Consider, for example, Darwin's circular reasoning in his "Compensation and Economy of Growth" section in Chapter 5 of Origins. In my previous post I explained that Darwin begged the question. Darwin argued that natural selection could bring about change which he presupposed had evolved. Here is his argument broken out into the four points Darwin made:
1. when a cirripede is parasitic within another cirripede and is thus protected, it loses more or less completely its own shell or carapace. This is the case with the male Ibla, and in a truly extraordinary manner with the Proteolepas: for the carapace in all other cirripedes consists of the three highly-important anterior segments of the head enormously developed, and furnished with great nerves and muscles; but in the parasitic and protected Proteolepas, the whole anterior part of the head is reduced to the merest rudiment attached to the bases of the prehensile antennae.
2. Now the saving of a large and complex structure, when rendered superfluous, would be a decided advantage to each successive individual of the species; for in the struggle for life to which every animal is exposed, each would have a better chance of supporting itself, by less nutriment being wasted.
3. Thus, I believe, natural selection will tend in the long run to reduce any part of the organisation, as soon as it becomes, through changed habits, superfluous, without by any means causing some other part to be largely developed in a corresponding degree.
4. And, conversely, that natural selection may perfectly well succeed in largely developing an organ without requiring as a necessary compensation the reduction of some adjoining part.
Or, simply put:
1. Many species have undergone significant loss of components.
2. Such loss can be advantageous, and so increase fitness.
3. Therefore natural selection will select such loss.
4. In the same way, natural selection develops new components. That is, natural selection can add components as well as remove components.
Notice that in Step 1, Darwin takes as his premise that the male Ibla and Proteolepas have undergone evolutionary change. That is, he assumes that they have lost their carapace. His conclusion, that natural selection brings about such change, is presupposed in his premise.
Furthermore, notice that in Step 4, Darwin equates loss of components with addition of components. But there is no basis for this assumption.
This reasoning is fallacious, and therefore not scientific (science requires logical reasoning). Yet every time evolutionists are presented with their own fallacies, they deny there is any such problem.