Now it has been alleged by competent naturalists that there are several such cases in organic nature. We have already noticed in a previous chapter (pp. 58, 59), that Mr. Mivart has instanced the eye of the cuttle-fish as not only analogous to, but also homologous with, the eye of a true fish--that is to say, the eye of a mollusk with the eye of a vertebrate. And he has also instanced the remarkable resemblance of a shrew to a mouse--that is, of an insectivorous mammal to a rodent--not to mention other cases. In the chapter alluded to these instances of homology, alleged to occur in different branches of the tree of life, were considered with reference to the process of organic evolution as a fact: they are now being considered with reference to the agency of natural selection as a method. And just as in the former case it was shown, that if any such alleged instances could be proved, the proof would be fatal to the general theory of organic evolution by physical causes, so in the present case, if this could be proved, it would be equally fatal to the more special theory of natural selection. But, as we have before seen, no single case of this kind has ever been made out; and, therefore, not only does this supposed objection fall to the ground, but in so doing it furnishes an additional argument in favour of natural selection. For in the earlier chapter just alluded to I showed that this great and general fact of our nowhere being able to find two homologous structures in different branches of the tree of life, was the strongest possible testimony in favour of the theory of evolution. And, by parity of reasoning, I now adduce it as equally strong evidence of natural selection having been the cause of _adaptive_ structures, independently developed in all the different lines of descent. For the alternative is between adaptations having been caused by natural selection or by supernatural design. Now, if adaptations were caused by natural selection, we can very well understand why they should never be homologous in different lines of descent, even in cases where they have been brought to be so closely analogous as to have deceived so good a naturalist as Mr. Mivart. Indeed, as I have already observed, so well can we understand this, that any single instance to the contrary would be sufficient to destroy the theory of natural selection _in toto_, unless the structure be one of a very simple type. But on the other hand, it is impossible to suggest any rational explanation why, if all adaptations are due to supernatural design, such scrupulous care should have been taken never to allow homologous adaptations to occur in different divisions of the animal or vegetable kingdoms. Why, for instance, should the eye of a cuttle-fish _not_ have been constructed on the same ideal pattern as that of vertebrate? Or why, among the thousands of vertebrated species, should no one of their eyes be