The genetic and the operative evidence relating to secondary sexual characters — Key Ideas to Explore

Castration and Feather Type in Sebrights

Morgan begins Part I by recounting castration experiments on Sebright males. He notes that a castrated Sebright did not become cock-feathered, contrary to what might be expected if the testes suppress male plumage. This negative result is presented without fanfare, yet it undermines a simple hormonal explanation. He also describes transitional feathers—intermediate between hen and cock types—that appear in some birds, suggesting a graded rather than binary control. The section includes a cross between Sebright and Black-Breasted Game bantams, producing F₁ and F₂ generations that reveal how hen-feathering and color are inherited. Morgan tracks color factors methodically, using back-crosses to estimate the number of genes involved. The reader sees a geneticist at work, combining breeding data with surgical intervention to dissect a trait that blurs the line between sex-limited and sex-linked inheritance.

Courtship Experiments in Drosophila

In Part II, Morgan shifts to insects, describing Alfred Sturtevant's experiments with fruit flies to test Darwin's theory of female choice. The key passage reports that when a male's wings were clipped, he still mated, but normal males achieved copulation more quickly. In competition, the clipped male mated 53 times versus 72 for the normal male—a modest difference. Morgan interprets this as showing that wing display shortens the time to mating but does not give the displaying male an exclusive advantage; the female, once aroused, mates with either male. He concludes that an aesthetic sense in the female need not be postulated. The section also includes a striking observation: a male Drosophila, after copulation, mounts the female's back, but mounting never occurs before actual copulation—a reversal of the sequence in many other flies. These details ground the theoretical discussion in concrete behavior.

The Genetic and Operative Evidence: A Synthesis

Part III opens with Morgan's summary of the evidence from mammals, birds, amphibians, crustaceans, and insects. He argues that secondary sexual characters arise from diverse physiological processes, not a single mechanism. Some are sex-linked, some sex-limited, some depend on the gonad (ovary or testis), and others are direct products of the genetic constitution. He warns that theories based on one set of data are often contradicted by another. The section is organized taxonomically, with subsections for each vertebrate and invertebrate group. Morgan's tone is cautious and integrative; he does not claim to have resolved all contradictions but insists that any adequate theory must account for the full range of cases. The reader gains a sense of the field's complexity in 1919, with genetics and endocrinology still in their early stages.

Darwin's Theory Under Scrutiny

Throughout the monograph, Morgan engages critically with Darwin's sexual selection. In Part II, he reviews alternative theories—including those based on display as a stimulus to the female rather than a basis for choice. The Drosophila experiments are presented as a direct test: if female choice were operating, clipped males should fare much worse than they did. Morgan also discusses the display of male birds, noting that in some species the male's ornamentation may serve to synchronize reproduction or to intimidate rivals. He does not dismiss Darwin entirely but reframes the problem in physiological and genetic terms. The closing pages of Part IV summarize the conclusions: secondary sexual characters are heterogeneous, and future work must distinguish between genetic, endocrine, and behavioral factors. The bibliography and plates provide a roadmap for further study.

Morgan's monograph rewards a reader who attends to the interplay of breeding data, surgical results, and behavioral observation. The excerpts show a scientist testing hypotheses with whatever tools were available—castration, cross-fostering, wing-clipping—and refusing to overgeneralize. Readers new to early genetics will find the poultry crosses instructive, while those interested in animal behavior will appreciate the Drosophila courtship trials. The work stands as a document of a transitional moment in biology, when the mechanisms of sex determination and differentiation were being mapped onto chromosomes and hormones.

Morgan’s hens, feathers and hormones, always made me think of a quieter sort of inquiry—how the body’s hidden signals steer its visible self. That same patience rides through Physiological Researches on Life and Death — Inside the Classic, where life’s mechanics are weighed with a steady hand. They share a fondness for the unfinished story, the pulse beneath the page.