Darwin and Wallace came to the same breakthrough idea: natural selection. What natural selection did was provide an engine for change, an external force to drive species to change, a force analogous to Newton's gravity. In a way, the two did little more than take a natural step that their predecessor, Charles Lyell, had avoided. Lyell's uniformatarian-ism, his conclusions that the earth changed slowly, gave them the force. Darwin and Wallace simply came to the inevitable conclusion: If the geologic environment changed, so too must the living things in it.
Natural selection means that organisms change, generation by generation, as dictated by the environments they live in. The change comes about through the survival and reproduction of the best adapted (the most fit) and the failure to reproduce of those not so well endowed. If the organisms are already fit and adapted, there may be no change. But if they are not, there will be change or, if change does not occur, there will be extinction. These slow and imperceptible changes will mean the organisms take on new forms—forms that ultimately mean the development of new and separate species.
Like Newton, Wallace and Darwin took an external approach. Species did not change from within, but because external forces (specifically, changing environments) forced them into changing. Malthus gave them the means—they applied his essay on human populations to all organisms, and it set for them a problem only evolution could solve. It was clear that all species, even the ones slowest to reproduce (such as the elephant), would overrun the earth if left alone. If populations did not increase uncontrolled there must be checks, and they found them: predation, starvation, and disease. These natural pressures would winnow away numbers, leaving a few to survive. And which few? As Wallace asked, "Why do some live and some die?" That question focused on the why of species transmutation like a laser. Wallace and Darwin answered by pointing out that the ones that live must be different: faster, stronger, bigger, smaller, or perhaps just more resourceful—in a word, fitter—than those that die.
Several points deserve special mention in this theory. First, there is no impetus of design in this theory. That is, there is no overarching idea or plan; change occurs if and only if it means better survival and reproduction. There is no direction to the change; organisms do not necessarily become more complex or more intelligent unless those characteristics contribute to survival and reproduction. Second, there is no mention of how these changes occur. Genetics was an unborn science at the time of Darwin and Wallace. Third, there are some implicit requirements that Darwin recognized and knew had to be established in order to make the theory work.
He had a monumental task, which he recognized. Evolution, a word not used by Darwin or Wallace (evolution at the time meant only "to roll out, as in a scroll" and was thus not without its own implications of design and plan), was not something that could be proved the way Newton could prove gravity. Organisms ultimately would not act as predictably as did inanimate objects. Plus, there was a time element—all these changes would require periods far longer than a single human life span, thus there would be no experiment possible that would prove evolution. This was clear to Darwin. He had to invent a new approach to establish a new kind of science.
Concurrently in Europe there were physical scientists looking at analogous problems. Studies of gases and fluids were under way, and scientists were realizing they would have to predict behavior on the basis of populations of molecules. They began thinking in terms of trends. At the same time, mathematicians were working out the rules of probability and statistics. Both of these new studies would have helped Darwin, if only because they represented a new way of thinking about proof. But they were not well enough developed, and Darwin lacked the aptitude for math and physics (later on he admitted regretting the lack of discipline to apply himself to studying math as a student) to be aware of their emergence.
Even with these tools, he still wouldn't have had a complete solution. Living systems are what we now call complex systems, and as such there are few simple, linear solutions to problems concerning them. The mathematics needed to be able to predict the behavior of complex, chaotic systems is only now beginning to catch up with messy reality.
So what was Darwin's solution? Here was a theory he knew was correct, but he also knew few would accept it. The science of his time was unwilling to either believe in such an indeterminate idea or to accept a scientific proof of such vagueness. He could neither offer the clean precision of Newton nor the reassuring comfort of Genesis. He would have to bludgeon his idea through; he would have to provide "one long argument."
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