The structural differences between Axolotl and Amblystoma are considerably greater and of more importance than those between allied genera, or indeed than between the families of the Urodela. The genus _Siredon_ undoubtedly belongs to a different sub-order to the genus _Amblystoma_ into which it occasionally becomes transformed. Strauch, the most recent systematic worker at this group, distinguishes the sub-order _Salamandrida_ from that of the _Ichthyodea_ by the possession of eyelids, and by the situation of the palatine teeth in single rows on the posterior edge of the palatal bone: in _Ichthyodea_ the eyelids are wanting and the palatine teeth are either “situated on the anterior edge of the palatal bone,” or “cover the whole surface of the palatal plates in brush-like tufts.” How is it possible to regard such widely divergent anatomical characters as changes suddenly produced by the action (but once exerted) of deviating conditions of life? Hand in hand with the shedding of the old and the appearance of new palatine teeth, there occurs a change in the anatomical structure of the vertebral column, and also--as we may fairly conclude from Kölliker’s correct observation of the cessation of the slimy secretion--in the histological structure of the skin. Who would undertake to explain all these profound modifications as the direct and sudden action of certain external influences? And if any one were inclined to explain such changes as a consequence of the disappearance of the gills, _i.e._ as correlative changes, what else is such a correlation than the phyletic vital force under another name? If from one change arising from the direct action of external agencies, the whole body can in two days become transformed in all its parts, in the precise manner which appears best adapted for the new conditions of life under which it is henceforward to exist, then the word “correlation” is only a phrase which explains nothing, but which prevents any attempt at a better explanation, and it would be preferable to profess simply the belief in a phyletic vital force.