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"Difficulties on Theory" is the sixth chapter of On the Origin of Species by Means of Natural Selection by Charles Darwin. In this chapter, Darwin explains some of the challenges to his theory, and potential ways to disprove it. Creationists sometimes note that a book that includes an entire chapter on why it might be wrong probably is, while evolutionists view this chapter as great science: an attempt to display all the flaws right up front, so no one else has to think of them all over again.

Darwin starts this chapter by classifying objections to his theories into four areas. The first two objections are dealt with in this chapter, while the third and fourth are dealt with in the next two chapters, "Instinct" and "Hybridism." The first objection is the lack of transitional species. "[W]hy," Darwin writes, "if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?" Steve Jones, in his book Darwin's Ghost, expresses the answer better than Darwin: As new species rise, older ones are often made extinct by their inability to compete with the new, better-developed species. "As a result," Jones writes, "at any time, just the tips of the twigs of the evolutionary tree are on view." Even so, in some animals we can see a variety of transitional forms. Jones explains that squirrels, for instance, vary "from animals with their tails only slightly flattened, and from others with the skin on their flanks rather full, to the so-called flying squirrels that have their limbs and even the base of the tail united by a broad expanse of skin, which serves as a parachute and allows them to glide through the air to an astonishing distance from tree to tree."

Darwin’s second objection questions whether an animal "could have been formed by the modification of some animal with wholly different habits." There are, in fact, recorded cases of this happening. For instance, as the United States grew up, colonists planted apple orchards. "In the 1860s," Jones writes, "in the valley of the Hudson, the crop was attacked by a new pest, the apple maggot fly." This pest was at first thought to have come from Europe, but was in fact a quickly evolved descendent of the hawthorn fly, which is native to America. It continues to exist and, according to Jones, does millions of dollars of damage each year.

There are also known examples of animals that are midway in the process of modification from one environment to another. Darwin give the example of the Mustela vison, an otter-like creature that has webbed feet. "[D]uring summer," he writes, "this animal dives for and preys on fish, but during the long winter it leaves the frozen waters, and preys like other polecats on mice and land animals." One can imagine this animal evolving into either a land or sea animal, given enough time.

In "Difficulties on Theory," Darwin also presents conditions under which evolution by natural selection could be disproved. For instance, Darwin writes that "[i]f it could be proved that any part of the structure of any one species had been formed for the exclusive good of another species, it would annihilate my theory, for such could not have been produced through natural selection." As Darwin notes, "some authors suppose" that the rattlesnake "is furnished with a rattle for its own injury, namely, to warn its prey to escape." He dismisses this idea, but does not fully explain why. According to several web sites, including that of the Alabama Wildlife Federation, the rattle is now thought to have evolved as a warning device to prevent snakes from being crushed by large mammals.

"Difficulties on Theory" is an interesting chapter because it deals with scientific skepticism; any theory can be proven wrong. The fact that objections to and attempts to disprove evolution are still highly questionable shows the strength of the theory.

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