Thought experiments, imaginary situations designed to ponder conclusions of a theory, have been used throughout history. There is even a Stanford encyclopedia article about them. But the famous ones that come to mind are either from physics (Archimedes's overturning of the Earth, Newton's bucket, Maxwell's demon, Einstein's elevator, etc.) or philosophy/psychology (Plato's cave, Descartes' evil demon, Leibniz's mill, etc.). The article mentions that more recently "noteworthy contributions have been made exploring the importance of thought experiments in disciplines other than mathematics, philosophy, or physics. They include history, the social sciences, and Christian theology".
Conspicuously absent are natural sciences other than physics. Biology is not even mentioned, and chemistry "has none of note at all. Why is this the case? Perhaps it is merely an historical accident that chemists never developed a culture of doing thought experiments".
What kinds of thought experiments appear in biology? Is it true that they were never entertained in chemistry? If so, are there explanations better than "historical accident"?
EDIT: This paper discusses possible "fundamental difference" between physics and chemistry as an explanation for scarcity of thought experiments in the latter. Darwin's thought experiments are discussed here, see also HDE's answer and general discussion on Biology SE.