The idea of Darwin's based on fossils and modern organisms that says genetic changes are inherited by later generations is called descent with modification.

Here's a breakdown:

* Fossils: Darwin observed that fossils showed a progression of life forms, with older fossils being simpler and more primitive than newer ones. This suggested a gradual change over time.

* Modern organisms: He also observed variations within species, and that some variations were more advantageous for survival in a particular environment.

* Inheritance: Darwin reasoned that these advantageous variations were passed down to offspring, gradually changing the characteristics of a species over many generations.

He didn't know the exact mechanisms of inheritance (as Mendel's work on genetics was not widely known at the time), but he understood that some traits were passed from parents to offspring, allowing for the accumulation of changes over time.

This concept, descent with modification, is a key element of Darwin's theory of evolution by natural selection. It emphasizes the gradual change and adaptation of organisms over time, driven by the inheritance of beneficial traits.