THE GENETIC RELATIONS OF THE CETACEANS. 21 
excusable, for wresting such an interpretation therefrom, especially 
my reference to their systematic places of the extinct typical 
Cetaceans was overlooked. . 
Methods involved in discovery.—In dealing with genetic prob- 
lems, there are facts and inferences from facts to be considered. 
As facts, the Zeuglodonts are less aberrant in structure and 
more related to the ordinary quadrupeds than are the existing 
Cetaceans, and they are not living, and their remains have only 
been found (or at least identified) in the Tertiary epoch. 
As other facts, the Cetaceans of the present epoch share with 
the Zeuglodonts: the special features which differentiate them as 
Cetaceans from other mammals, and superadd other specialized 
characteristics. 
As facts, then, the Zeuglodonts (only yet known from tertiary 
beds) bridge over the gap between the Carnivores (or normal 
quadrupeds) and the existing Cetaceans, that is, N are more 
like the former than are the latter. 
As inferences from these facts, it seems “most probable that the 
known Zeuglodonts represent a stock relatively near the original. 
stem or line of descent, and comparatively little differentiated (in 
at least the jaws, teeth, olfactory apparatus, members, etc.) from 
the generalized cetacean progenitors of the Denticetes and Mys- 
ticetes. Whether the restricted characters which might be applied 
to all the known Zeuglodonts could be extended to those atavan 
forms is questionable, but that the latter had the jaws, nasal aper- 
tures and teeth attributed to the suborder in my article is, I think, 
a perfectly legitimate inference from the facts and, therefore, it 
may with confidence be said that the Denticetes and the Mysti- 
cetes have originated from the generalizec: Zeuglodont stem (not 
Zeuglodonts) thus understood. 
But when they originated is entirely another question, and for 
the, solution of whieh we have no data. They — or one, or the 
other of them— may have become differentiated in the Cretaceous, 
or the Jurassic, or a still earlier age. I should probably in the 
main agree with Dr. Brandt, however remote he might place the 
date of origin* and at least would have ho direct evidence to 
Especially as Dr. Brandt concedes that the Sirenians may hav e originated little 
wales the Miocene (perhaps before the Eocene), with the Halitheriids as Witnesses of 
the high eis of specialization as Sirenians whieh the Miocene forms had already 
attai 
Ste, 
t Hiren niorum, ab initio verisimiliter efi ed Aces 
5 \ 3 
