How elementary could the flaws in Darwin's logic be? How's this?

Only one or two of each living creature's progeny get to reproduce and pass on their genes (natural selection). Every few hundred million years there's a huge meteor impact. Therefore, natural selection causes meteor impacts.

Compare that with the following:

Only one or two of each living creature's progeny get to reproduce and pass on their genes (natural selection). Over millions of years species of living creatures tend to evolve. Therefore, natural selection causes evolution.

I think the logic in both is the same. Both are examples of post hoc ergo propter hoc, the fallacy of assuming that because one thing happens before another it must have caused the other.

I suspect that few people will appreciate this, because natural selection and evolution have come to seem logically linked. But let's try this another way.

Consider two worlds, both with living creatures that reproduce genetically. In both worlds creatures' genes vary and there will be a greater tendency for creatures that are better genetically endowed to survive to reproduce (natural selection). In one world, natural selection contributes very little to evolution, in the other it is the primary cause of evolution.

Is that possible? I think it is. I see natural selection logically as inevitable, it is bound to occur. But its effect could be negligible. Compare natural selection to friction in driving a car. Friction is inevitable. But must it be what drives the car? Every so often while driving a car one will park on a hill and engage the brake to exploit friction. In this instance friction helps us manage the motion of the car. But the rest of the time friction has much less effect than the engine. Similarly the effect of natural selection could be always present, sometimes beneficial, but usually with negligible effect. The mere fact that the effects of natural selection and evolution tend to operate in the same direction does not mean one causes the other.

Which world do we live in?

For a far better critique of the flaws in Darwin's work see my review of Gertrude Himmelfarb's book on the subject.

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