The biologist and geneticist J.B.S. Haldane once was asked what inference one might draw about the nature of God, based on a study of His works. Haldane replied: "An inordinate fondness for beetles."

It's easy to see what Haldane was getting at. All beetles are insects, while not all insects are beetles. Even so, although the exact number of species of beetles in the world is still unknown, it is probably around a million. And these are, remember, distinct forms, each one different enough from all other kinds of beetle that they cannot inter-breed and produce fertile offspring. It is not difficult to distinguish, say, a ladybug from a glowworm beetle or a huge antlered stag beetle, but many species differ outwardly in such tiny ways that only a specialist can tell the difference.

Now imagine, if you can, a beetle specialist who has never in her whole life seen a dog. If you take her to a dog show, she will easily distinguish more than a hundred different kinds, ranging from the hairless Chihuahua to the thickly furred Samoyed, and from the 250-pound English Mastiff to the two-pound Toy Poodle. She would probably argue that the diversity of appearance implies at least scores of different species. And she would be astonished, as we should be astonished, by the fact that all these different sizes and shapes are members of just one species.

Setting aside the practical physical difficulties arising from their very different sizes, if you were to mate any pair of breeds of dog, the mongrel puppies would be healthy and fertile. The reason for the diversity of dogs is simple. We humans have been breeding them selectively for a long time. No one knows how long, just as no one knows how the domestication of dogs from wolves began. Certainly, Egyptian monuments display several different types of dog, so the process of "unnatural selection," as opposed to natural selection, was already well developed. Dogs may have been human companions for as long as 50,000 years, though most archaeologists would probably place it as more like 20,000 years.

This may sound like a long time, but not when we are talking of such radical changes of form. Compare dogs with cats, which have been domesticated for at least 5,000 years. You can breed a cat with more or less hair, or with different patterns of fur, or somewhat larger or smaller. But that is about it. A cat is a cat is a cat, easily recognized and accepted as a single species. The same is true of a horse. Bigger or smaller, shaggier or smoother, and that is about the limit of the range of equine variation. The dog, among domesticated animals, seems uniquely malleable. It can be bred to a huge variety of shapes and sizes, long-muzzled or flat-faced, wire-haired or smooth-haired, big or small ears that stick up or flop down. The dog can also, with no difficulty, be "bred back" to the original form. If you perform random mating of dog breeds for four or five generations, you arrive at something very like a wolf.

This suggests a series of questions that at the moment we cannot answer. First, what is it that decides the plasticity of form of different plants and animals? Is it some basic difference of genetic makeup, which confers stability of structure on some organisms, while permitting great variability to others? If so, what are the evolutionary advantages and disadvantages of both?

It is easy to see how the ability of an animal to "morph" to a different shape or size in a relatively short time span can be useful. The dog, highly variable thanks to human actions, exists comfortably as a work animal and a companion to people from pole to pole. The more rigid feline form has a correspondingly more limited geographic range. As for the idea of cats as controllable work animals, the expression "like herding cats" about covers it.

Natural environmental changes provide a driving force for natural selection, just as humans have provided for dogs a driving force for unnatural (or artificial) selection. Charles Darwin, in developing the theory of evolution, was guided in his thinking by the planned breeding of pigeons, another animal which can adopt an extraordinary variety of shapes (though not of sizes). It would seem that a capacity for rapid change ought to be such an advantage that all organisms would have developed it. Yet the feline forms do very nicely. Clearly, there must be compensating advantages to a resistance to changes of form.

Dogs themselves suggest one possible such advantage to resisting form changes. After a few thousand years, dogs of different breeds can still mate in principle and produce offspring, but in practice they have become too physically different to do so. The genetic material of organisms that do not mate with each other still continues to suffer mutations, and if this goes on long enough the physical ability to mate becomes academic, because the animals have become different species. Thus the capacity for rapid change acts as a mechanism for speciation - the development of new species. This is certainly not a good thing if the total population of a species is in danger of dipping low enough for the adverse effects of inbreeding to become a factor.

Cats and dogs illustrate a general principle that we often find in Nature, where two factors are at work in opposite directions. When living conditions become worse, it's a good thing to be able to change fast, so as to make the best use of a bad situation. Be a dog. On the other hand, if conditions are so bad that populations dip toward the danger level, then it's desirable for every member of a species to be able to breed with every other. Be a cat.

I have one final question. Where do humans sit in terms of stability of form? Certainly, we are all one species. The bush people of the Kalahari may have little in common with the Inuit when it comes to lifestyle, but they could interbreed if they chose to do so.

I doubt if we will ever have an answer to my question, because human beings have adopted and are now totally committed to a different approach to the problem of varying physical circumstances. It is simple. If the environment changes, don't try to change yourselves to fit it. Change the environment.

Whether this is an effective long-term solution remains to be seen.

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