If, however, the Reader thinks that teeth are parts peculiarly qualified for rapid variation (in which view the Author cannot concur), he is requested to suspend his judgment till he has considered the question of the independent evolution of the _highest organs of sense_.

If this seems to establish the {70} existence of some other law than that of "Natural Selection," then the operation of that other law may surely be also traced in the harmonious co-ordinations of dental form.
The other difficulty, kindly suggested to me by the learned Professor, refers to the structure of birds, and of extinct reptiles more or less related to them.
The class of birds is one which is remarkably uniform in its organization.
So much is this the case, that the best mode of subdividing the class is a problem of the greatest difficulty.

Existing birds, however, present forms which, though closely resembling in the greater part of their structure, yet differ importantly the one from the other.

One form is exemplified by the ostrich, rhea, emeu, cassowary, apteryx, dinornis, &c.

These are the _struthious_ birds.