The Evolution Theory, Vol. 2 of 2

In Short

This second volume of August Weismann's The Evolution Theory serves as a grand synthesis of neo-Darwinian thought, setting out to prove that natural selection is the driving force of life. Expanding far beyond Charles Darwin's original observations, Weismann systematically examines the cellular mechanisms of heredity, the roles of sexual reproduction and isolation, and the phenomenon he calls "germinal selection." The work surveys a vast spectrum of the animal kingdom—from microscopic Protozoa and parasitic crustaceans to caterpillars, ants, and humans—to explain how complex adaptations arise and how unused structures disappear. It has lasted as a foundational text because it established the concept of the "germ-plasm," challenging the inheritance of acquired traits and paving the way for modern genetics.

The Story

The narrative of Weismann's work follows the logical progression of an empirical scientific argument, systematically addressing the mysteries of inheritance, variation, and speciation. He begins by examining the mechanics of cellular regeneration and tissue differentiation. Observing how a Planarian worm can be sliced into multiple pieces and still regenerate, or how a crab voluntarily casts off a trapped leg through autotomy, Weismann asks what internal blueprints allow these complex recoveries. He rejects the notion that every cell holds a complete copy of an organism's developmental instructions throughout its life, arguing instead that specialized tissue cells carry only specific units of hereditary material, while the true primary constituents are safeguarded within the germ-cells.

Moving deeper into the mechanics of inheritance, Weismann confronts the nature of "amphimixis"—the sexual mingling of parental germ-plasms. He refutes the popular idea that fertilization is merely a physical spark or a simple means of reproduction, pointing out that single-celled organisms multiply rapidly by division long before engaging in conjugation. Instead, he demonstrates that amphimixis exists primarily to create endless new combinations of hereditary units, known as "ids" and "determinants." This constant mixing supplies the necessary variety upon which natural selection acts. Where amphimixis is bypassed, as in the parthenogenetic life cycles of gall-wasps or the asexual propagation of cultivated plants, specific adaptive advantages like rapid seasonal population growth emerge, though sometimes accompanied by the degeneration of unused reproductive organs.

Weismann then broadens his lens to examine how species form, diversify, and vanish in nature. He evaluates environmental influences and geographic isolation, analyzing island populations such as the snails of Hawaii and the thrushes of the Galapagos. While acknowledging that new conditions can stir up periods of increased variability or "mutations," he firmly rejects Jean-Baptiste Lamarck's theory that use and disuse directly alter inherited traits. He refutes claims that acquired injuries or somatic variations, such as the effects of alcoholism, can rewrite the germ-plasm. To explain how disused structures—like the vestigial hind-limbs embedded inside whales or the embryonic finger bones of birds—gradually wither away over millions of years, he introduces his theory of "germinal selection." He proposes a continuous internal struggle for nutrition among the tiny determinants within the germ-plasm itself, where weaker units regress and stronger ones flourish.

The final arc of the argument scales up from microscopic germinal units to the grandeur of total evolutionary history, culminating in a defense of pan-selectionism. Weismann categorizes the tiers of selection, showing how personal selection operates on individuals, sexual selection crafts ornamental traits, and "cormal selection" operates on entire social colonies like ants, bees, or coral polyps. In his conclusion, he addresses spontaneous generation and the ultimate limits of evolutionary theory. Applying his framework to human society, he argues that while human intellectual capability is bounded by the natural struggle for existence, unique individual geniuses arise from rare, chance combinations of hereditary units. Ultimately, Weismann closes with the firm declaration that every permanent and functional structure in the living world is an product of selection and adaptation.

How It Unfolds

The mechanics of regeneration Weismann explores how lower animals repair damaged tissues, citing planarians that regenerate after division and crabs that discard legs to escape predators. He argues these adaptations depend on specialized cellular structures rather than a universal capacity in every cell.

The role of amphimixis Analyzing the fusion of germ-cells across plants, Protozoa, and higher animals, he proves that sexual mingling is distinct from simple reproduction. Its chief function is to reduce the number of internal "ids" by half and recombine them to generate continuous variation.

Evaluating environmental effects The author addresses external influences, explaining that while changing environments can excite latent variability in the germ-plasm, they do not directly write somatogenic changes into the offspring. Supposed cases of inherited injuries or diseases are exposed as germinal infections or direct cellular exposure.

Isolation and species formation Examining island endemic species, such as Hawaiian snails and Galapagos thrushes, Weismann analyzes how geographic barriers foster distinct local forms. He maintains that while isolation allows new variations to gather momentum, natural selection remains the ultimate arbiter of species survival.

Germinal selection and degeneration To explain why unused organs—such as the hidden leg bones of whales—slowly disappear, Weismann presents a model of competition between microscopic determinants inside the germ-plasm. Plus- and minus-variations flourish or wither based on the internal allocation of nutrients.

Tiers of selection and human genius The argument reaches its climax by expanding selection theory across four distinct levels, including cormal selection in social insect colonies. He concludes by analyzing human intellect, attributing extraordinary human geniuses like Shakespeare or Beethoven to rare, non-heritable combinations of mental determinants.

The People

August Weismann The author and chief strategist of the text acts as the guiding intellectual force throughout the argument. He seeks to build a comprehensive, logical framework for evolutionary theory that eliminates Lamarckian assumptions of acquired inheritance, facing opposition from naturalists who credit environment or use with direct evolutionary power. Ultimately, his rigorous focus on the germ-plasm establishes a durable foundation for modern evolutionary theory.

The Lamarckians Represented in the text by contemporary naturalists like Louis Zehnder and M. Nussbaum, this group seeks to prove that physical strain, exercise, or environmental impacts on an adult body can alter its offspring. Weismann systematically counters their proposals by demonstrating the biological impossibility of skin nerves sending specialized nutritive signals back into the germ-cells.

Hugo de Vries A vital contemporary investigator whose empirical work with the evening primrose (Oenothera) is closely analyzed by Weismann. De Vries seeks to prove that new species arise through sudden "mutations" during distinct variational periods. Weismann accepts his observations of sudden variations but reinterprets them as the result of environmental factors stirring the internal nutritive stream of the germ-plasm.

The Social Insects Serving as crucial collective figures in the book's argument, sterile worker ants, bees, and termites pose a classic challenge to selection theory because they cannot pass on traits directly. Weismann demonstrates that their complex anatomical adaptations, such as the specialized comb-legs of bees or the distinct worker castes of ants, are preserved and refined through selection acting on the fertile queens and males ("cormal selection").

In Its Own Voice

"Thus even the minute spermatozoon of Man can contain microbes, and transmit them to the germ of a new individual."

Weismann relies on microscopic evidence to clarify that disease transmission through reproductive cells represents a simple biological infection of the germ rather than the true inheritance of an acquired bodily condition.

"They can therefore never have been caused to vary through use in the course of the phyletic development of species and genera, and the Lamarckian principle can have no part in their transformations."

In analyzing the complex chitinous wing-veins of insects, the author emphasizes that dead, non-renewing structures cannot be modified by active physical exercise during an individual's lifetime.

"We venture to maintain that everything in the world of organisms that has permanence and significance depends upon adaptation, and has arisen through a sifting of the variations which presented themselves, that is, through selection."

Near the conclusion of the text, Weismann summarizes his central philosophical position, asserting that natural selection governs all persistent biological structures from the microscopic level to whole animal societies.

What It's Really About

At its core, The Evolution Theory is a sustained defense of selectionism against rival mechanisms of evolutionary change. Weismann sets out to answer a single fundamental question: how do complex biological adaptations originate, persist, and eventually disappear without relying on the inheritance of acquired traits? To resolve this, he takes the argument beneath the visible surface of plants and animals into the microscopic realm of the germ-plasm.

The book argues that the physical body (the soma) and the reproductive material (the germ-plasm) are fundamentally distinct. Changes wrought upon the body by habit, injury, or climate cannot modify the genetic instructions carried in the germ-cells. Instead, evolution is propelled by a multi-layered process of selection operating at every scale of existence. By demonstrating that variation stems from internal germinal combinations rather than external physical habit, Weismann reframes the origin of species as a continuous, elegant sifting of microscopic units.

Why Read It Today

The Evolution Theory will deeply satisfy historians of science, evolutionary biologists, and readers fascinated by the intellectual foundations of modern genetics. Reading Weismann feels like stepping into a vast, Victorian natural history museum alongside an exceptionally articulate curator. His prose is marked by methodical clarity, patience, and a genuine awe for the intricate machinery of the natural world. Rather than presenting abstract formulas, he grounds every theoretical claim in vivid physical examples: the regrowing lens of a newt's eye, the self-mutilating leg of a crab, or the intricate brush-and-comb structures on the leg of a honeybee.

The work does present distinct period challenges that require patience from a modern audience. As a late 19th-century text translated from German, the prose is dense, formal, and organized into long, multi-page theoretical lectures. Readers must also navigate outdated biological terminology, such as Weismann's hypothetical framework of "ids," "biophors," and "determinants," which preceded the modern discovery of DNA and molecular genetics. Furthermore, the text reflects period attitudes toward human racial hierarchies and historical development. Yet, despite these historical hurdles, what stays with the reader is Weismann's extraordinary foresight. His insistence on the continuity of the germ-line predicted the core tenets of 20th-century genetics, making this book a thrilling monument to the power of pure scientific deduction.