The remaining Eutheria are obviously all to be referred to one great division with the possible exception of the Whales, whose affinities form one of the principal difficulties to the student
of this group. A short résumé of what is at present thought of the systematic position of this anomalous order is appropriate here. Albrecht went so far as to regard the Cetacea as the nearest group of animals to the hypothetical Promammalia.[[66]] But discounting his arguments by the removal of such of them as relate to structure plainly altered by the singular mode of life of these creatures, there is really a great deal to be said in favour of his view.
The chief facts which argue a primitive position among mammals for the Cetacea are perhaps: (1) the slight union of the rami of the lower jaw; (2) the occasionally rather marked traces of the double constitution of the sternum; (3) the long and simple lungs; (4) the retention of the testes within the body-cavity; (5) the occasional presence (in Balaenoptera) of a separate supra-angular bone. These points, however, are but few, and are not of such great weight as those which ought to be present to establish a claim to separate treatment for the Cetacea as opposed to the Eutheria. If this group of mammals can be tacked on anywhere, it appears to us that the nearest relatives are not, as is sometimes put forward, the Ungulata or the Carnivora, but the Edentata. There are quite a number of rather striking features in which a likeness is shown between these apparently diverse orders of mammals. The chief ones are these: (1) the existence of traces of a hard exoskeleton, of which vestiges remain in the Porpoise; (2) the double articulation of the rib of the Balaenopterids to the sternum, with which compare the conditions obtaining in the Great Anteater; (3) the concrescence of some of the cervical vertebrae; (4) the share which the pterygoids may take in the formation of the hard palate; (5) the fact that in the Porpoise, at any rate, as in many Edentates, the vena cava, instead of increasing in size as it approaches the liver, diminishes.
Another group which is perfectly isolated is that of the Sirenia. The alliance advocated by some with the Cetacea, and quite recently renewed by Professor Haeckel, is contradicted by so many important features that it seems necessary to abandon it. The recent discovery of a fossil Sirenian jaw by Dr. Lydekker with teeth highly suggestive of those of Artiodactyla, may prove a clue. A third group which is so isolated as to have been placed in a
primary division, proposed to be called Paratheria, is that of the Edentates. Probably the group so called should really be divided into the Edentata and the Effodientia, the latter containing the Old World forms. Whether or not it be ultimately shown that the Ganodonta are ancestral Edentates (sensu strictiori), the connexion of the group with others is not at present plain. The same is the case with the extensive order of Rodents. It is true that the extinct order of the Tillodontia shows certain Rodent-like characters on the one hand, and likenesses to Ungulates on the other. Certain likenesses shown by such apparently diverse animals as the Rabbit and the Elephant used to be insisted upon by Professor Huxley. For the present, however, the Rodents must remain as an isolated group with only very dubious affinities to others. The remaining groups of existing mammals are easier to connect. At first the differences between a Cat and a Horse seem to be quite as wide as those which separate any two of the higher Eutherian orders. But it seems to become clearer and clearer, as palaeontological investigation proceeds, that the bulk of the Ungulate and the Carnivorous, Insectivorous, and perhaps Lemuroid stocks converge into the early Eocene Creodonta. From the Lemuroid branch the higher Primates can be derived. The only "Ungulates" which cannot be fitted in with some reasonable probability is the group of the Proboscidea. But of the early forms of this division we have at present no knowledge.