Darwin, and After Darwin, Volume 3 of 3: Post-Darwinian Questions: Isolation and Physiological Selection

In Short

This volume serves as a sophisticated post-Darwinian inquiry into the mechanisms of speciation. It argues that natural selection alone cannot account for divergent or "polytypic" evolution—the branching of one species into many—because free intercrossing would effectively "swamp" any emerging differences. Instead, the author proposes that physiological selection, or the development of cross-infertility, provides the necessary barrier to allow new types to stabilize. A seminal text for students of evolutionary history, it captures the intense scientific debates of the late nineteenth century regarding heredity and isolation.

The Story

The narrative arc begins by challenging the sufficiency of natural selection as a complete explanation for the origin of species. The author distinguishes between "monotypic" evolution—the gradual, linear transformation of a single species—and "polytypic" evolution, which involves the formation of multiple, simultaneous lines of descent. While acknowledging that natural selection is perfectly adequate for the former, the argument insists that it fails to explain the latter. Without some form of isolation to prevent the homogenizing effects of free intercrossing, the author contends that diverse adaptations cannot persist or accumulate in a population.

The core of the work is dedicated to "physiological selection," a theory suggesting that variations in the reproductive system can create a barrier between segments of a species. This internal, physiological isolation allows different groups to diverge into new species even while inhabiting the same geographical area. The author painstakingly contrasts this with geographical isolation, where physical barriers like oceans or mountains enforce separation. He notes that while naturalists like Moritz Wagner correctly identified the importance of geographical isolation, they often failed to see the broader implications of isolation as a necessary condition for all divergent evolution.

The book moves through a series of technical arguments, addressing potential objections from prominent contemporaries like Alfred Russel Wallace. A central conflict emerges regarding the fertility of hybrids; the author observes that the "swamping" effect of intercrossing is absent in domesticated breeds precisely because artificial, human-directed isolation keeps them separate. In nature, where no such artificial hand exists, the author argues that cross-infertility must be the primary agent of segregation.

The final chapters synthesize various lines of evidence, including the geographical distribution of species and the phenomenon of "prepotency"—the tendency of certain individuals to be more successful in reproduction. By the end, the author positions physiological selection not as a replacement for Darwinism, but as a critical, logical necessity required to fill a gap in the original theory. He concludes that the existence of closely allied species living in intimate association is strong, empirical proof that reproductive barriers play an active, formative role in the history of life. The book finishes with a call for more precise, experimental research to map these hidden barriers, leaving the reader with a clearer, more nuanced understanding of the biological architecture that maintains the world’s vast diversity.

How It Unfolds

The distinction of evolutionary paths The author sets the stage by defining the difference between monotypic and polytypic evolution. He asserts that while natural selection drives the improvement of a single line, it is inherently unable to cause divergence if intercrossing remains unchecked.

The necessity of isolation The argument shifts to the role of barriers, asserting that isolation—whether geographical or physiological—is the essential "concomitant" of new species formation. The author reviews the contributions and errors of previous thinkers like Wagner and Weismann.

The theory of physiological selection The author introduces his own concept, arguing that variations in reproductive compatibility create natural barriers. He uses these to explain how species can differentiate while occupying the same territory, avoiding the "swamping" of new traits.

Engagement with critics The latter half of the text features a robust defense against critics, particularly Alfred Russel Wallace. The author uses letters and past publications to clarify that he sees physiological selection as a fundamental pillar required to support, rather than dismantle, the Darwinian framework.

Synthesis of evidence The final sections pull together diverse observations from botany and zoology to solidify the argument. The author concludes that the evidence of cross-infertility among closely related species provides the strongest possible validation for his theory.

The People

The primary intellectual presence is the author, George John Romanes, who acts as a careful mediator and expander of the Darwinian tradition. He is driven by a desire for logical consistency, seeking to resolve the "difficulty" of why varieties do not always become species.

He is joined in spirit by J. T. Gulick, whose work on "cumulative segregation" and positive isolation provides the empirical backbone for the theory. Romanes treats Gulick as a primary collaborator in identifying the necessity of reproductive barriers.

Opposing these views is Alfred Russel Wallace, whom Romanes respects but frequently challenges. Wallace represents the "pure" Darwinian stance, often skeptical of factors that complicate the primacy of natural selection. Their debate forms the book’s most tense and intellectually stimulating thread, as they clash over whether reproductive infertility is a cause of speciation or merely a consequence of it.

Finally, the presence of Charles Darwin looms over the entire text. Romanes treats Darwin’s writings as the foundational, yet incomplete, map of the field. Romanes’ goal is not to prove Darwin wrong, but to "complete" the map by addressing the specific, lingering problems that Darwin himself identified but could not fully solve.

In Its Own Voice

"If from any cause a section of a species is prevented from intercrossing with the rest of its species, we might expect that new varieties should arise within that section, and that in time these varieties should pass into new species."

This sentence summarizes the author's fundamental premise that isolation is the prerequisite for all divergent evolution.

"And this is just what we do find."

Following the previous observation, the author offers this concise, confident claim that his theory aligns with the actual, observable patterns in nature.

"The bar of mutual sterility, whenever and wherever it occurs, must always render polytypic evolution possible."

This quote captures the core of the author's thesis, identifying the specific mechanism by which one species splits into many.

What It's Really About

At its heart, this is a treatise on the logic of diversity. The underlying question is not merely "how do species change?" but "how do they stay separate?" The author probes the tension between the fluid, mixing nature of interbreeding populations and the static, distinct reality of classified species. It is a work about the limits of utility; the author argues against the "a priori dogma" that every single characteristic of an animal must be useful, suggesting instead that some traits—like cross-infertility—are merely structural consequences of evolutionary divergence. Ultimately, the book is a study in consistency, attempting to construct a coherent, totalizing explanation for the branching patterns of life that simple natural selection cannot fully explain.

Why Read It Today

Readers interested in the history of science will find this volume deeply rewarding, as it provides a rare, "behind-the-scenes" look at how the Darwinian theory was refined in the decades immediately following the publication of The Origin of Species. The prose is intellectual and precise, reflecting the Victorian standard for rigorous, long-form scientific argument. It is not, however, a light read; the text is dense with the formal, often repetitive terminology of late nineteenth-century biology, and the author’s tendency to engage in exhaustive point-by-point rebuttals of his peers may feel exhaustive to a modern audience.

Nevertheless, those who appreciate a slow, deliberate excavation of ideas will find the effort worthwhile. There is a quiet beauty in watching the author "think through" the problems of his field, meticulously piecing together evidence to build a case for the importance of physiological barriers. If you are patient with the period-specific language and the intricate, sometimes pedantic, nature of the debates, you will walk away with a profound appreciation for the complexity of the evolutionary process. The book remains a testament to the fact that scientific "truth" is often a layered, collaborative, and long-contested endeavor.