Darwin noticed that many organisms seemed well suited to

survive in their environment

Explanation:

According to Darwin's theory of evolution, natural selection is the main mechanism. Darwin explains that organisms that have heritable traits that help them survive and reproduce, will be favorable by natural selection. Those favorable traits will enable organism to better adapt to their environment and to pass more genes on to the next generation (offspring).

Organisms adapt to their environment by changing their behavior, structural traits or physiology as a response to environmental change, so that they become well suited to it.