用方式相同，所以研究现在正在进行的地质作用，就可以明了过去的地球历史。这样的思想后来已经有所修正。与其相反的学说是由亚伯拉罕·韦纳所提出的灾变论（Catastrophism）。

Uniformitarianism was an assumption that the same natural laws and processes that operate in the universe now have always operated in the universe in the past and apply everywhere in the universe. It has included the gradualistic concept that "the present is the key to the past" and is functioning at the same rates. Uniformitarianism has been a key first principle of geology and virtually all fields of science, but naturalism's modern geologists, while accepting that geology has occurred across deep time, no longer hold to a strict gradualism.