The work examines how organisms become fitted to their environments by surveying evidence for evolution and scrutinizing natural and sexual selection. It evaluates comparative anatomy, embryology, the fossil record, and experimental observations, and considers variation, competition, and heredity alongside objections such as sterility between species and limits of selection. Special attention is given to regenerative abilities and internal developmental factors as possible alternatives or complements to selection. The author distinguishes well-supported conclusions from unresolved problems and suggests directions for further empirical investigation into the mechanisms that produce adaptive form and function.