These perhaps obscure Old Testament words carry a contemporary resonance. Public debate about genetically modified food rages. The spectre of 'Frankenstein foods' stalks the popular imagination, the latest episode being fears about how far pollen from trials of GM crops can travel and what it might affect.
So did the authors of the Pentateuch know something our generation has forgotten or overlooked? I'm not sure. Leviticus 19:19 is essentially a piece of common sense. It did not necessarily rule out breeding of mules (a cross between a horse and a donkey) since later on the Hebrew kings would ride a mule on ceremonial occasions. For ordinary people there simply were more economic benefits from breeding donkeys or horses than mules because mules cannot reproduce. Likewise, mixing varieties of seed wheat or corn invites 'throwbacks', weeds and even crop failure.
Selective breeding to produce new strains of animals and plants has been commonplace for millennia. There are even hints of it in the Book of Genesis. As a farmer's son I hail the benefits of sustained experiment and research: higher yielding crops, fine wools, wonderful wines, an ever-bigger array of nutritious cereals and fruits, better meats.
Genetic modification of food represents a further step. There are no proven health hazards yet, and it could bring benefits - it could improve quality and introducing drought resistant crops to barren parts of the world could ensure global food security. But there is no reason to hurry GM foods into general use. The world's food security problems are as much due to distribution failure as shortages. We can afford to wait as long as is necessary to ensure that these foods are completely safe.
Meanwhile we could benefit from a wider-ranging debate about the underlying crisis in world farming. A handful of companies, among them some of the strongest advocates of GM foods, hold virtual monopolies and make mega-profits. In contrast small family farms the world over struggle to survive. Millions of small farmers caught in a cycle of falling prices try to compen-sate by producing more and more, for which the cash returns diminish all the time. The result: environmental devastation, increasing debt and bankruptcies, and inevitable mountains of unwanted food.
