Charles Darwin proposed a mechanism for descent with modification which stated that organisms of particular species are adapted to their environment when they possess traits that increase their chances of survival and reproduction in that environment.

This is the core of Darwin's theory of natural selection:

* Variation: Individuals within a species have slight differences in their traits.

* Heritability: These traits can be passed down from parents to offspring.

* Survival and Reproduction: Individuals with traits that make them better suited to their environment are more likely to survive and reproduce, passing on those advantageous traits to their offspring.

Over time, this process leads to a gradual change in the species, as the traits that increase fitness become more common in the population. This is how organisms become adapted to their environment.