What is an organism that lives on or in another organism and derives its nutrition from that organism?

A parasite

Explanation:

Parasitism is a form of symbiosis where one organism lives on/in another organism, as in the question, and gets a benefit from that organism, while the other organism is harmed. This is denoted by +/-, which means that one is positively impacted and one is negatively impacted, as opposed to commensalism, which is +/0, or mutualism, which is +/+.

An example is tapeworms, which are flatworm parasites that live in the human  intestine; they benefit while the host suffers.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".

for example, no new evidence will demonstrate that the earth does not orbit around the sun (heliocentric theory), or that living things are not made of cells (cell theory), that matter is not composed of atoms, or that the surface of the earth is not divided into solid plates that have moved over geological timescales .