What is Darwin presenting in chapter 6?
In chapter 6, Darwin presents two problems to his theory and tries to address them.
The first problem is why do we not see a continuous gradation between all life, especially in the fossil record?
“But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?”
I think this is a fair question to ask and at least he tries to answer it. He responds by saying that the fossil record is imperfect.
“I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed”
“Consequently evidence of their former existence could be found only amongst fossil remains, which are preserved, as we shall in a future chapter attempt to show, in an extremely imperfect and intermittent record.”
The problem is that if one goes by the available evidence, it would not support his theory. So, rather than modify his theory, he says the evidence is not perfect. This is not a very persuasive argument. If evidence and theory do not match, it would make more sense to question the theory, rather than the evidence.
But, I grant that paleontology was in its infancy during Darwin’s time. And many more fossils have been yet to be discovered and perhaps it would fill in the gaps.
Next, Darwin addresses the problem of how can new features/functionality arise. He posits that animals have transitioned to changed features and functionalities. One example he gives is flying lemurs transitioning to bats. However, his argument is not based on any evidence, but simply his imagination.
“Nor can I see any insuperable difficulty in further believing it possible that the membrane-connected fingers and fore-arm of the Galeopithecus might be greatly lengthened by natural selection; and this, as far as the organs of flight are concerned, would convert it into a bat.”
Darwin also talks about complex organs. And this section contains two of the most quoted parts of the book (from the opponents of Darwin).
“To suppose that the eye, with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest possible degree.”
“If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.”
But, again, he presents no evidence of how natural selection can produce complex structures, but simply reiterates his belief in the powers of natural selection.
“I can see no very great difficulty (not more than in the case of many other structures) in believing that natural selection has converted the simple apparatus of an optic nerve merely coated with pigment and invested by transparent membrane, into an optical instrument as perfect as is possessed by any member of the great Articulate class.”
He uses the same argument for the conversion of a swimbladder into lungs.
“hence there seems to me to be no great difficulty in believing that natural selection has actually converted a swimbladder into a lung, or organ used exclusively for respiration.”
He then adds by saying that he will address this issue more in a future book.
“Although we must be extremely cautious in concluding that any organ could not possibly have been produced by successive transitional gradations, yet, undoubtedly, grave cases of difficulty occur, some of which will be discussed in my future work.”
One final note about this chapter is that Darwin makes a statement that reveals that one cannot make predictions regarding natural selection.
“we are much too ignorant in regard to the whole economy of any one organic being, to say what slight modifications would be of importance or not.”