Genetics: Teratology, the science of monsters, has inspired a specialised study of genetics that makes this masterly book as fascinatingly horrible as an old- fashioned circus freak show.
Step up and see the bearded lady, the single skeleton with two heads, the baby with one eye-socket and no nose! The slightest garbling of DNA instructions to a fertilised human egg can produce grotesque distortions terrible to contemplate, yet anomalously consoling. There but for the grace of God . . .
Dr Armand Marie Leroi, reader in evolutionary developmental biology at Imperial College in the UK, has written a lucidly scientific, comfortingly humane study of morbid mutation which implies that any person born in a form recognised as normal is very lucky.
What he calls "varieties and errors of the human body" are rare. It is amazing how microscopic originators of life are able to transmit such complicated genetic messages from generation to generation with so few mistakes. But accidents do occur, and Leroi explains some of the worst of them.
Every foetus, he shows, is vulnerable - more or less - to maladjustments.
"The average newly conceived human," he writes, "bears 300 mutations that impair its health in some fashion. No one completely escapes this mutational storm. But - this is necessarily true - we are not all equally subject to its force. Some of us, by chance, are born with an unusually large number of mildly deleterious mutations, while others are born with rather few. And some of us, by chance, are born with just one mutation of devastating effect where most of us are not."
Paraphrasing an Orwellian aphorism, he writes: "We are all mutants. But some of us are more mutant than others."
Francis Bacon, far ahead of his time in the 17th century, said natural history could be divided into the study of normal nature, aberrant nature and nature manipulated by man, and that analysis of monsters and other prodigies might help to achieve elimination of causes of abnormality.
"In a sense," Leroi promises, "this book is an interim report on Bacon's project."
Now geneticists know the human genome contains about 30,000 genes - in Leroi's words, "the instruction manual for making a human". (Even the ordinary little fly can be put together only under the direction of about 13,000 of them.) It is hoped that genetic engineering may neutralise genes that cause various defects and susceptibility to diseases, and thus may extend healthy life far beyond the present usual span, at the same time preserving enough of the brain's 100 billion neurons to save the senescent from dementia. Old age isn't much good if you can't find your spectacles or remember why you moved from one room to another.
Like a mathematician praising the nicety of a perfect theorem, Leroi is able to perceive beauty in state-of-the-art genetics. He writes with such intellectual fervour, precision and clarity that he can make even statistics exciting. Readers whose biology at school progressed little past pressing wild flowers, collecting butterflies and dissecting frogs may find themselves helped by Leroi to understand something of genetic mysteries that were, until recently, considered inscrutable.
In the meantime, in between passages of esoteric scientific exposition, there are all those enchanting horrors to make you grateful if you have been personally spared.
Leroi describes some of the bizarre exhibits in the Willem Vrolik teratological collection in Amsterdam. Acquired by the city in 1890 and now open to the public, Vrolik's 5,103 specimens allow visitors to contemplate what are termed the limits of human form.
"The only visual referent that suggests itself," Leroi comments, "are the demonic creatures that caper across the canvases of Hieronymus Bosch . . . Of course, there is a difference in meaning. Where Bosch's grotesques serve to warn errant humanity of the fate that awaits it in the afterlife, Vrolik's are presented with clinical detachment, cleansed of moral value. It is a Last Judgement for the scientific age."
The good Dr Leroi, encouragingly, further suggests that geneticists may offer redemption.