A tone of reverence for life in its complexity, and variety, and persistent coherence; a tone of cool-headed compassion, and an energetic and exacting need to know.

The Listener, New Zealand

More things in heaven and earth

When I was a student I had what might have been the very best holiday job. I worked in the National Museum - the old museum on Buckle St in Wellington. One day the custodian gave me a spare set of keys to all the building's internal doors. I took to wandering in my spare time - this was when jobs had spare time - around the basements, browsing the collections. I particularly liked the vast wet collection. I spent quite a bit of time peering into jars at birds' eggs, tapeworms, seaweed, a shark's gonads. Many of the specimens were examples of typical things, but some were atypical. For instance, one sizable jar contained conjoined twin lambs, their wool stained orange by the medium in which they were immersed. Museums and private collectors have for centuries preserved specimens of monsters and mutants. Armand Marie Leroi's book Mutants: On the form, varieties & errors of the human body is, in part, a history of that collector's curiosity, a curiosity made up of fear, dread, prurience, pity, compassion and the plain need to know why it is that sometimes what normally happens fails to happen. To know why nature presents families with tragic and puzzling mistakes. It is Leroi's proposition that, by working from examples of the exceptional in embryonic development, we are led to discover how it is that embryos grow.

The book is, in part, a cultural history of how people in the past have thought about mutation. In its pages we meet and hear the histories of people like the conjoined twins Christina and Ritta Parodi; and the Orvitz family - particularly Elizabeth Orvitz, a survivor of Mengele's torturous Auschwitz experiments. We meet Alexina Barbin, who became Abel Barbin when it was decided that Abel's testicles counted for more, legally, than Alexina's vagina. We are introduced to a hairy family belonging to the royal court of Burma; to the dwarf Joseph Boruwlaski (the story of whose happy life is quite encouraging); and to poor Harry Eastlake, whose every injury formed bone instead of scar tissue.

These accounts usually begin in a short life story. Leroi describes the first appearance of the Parodi twins, or Boruw-laski, or Eastlake, the first exhibited by their poverty-stricken parents, the last coming to the attention of doctors when, at five, he developed a stiff hip after breaking his leg. Leroi traces these initial appearances in the public record, and talks about the attention accorded such individuals by the scientific community of their time. He discusses the ways in which the thinking of society about mutations has changed over the ages, offering thoughtful and admiring accounts of pre-scientific thinking about nature - the imaginative and metaphorically intuitive, if incorrect, ideas of the likes of Aristotle and Ptolemy. He outlines our ancestors' attempts to come to grips with the prodigious in nature. He tells how mutations were taken by communities as signs and omens, or punishment for moral failing, or as evidence of "maternal impressions" - unfortunate encounters of pregnant women with seals or goats or wall-eyed clergymen. In each case Leroi explains what happens in that mysterious and hidden place, the womb, within the first few weeks of conception. Mutants is a study of what happens in the womb, what normally happens, and what sometimes goes wrong. The book explores what genetic mutations reveal about the growth of different parts of the body - limbs, skeleton, genitals, skin. "We are all mutants," says Leroi. Each of us is the sum of our DNA. On average, we each have about 100 genetic mutations we did not inherit from our parents. "Of these hundred mutations, about four will alter the meaning of genes by changing the amino acid sequences of proteins. And, of these four content-altering mutations, about three will be harmful." Leroi explains, rather reassuringly, that these "loss-of-function" mutations might even confer compensatory advantages. For instance, because their MC1R is inactive, red-haired people are vulnerable to sunburn - but then again we can all enjoy their beautiful colouring.

Leroi is an accomplished explainer of scientific facts and methods. Having read Mutants I now feel that I understand why our hearts are generally in the right place - the left side of the body - and how it is that I have separate fingers on my hands, even though the bones in my palm, of which the fingerbones are a progression, are encased in skin and muscle. Leroi patiently explains the mysteries of modern genetic science, revealing the influence in our lives of retinoic acid and free radicals, of sonic hedgehogs and signalling cilia on the embryonic "organiser" - and of many other things. The way he tells it, it all makes sense. A humane exploration of the science of human development, Mutants doesn't flinch from acknowledging the extent to which our knowledge of genetics comes from experiments on animals - newts, mice and nematode worms. Leroi is honest in talking about this, and in admitting what else he would like to be able to know, if scientists were allowed to design experiments in order to find out. He takes the bull by the horns in talking about the barriers to scientific exploration - baldly, "soi-disant ethicists, dialectical biologists and bishops", people he clearly doesn't have much time for.

I understand that there is a television series of Mutants. I'm curious to see it, given that television will probably show us Willem Vrolik's anatomical collection in the University of Amsterdam, and the distorted mice, and the lopsided newts that the Leroi only tells us about in his book. I am curious to see whether the series manages the book's delicacy, sensitivity and reverence about its difficult subject. Leroi writes that, "To seek out, look at, much less speak about deformity brings us uncomfortably close to naive, gaping wonder (or, to put it less charitably, prurience), callous derision, or at best a taste for thoughtless acquisition. It suggests the menageries of princes, the circuses of P T Barnum Š or simply the basements of museums in which exhibits designed for our forebears' apparently coarser sensibilities now languish."

So - what was I thinking when I went about peering in the jars of Buckle St's wet collection? I was probably gaping, but I was also wondering, "What is this? And why was it preserved?" I thought about the interests of the collectors. I tried to work out which specimens represented an average, and which were prodigious. I was taken by the contents of the collection - the bottled plants, and creatures, and bits of creatures. I was taken by the room, light magnified in glass curves and golden liquid, and shadows of matter in the light: of leached, waxy, organic and often unrecognisable things. I was taken by the room as a kind of art direction and, equally, by its reason for being. I was taken by the museum, the fact of the existence of museums in the world, and their devotion to the collection and preservation of evidence (as well as treasure). Because - it seemed to me - natural history collections in museums were evidence of human wondering still at work, the act of wondering having somehow survived in the objects of study.

Musing on the history of investigations into human development, Leroi often acknowledges with admiration the work of scientific and pre-scientific collectors who anatomised, for instance, the poor infant known as the monster of Ravenna, or boiled clean the skeleton (or skeletons?) of the Parodi twins. He is full of praise for Dutch collector Vrolik's lithographs of a "beauty and veracity that have never been surpassed". In such sympathy with his scientific predecessors he sets the tone of his book. A tone of reverence for life in its complexity, and variety, and persistent coherence; a tone of cool-headed compassion, and an energetic and exacting need to know.

Elizabeth Knox