Darwin then cited two particular species which he claimed supported his position. Most readers were probably impressed with the wealth of biological details that Darwin presented in his argument, but his logic was circular.
Darwin argued that the unique features of these two species show that the law of compensation does not hold in the wild because, after all, such unique features must have evolved. Darwin presupposed the truth of evolution in order to find evidence for evolution.
Let’s have a look at Darwin’s argument. Cirripedia, a class in the crustacea phylum, are sessile and highly unique barnacles upon which Darwin had completed a major systematic study. Over against Goethe and Geoffroy, Darwin argued that natural selection, rather than any internal law of biology, could bring about changes he argued were evident in two species named Ibla and Proteolepas:
If under changed conditions of life a structure, before useful, becomes less useful, its diminution will be favoured, for it will profit the individual not to have its nutriment wasted in building up a useless structure. I can thus only understand a fact with which I was much struck when examining cirripedes, and of which many analogous instances could be given: namely, that 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. 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.
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. 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. [Origins, 6th Ed]
Darwin was not simply applying his theory to a set of observations. He was not illustrating how evolution might have formed these species. Rather, Darwin was building an argument for natural selection. But in constructing his argument, he presupposed that the species had evolved.
Darwin was unquestionably an expert on barnacles, and he could present a detailed example of highly-modified species that most people have never even heard of. But Darwin begs the question when he says that the unique structures of the Ibla and Proteolepas evolved and therefore natural selection can effect such changes.