He has done so on the ground that the probable common ancestor of mammals and of osseous fishes was a vertebrate animal of so low a type that it could not be supposed to have possessed a skull differentiated into distinct bony elements--even if it was bony at all.

If this was so, then the cranial bones must have had an independent origin in each class, and in this case we have the most strikingly harmonious and parallel results from independent actions.

For the bones of the skull in an osseous fish are so closely conformed to those of a mammal, that "both types of skull exhibit many bones in common," though "in each type some of these bones acquire special arrangements and very different magnitudes."[160] And no investigator of homologies doubts that a considerable number of the bones which form the skull of any osseous fish are distinctly homologous with the cranial bones of man.

The occipital, the parietal, and frontal, the bones which surround the internal ear, the vomer, the premaxilla, and the quadrate bones, may be given as examples.
Now, if such close relations of homology can be brought about independently of any but the most remote genetic affinity, it would be rash to affirm dogmatically that there is any impossibility in the independent origin of such forms as centetes and solenodon, or of genetically distinct batrachians, as similar to each other as are some of the frogs of South America and of Europe.

At the same time such phenomena must at present be considered as very improbable, from the action of ancestral habit, as before stated.
We have seen, then, that the geographical distribution of animals presents difficulties, though not insuperable ones, for the Darwinian hypothesis.
If, however, other reasons against it appear of any weight--if, especially, there is reason to believe that geological time has not been {154} sufficient for it, then it will be well to bear in mind the facts here enumerated.