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Karl von Nägeli's "A Mechanico-Physiological Theory of Organic Evolution" presents a detailed exploration of the evolution of organisms from a mechanistic and physiological perspective. Nägeli proposes that organic evolution is driven by the formation and transformation of micellar structures within plasma, which he terms idioplasm. This idioplasm is responsible for the growth and development of organisms, as it dictates the arrangement and function of cells. Nägeli argues that evolution is a result of both internal processes, which he calls the automatic perfecting process, and external stimuli that lead to adaptation. He emphasizes that the idioplasm undergoes gradual changes, leading to the development of new characteristics over time. Nägeli also discusses the role of reproduction in evolution, highlighting the continuity of idioplasm through generations and the influence of sexual reproduction on genetic variation. He distinguishes between varieties, races, and modifications, noting that varieties arise from slow idioplasmic changes, while races result from crossing and pathological influences. Nägeli's theory challenges the Darwinian view by suggesting that natural selection plays a limited role in evolution, acting more as a pruning mechanism rather than a creative force. His work underscores the complexity of evolutionary processes and the intricate interplay between internal and external factors in shaping the diversity of life.

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