The saints of science are also its targets. Accompanying the veneration of Isaac Newton, especially in the 19th century, was an unceasing search for holes in the solid structure of his mechanics. Recognition of Albert Einstein’s achievement became worldwide soon after 1905. But to this day, hopeful theorists hunt for gaps and failures in his work, early and late.

The strength of science lies in relentless probing for weaknesses in its commanding theoretical structures. Successful naysayers can themselves become candidates for glory. The finder of a breakdown in General Relativity would become as famous as Einstein.

Darwin, too, has never lacked volunteers to fix his theory. Mostly, this is honest naysaying, repair work done with respect. Usually, but not always, it has been based upon new knowledge. The “Modern Synthesis” of the 1940s, for example, was a major fix that provided the critical understanding of heredity and populations missing from Darwin’s original theory. This is in sharp contrast to hostile and usually incompetent naysaying from outside science, as, for example, in the perennial farce of creationist attacks on evolutionary biology (or, as they insist on calling it, “Darwinism” – as though nothing has happened since 1859).

“The Plausibility of Life” (Yale University Press, 336 pages, $30), a new book by two distinguished developmental biologists, Marc Kirschner of Harvard and John Gerhart of Berkeley, comes very much from inside evolutionary and developmental biology. Their subtitle is “Resolving Darwin’s Dilemma”; it is another of those kindly proffered repairs. But the response of many biologists who have not yet read the book might be: “What dilemma?

Have they found some heretofore hidden flaw in evolutionary biology? Is there a “dilemma” about this, the center of our understanding of life’s history on Earth? To such questions, the answer is an unhesitating “no.” Darwin rests easy in his scientific sainthood. The core theory – that life’s diversity is due to descent with modification, and that a key mechanism of that diversification is natural selection – has been rock solid for 150 years.

What these authors present is rather a “completion” of evolutionary theory, an extended explanation of how novelty in body architecture – the actual design – of plants and animals emerges. They take as their subject the fundamentals of biological variation, the ineluctable differences arising between individuals of the same species, found in all populations of all living things. That they do so in a carefully argued, skillfully written work, accessible to the general reader, is part of their accomplishment.

There have been just a few deep and cataclysmic novelties in the design of life on Earth, and we know now that all of them happened long ago, probably at least a billion years ago. But abundant novelty has, demonstrably and unquestionably, arisen in the course of life’s history on this planet.

First was the transition from a (relatively) simple unicellular way of life to a more complex and efficient (but still unicellular) form: the “eukaryotic” cell, with nucleus, chromosomes, and mitochondria. At some point, eukaryotic cells discovered the advantages of multicellularity, then differentiation and division of labor among the conjoined and cooperating cells, making possible much more complex body structures.

Later, reproductive processes developed by which such life forms produced offspring via a cycle that builds a new body (development) from just one reproductive cell, or from the fused cells and genes of two individuals – the beginning of sex. From these process there arose, via continuous variation and the selection of reproductively successful variants, the whole of Earth-life’s diversity.

The authors assert that we can see now how that novelty, that vast range of variation, comes about. Key to the authors’ argument is better knowledge of gene expression in development, and the critical contributions to it from physiological (internal) and environmental (external) signals in the course of body-building. During the last 30 years, basic biology has been transformed by fusion of the fields of molecular genetics, cell biology, and embryology.

Before this, the authors contend, we were not on truly solid ground when we extrapolated from elementary Mendelian and population genetics to the necessary range and rate of observed variation. It was not that we were wrong: We knew the genetic basics (including mutation frequencies), the inevitability of environmental change and pressure on the biota, and the reality of natural selection. We observe the latter every day in, for example, the induction of drug resistance in microbial pathogens that cause disease. And we have observed species formation under selection, in nature and in the laboratory.

What was missing, the authors argue, was a sound theory of how the actual structural novelties – explicit and significant variations in body design – came into existence. For it is on existing microbes, animals, and plants – and therefore on their immediate offspring – that the mechanisms of evolution must work. Natural selection must work on “phenotypes,” the functioning bodies.

In the standard theory, we have the wellspring of heritable changes: mutation, recombination, genetic drift – all the inevitable and essentially random changes in DNA and the genome.Then there is the creative action of natural selection, which is distinctly non-random, and which discards changes that reduce reproductive success but preserves any variations that boost it. If the theory of this book is sustained, however, we have unearthed the mechanisms by which those raw genetic changes produce, at high frequency, not just mistakes but meaningful and functional variations in body configuration.

All these processes are the results of gene action, and they were already complex, highly redundant (with many spare and alternate parts), and flexible very early in the history of life. They got to be that way by evolution of the component basic chemistry – a kind of molecular evolution we can mimic in the laboratory. We know many of those processes now: They are referred to as the genetic (or developmental) “toolkit.”

Salient features of this toolkit are its great redundancy and flexibility, which means that many mutations – the inevitable accidents of quotidian life and reproduction – can be managed, even if the operation of the toolkit then causes some structures or component reactions to change from the norm; in other words, to vary within limits.

Knowing how to look,we can now see this happening. What we see, through the lens of molecular developmental biology, is the conservation and constraint of the core chemistry of life in responding to internal changes (accidental or otherwise), and to the vagaries of environment, thus continually producing phenotypic variation. That variation is seen to be of kinds, and in quantities, that offer to natural selection an inexhaustible supply of options for new versions of living forms. The result: New and viable body designs are available for selection to work upon. The theory is testable and it is being tested (by introducing known changes in the “toolkit” genes, for example).

A fully plausible mechanics of novelty-production would indeed “complete” evolutionary theory. Incidentally, it would also make the “Darwinian” evolution of life on Earth, in its incalculable diversity, so highly “plausible” as really to approach certainty – at least as good a bet as any in all of physics.The accomplishment of this book is to make not evolution – which is a fact – but diverse life itself, in a universe that began as hydrogen and helium, plausible.

Mr. Gross is university professor of life sciences emeritus at the University of Virginia, and co-author with Barbara Forrest of “Creationism’s Trojan Horse” (Oxford University Press). His review of K-12 science standards of 50 states has just been published by the Thomas B. Fordham Institute.