Charles Darwin was a biologist during the late 1800s. His most famous publication is the Origin of Species. In it, he describes his theory on how animals and species can adapt and change over a period of time. He explained how an animal may have certain adaptations that allow it to be more fit for a particular environment. Better fitness means a greater likelihood of survival, which is why it is called survival of the fittest. Those animals then pass on their traits through the process of natural selection to the next generation, and what is seen after many generations is an overall change in the characteristics of a population of animals.
Modernism basically describes a series of reforming movements across all disciplines (art, music, literature, science, etc.) that began occurring during the late 1800s and into the early 1900s. In general, modernism throws away the traditional thinking of the 1800s, which is why Darwin can be placed amid modernists. Previously it was thought that species do not change. They have been static since their creation. The other major change with Darwin is that he said humans were no different than any other species that is out there, subject to the same laws of natural selection and fitness adaptations. Many contemporaries of Darwin did not appreciate that because it linked humans with what they considered the "lower animals."