TIME IN EVOLUTION—ZEUNER 251 
These ridges run into the upper posterior edge of the processus 
postorbitalis in a certain number of fossil specimens, while in recent 
Marmota marmota they have moved to the upper side of the processus. 
That this character has become practically fixed since the Upper 
Pleistocene, is shown by the following figures: 
TaBLeE I 
Marmota Inter- Primitive 
tyz.e mediate type 
(Percent) (Percent) (Percent) 
Wppersbleisvocenes 32. 4- =) = oe a IS 33 53 13 
FES COT Gee ae a a res ee a ae Byer eye I a 98 LM, 2 
Forms of this degree of variation would, in the recent fauna, be re- 
garded as subspecies. 
Going back to the Middle Pleistocene, about 250,000 years ago, 
the differences become more conspicuous, but they are still treated 
as subspecific by most taxonomists. It is only in the Lower Pleisto- 
cene, about 500,000 years ago, that we encounter ancestral forms on 
which the taxonomists agree that they must be classified as distinct 
species. This applies, for instance, to the European elephants, 
Elephas antiquus, and the mammoth, which Soergel has shown to 
have evolved from an early Pleistocene common ancestor, Hlephas 
meridionalis. If this view is correct, nearly half a million years 
were required to produce a new species by way of a gradual change. 
If it is not correct, the point of divergence lies farther back in the 
past and the rate of species change is longer. In the lineages of 
rhinoceroses, bears, and deer, similar evidence is available and there 
are cases in the Recent fauna which have been explained as the results 
of geographical isolation during the glaciations (the common crow 
and the hooded crow, for instance). But again there are species 
which have changed much less since the Lower Pleistocene, which 
are regarded as no more than subspecifically distinct from their 
Recent descendants and, therefore, must have a slower rate of species 
formation. In the Cromer Forest Bed (about 500,000 years ago), 
14 percent of the species are regarded as “Recent,” though it is quite 
likely that subspecific differences will be detected by future revisers. 
Five hundred thousand years, then, appears to be a fast rate of 
species evolution in terrestrial mammals and no faster rate has, to 
my knowledge, yet been found in other groups of the animal king- 
dom. Let us now consider the question, how long have species re- 
mained stable or unchanged? To answer it we have to resort to ma- 
rine groups. Lower Miocene Mollusca of Java, for instance, are 
regarded as conspecific with Recent forms. They have been stable 
for 30 million years. Surveying fossils in general, this has to be 
regarded as a high figure, but it need not be a maximum. 
