"I would rather you worked the mechanism out for yourself. So I ask: If you had three cows, but only enough fodder to keep two of them alive, what would you do?"
"I suppose I'd have to slaughter one of them."
"All right... which one would you slaughter?"
"I suppose I'd slaughter the one that gave the least milk."
"Would you?"
"Yes, that's logical, isn't it?"
"That is exactly what mankind had done for thousands of years. But we haven't finished with your two cows yet. Suppose you wanted one of them to calve. Which one would you choose?"
"The one that was the best milker. Then its calf would probably be a good milker too."
"You prefer good milkers to bad, then. Now there's one more question. If you were a hunter and you had two gundogs, but had to give up one of them, which one would you keep?"
"The one that's best at finding the kind of game I shoot, obviously."
"Quite so, you would favor the better gundog. That's exactly how people have bred domestic animals for more than ten thousand years, Sophie. Hens did not always lay five eggs a week, sheep did not always yield as much wool, and horses were not always as strong and swift as they are now. Breeders have made an artificial selection. The same applies to the vegetable kingdom. You don't plant bad potatoes if there are good seed potatoes available, and you don't waste time cutting wheat that yields no grain. Darwin pointed out that no cows, no stalks of wheat, no dogs, and no finches are completely alike. Nature produces an enormous breadth of variation. Even within the same species, no two individuals are exactly alike. You probably experienced that for yourself when you drank the blue liquid."
"I'll say."
"So now Darwin had to ask himself: could a similar mechanism be at work in nature too? Is it possible that nature makes a 'natural selection' as to which individuals are to survive? And could such a selection over a very long period of time create new species of flora and fauna?"
"I would guess the answer is yes."
"Darwin could still not quite imagine how such a natural selection could take place. But in October 1838, exactly two years after his return on the Beagle, he chanced to come across a little book by the specialist in population studies, Thomas Malthus. The book was called An Essay on the Principle of Population. Malthus got the idea for this essay from Benjamin Franklin, the American who in-vented the lightning conductor among other things. Franklin had made the point that if there were no limiting factors in nature, one single species of plant or animal would spread over the entire globe. But because there are many species, they keep each other in balance."
"I can see that."
"Malthus developed this idea and applied it to the world's population. He believed that mankind's ability to procreate is so great that there are always more children born than can survive. Since the production of food can never keep pace with the increase in population, he believed that huge numbers were destined to succumb in the struggle for existence. Those who survived to grow up-- and perpetuate the race--would therefore be those who came out best in the struggle for survival."
"That sounds logical."
"But this was actually the universal mechanism that Darwin had been searching for. Here was the explanation of how evolution happens. It was due to natural selection in the struggle for life, in which those that were best adapted to their surroundings would survive and perpetuate the race. This was the second theory which he proposed in The Origin of Species. He wrote: The elephant is reck-oned the slowest breeder of all known animals,' but if it had six young and survived to a hundred, 'after a period of from 740 to 750 years there would be nearly nineteen million elephants alive, descended from the first pair.' "
"Not to mention all the thousands of cods' eggs from a single cod."
"Darwin further proposed that the struggle for survival is frequently hardest among species that resemble each other the most. They have to fight for the same food. There, the slightest advantage--that is to say, the infinitesimal variation--truly comes into its own. The more bitter the struggle for survival, the quicker will be the evolution of new species, so that only the very best adapted will survive and the others will die out."
"The less food there is and the bigger the brood, the quicker evolution happens?"