The Variation of Animals and Plants Under Domestication, Volume II (of 2)

In Short

This monumental natural history text synthesizes vast empirical data to uncover the fundamental laws governing how domestic plants and animals change over time. Charles Darwin systematically details the complex mechanics of heredity, examining how traits are inherited, lost, or unexpectedly revived across generations. The work investigates the subtle causes of variation, including the effects of environmental changes, climate, and the use or disuse of physical organs. Darwin evaluates agricultural practices, comparing unconscious and deliberate selection by breeders to show how distinct strains and new breeds are established or modified. Crucially, he addresses the paradox of self-sterility alongside cross-breeding vigor, detailing how close interbreeding impacts health and fertility. To tie these disparate biological phenomena into a unified framework, Darwin introduces his speculative theoretical model of pangenesis, proposing that microscopic cellular particles travel throughout organisms to transmit hereditary traits to future offspring.

The Story

The investigation opens with a rigorous examination of the laws governing inheritance. Physical traits do not descend by mere chance; rather, they adhere to subtle, pervasive principles of transmission. Peculiarities of every description—from asymmetrical bone structures and polydactylism in humans and salamanders to inherited eye diseases and structural defects in horses—are passed faithfully down family lineages. Yet inheritance remains fluctuating and complex. Traits frequently vanish in one generation only to reappear in another, a phenomenon known as reversion or atavism. Reversion displays its power when domestic animals, such as pigeons or fowls, are crossed, suddenly throwing off ancient ancestral traits like leg-stripes on horses or primitive plumage colors. Reversion also manifests in plants through bud-propagation, where a single branch or segment of fruit abruptly reverts to an older form, as seen when a nectarine tree produces a branch bearing peaches. Monstrosities and extra digits similarly point back to embryonic conditions or remote, multidigitate ancestral forms.

The inquiry turns next to how traits become fixed, how one parent exhibits prepotency over another in transmitting features, and how traits obey strict rules regarding sex and age. Certain peculiarities appear in offspring at the exact life stage in which they first emerged in the progenitor, demonstrated by silkworm caterpillars changing mark or comb structures developing in poultry. Darwin then evaluates the practice of crossing distinct breeds. Crossing serves as a double-edged sword: it can blend forms to eliminate differences, or, when paired with rigorous selection, it can produce entirely new races with desired traits. Historical examples reveal how breeders crossed greyhounds with bulldogs to acquire tenacity, or combined distinct sheep strains like Cotswolds and Leicesters to create robust new breeds like the Oxfordshire Downs.

Central to this study is the critical necessity of cross-fertilization versus the dangers of close interbreeding. Evidence drawn from agriculturalists, plant experimenters, and animal breeders proves that long-continued close interbreeding leads to reduced vigor, structural weakness, and eventual sterility. Conversely, an occasional cross between distinct strains or individuals infuses fresh vitality into offspring. Nature actively prevents continuous self-fertilization; hermaphrodite plants and animals possess elaborate structural adaptations to favor cross-pollination. In fascinating cases of self-impotence, plants such as certain Oncidium orchids produce healthy pollen and fertile ovules, yet are utterly incapable of fertilizing themselves, setting seed only when crossed with a distinct plant or species. Changing external conditions also affect reproduction in unexpected ways: wild animals in captivity, such as big cats, bears, and parrots, frequently become sterile despite perfect physical health, while domesticated species lose this susceptibility and become fully fertile together.

The text then shifts to analyzing how human intervention drives structural modification through methodical and unconscious selection. While methodical selection involves intentional breeding for specific ends, unconscious selection occurs naturally as humans preserve superior or more useful individuals without intending to alter the breed. Over centuries, this practice dramatically shifts the characters of pigeons, dogs, sheep, and crops. However, human selection is constantly checked or aided by natural selection, as subtle variations—such as the delayed leafing of a gooseberry or the shape of a flower petal—determine survival against cold, wind, or disease.

Exploring the ultimate causes and laws of variation, the work demonstrates that changed conditions of life directly or indirectly induce variability. When organisms experience altered environments, soils, or climates, their physical organizations are disturbed. This is seen in dahlias changing form with the seasons, plants altering foliage under different soils, or European crops adjusting to the heat of India through acclimatisation. Variability also follows established structural laws, including the effects of increased use or disuse of organs, correlated variations where modifying one part alters another, and the law of equable variability across allied species.

To synthesize these wide-ranging observations—from inherited diseases and bud-variations to latent traits and regeneration—Darwin presents his overarching hypothesis of Pangenesis. He posits that every unit or cell of an organism continuously throws off minute, self-multiplying granules called gemmules. These gemmules circulate freely throughout the system, aggregate in the reproductive organs or buds, and pass to the next generation. Gemmules can remain dormant for generations before developing, explaining reversion and latent traits. When environmental conditions or physical use alter an organism's cells, the gemmules thrown off by those cells are modified accordingly, transmitting changed structures to descendants. The treatise concludes by emphasizing that while organisms face physical limits to variation, the capacity for general modification under new conditions remains practically unlimited.

How It Unfolds

The laws of descent Inheritance follows definitive rules rather than random chance, reliably transmitting physical traits, peculiar habits, and hereditary diseases down generations. Even abnormal structural deviations like polydactylism recur across family lines, revealing deep underlying patterns of physical transmission.

Ancient ancestral returns Traits lost for generations unexpectedly reappear through reversion or atavism in pure breeds, hybrids, and bud variations. Cross-breeding frequently triggers these ancestral throwbacks, causing domestic stock to display primitive colors, markings, or structural forms long forgotten.

Timing and sexual limits Hereditary traits frequently observe strict timing, appearing in offspring at the exact stage of life they emerged in the parent. Certain characteristics remain constrained by sex or demonstrate prepotency, where one parent's traits consistently overpower those of the other.

Creating races by crossing Selective intercrossing alters old breeds and generates distinct new ones, provided it is paired with systematic selection. Historic breeding efforts with greyhounds, sheep, and cattle prove that crossing blends desired qualities like courage or hardiness into establishing unified strains.

The hazard of close breeding Continuous interbreeding between close relatives causes loss of constitutional vigor, diminished size, and sterility across animal and plant groups. Breeders must periodically introduce an occasional cross with another strain to maintain the health and fertility of their stock.

Adaptations for cross-fertilization Living organisms possess complex physical mechanisms designed to avoid continuous self-fertilization and ensure cross-pollination. In remarkable instances of self-impotence, perfect hermaphrodite flowers refuse their own pollen yet reproduce readily when crossed with distinct individuals.

Captivity and reproductive failure Altering an organism's living conditions profoundly disrupts its reproductive system, causing many wild animals to become completely sterile in confinement. Conversely, long-continued domestication eliminates this sensitivity, making domestic varieties fully fertile with one another.

Selection by human hands Human preference shapes populations through both methodical breeding and gradual, unconscious selection. By continuously preserving the most useful or pleasing individuals and eliminating the least valued, agriculturalists radically transform domestic species over centuries.

Environmental triggers of change Exposing organisms to changed conditions of life, new climates, or varied soils directly induces variability. Physical structure responds to environmental shifts, causing plants and animals to adjust their growth, leafing periods, or hardiness through acclimatisation.

Functional use and correlation Variability is governed by structural laws, such as the inherited effects of organ use and disuse, alongside correlated growth. Modifying one organ frequently induces unexpected, linked modifications in completely different parts of the body.

The speculative cellular theory To explain all phenomena of heredity, reversion, and development, the hypothesis of Pangenesis proposes that bodily cells shed microscopic gemmules. These dormant or active particles circulate, aggregate in reproductive tissues, and transmit both ancestral and modified traits to offspring.

The expansive horizon of variation Organisms retain an almost unlimited capacity for general variation under changing conditions, despite physical constraints on specific structures. Continued selection will keep modifying domestic and natural species, ensuring ongoing adaptation and evolutionary change.

The People

The Domestic Breeder Represented by agriculturalists and fanciers such as Bakewell, Youatt, Sir J. Sebright, and Nathusius, these practical experimenters drive the modification of animals and plants. They seek to improve livestock for yield, speed, or appearance by selecting desired traits over generations. Their primary obstacle is the constant threat of reversion to wilder forms and the insidious decline in vigor caused by close interbreeding. Through rigorous selection and strategic crossing, they prove that human agency can systematically reshape biological forms.

The Botanical Experimenter Embodied by historical botanists and hybridizers like Gärtner, Kölreuter, Naudin, Vilmorin, and Scott, these figures investigate the reproductive secrets of plants. Their work focuses on understanding hybrid fertility, self-impotence, and variation through seeds and buds. They contend with the unpredictable behavior of plant traits, such as sudden reversions in foliage or complete self-sterility in hermaphrodite flowers. Their experimental crossings reveal the delicate mechanisms governing plant inheritance and sexual constitution.

The Philosophical Naturalist Reflected in the analytical voice of Darwin himself, alongside insights from peers like Wallace, Geoffroy St. Hilaire, and Pallas, this figure seeks to uncover the universal laws behind biological diversity. Driven by a desire to unify disparate facts of inheritance, variation, and monstrosity, the naturalist faces the challenge of explaining complex biological paradoxes. Through methodical synthesis of global breeding data, natural history observations, and cellular hypotheses like Pangenesis, the naturalist constructs a coherent theoretical foundation for how life transforms over time.

In Its Own Voice

"So, again, in certain extinct reptiles, namely, the Ichthyopterygia, 'the digits may be seven, eight, or nine in number, a significant mark,' says Professor Owen, 'of piscine affinity.'"

This observation highlights how supernumerary digits in higher mammals may reflect ancient ancestral structures retained from early vertebrate ancestors.

"Oncidium sphacelatum has effective pollen, for with it Mr. Scott fertilised two distinct species; its ovules are likewise capable of impregnation, for they were readily fertilised by the pollen of O. divaricatum; nevertheless, between one and two hundred flowers fertilised by their own pollen did not produce a single capsule..."

This striking case illustrates the phenomenon of self-impotence, where a hermaphrodite plant possesses fully functional reproductive organs yet completely fails to fertilize itself.

"I assume that cells, before their conversion into completely passive or 'formed material,' throw off minute granules or atoms, which circulate freely throughout the system, and when supplied with proper nutriment multiply by self-division..."

Here, Darwin formulates the core mechanism of his Pangenesis hypothesis, proposing microscopic cell-gemmules to account for inheritance, growth, and reversion.

What It's Really About

At its core, this volume investigates the hidden mechanics of biological inheritance and the origin of physical diversity. While the theoretical framework of natural selection explains how useful traits are preserved in the wild, this work tackles the deeper question of how variations arise in the first place and how they are transmitted across generations. It explores the delicate tension between stability and transformation in living organisms—how hereditary patterns maintain consistency over centuries while remaining extraordinarily responsive to external influences, human selection, and environmental shifts.

The text addresses the fundamental puzzle of why offspring resemble their parents, why ancient ancestral traits suddenly re-emerge after generations of absence, and why close interbreeding causes physical degeneration while cross-breeding restores constitutional vigor. By compiling extensive empirical data from animal husbandry, horticulture, and comparative anatomy, the argument demonstrates that variation is not a chaotic process, but one strictly governed by biological laws. Ultimately, the work seeks to provide a unified physical theory of inheritance through Pangenesis, attempting to bridge the gap between microscopic cellular processes and the macroscopic transformation of species.

Why Read It Today

This work offers an extraordinary window into the foundational era of modern biology, capturing a world-changing thinker in the process of wrestling with the core mystery of genetics before the discovery of DNA. For readers interested in the history of science, evolutionary theory, or natural history, it provides an invaluable look at the sheer breadth of empirical evidence Darwin assembled to support his ideas. The prose is clear, measured, and intellectually rigorous, combining detailed observational case studies with bold theoretical synthesis. Reading it feels like sitting alongside a master detective of the natural world who examines every detail—from the plumage of pigeons to the self-sterility of orchids—to decipher the laws of life.

While the prose remains accessible and engaging, modern readers should be prepared for its dense, encyclopedic length and highly specialized agricultural detail. Long catalogs of breeding pedigrees, horticultural variations, and historical livestock records demand patient reading. Furthermore, the work reflects its nineteenth-century context, incorporating period terminology, early physiological concepts, and speculative theories—such as Pangenesis and the inheritance of acquired traits—that modern genetics has since recontextualized or superseded. Nevertheless, the book remains profoundly rewarding. Its rigorous methodology, historical importance, and deep appreciation for the complex beauty of living variations make it an enduring masterpiece of scientific literature.