Geological Aspects of Evolution 99 
disappeared with the close of the Mesozoic. In the Cenozoic there 
were long lines of animals that are more familiar to us, the 
mammals, including such forms as the horse, camel, dog, and 
elephant. 
Many invertebrates, too, changed in the course of geologic 
time. To take a single case, we shall compare the cephalopods 
of the Paleozoic with those of the Mesozoic. But first, let us 
examine figure 2 which combines the region of the Grand Canyon 
with that of the Colorado High Plateaus, and so make certain 
that the rocks of the Mesozoic actually overlie those of the 
Paleozoic and therefore consist of younger strata. The lower 
Paleozoic cephalopods all have straight or slightly curved suture 
lines, marking the divisions between the chambers. In shells of 
Upper Paleozoic cephalopods, the sutures are decidedly curved. 
The ammonite cephalopods, limited to the Mesozoic, have such in- 
tricately winding sets of sutures that you can easily distinguish 
the Mesozoic forms from the Paleozoic. If you examine all the 
Sutures of some ammonites, you will find that the sutures become 
increasingly complex with growth, so that the life history of the 
individual recapitulates the history of the cephalopods during 
400,000,000 years. 
The Quaternary or The Age of Man. The Quaternary: is 
often referred to as the Age of Man. It is not the purpose of 
the writer to discuss the antiquity of man’s body by an evaluation 
of the known fossil remains, as this belongs in the field of paleon- 
tology, but rather to attempt to bring out by a compilation the 
relation of the fossils and the implements to the geologic deposits 
containing them (See table 2). The Quaternary is divided into 
the Pleistocene which is the Glacial Period, and the Recent, which 
includes the present time. The estimates of most authorities place 
the beginning of the Pleistocene between 500,000 and 1,000,000 
years ago. During the Pleistocene there were four major ad- 
vances of the ice, which have been recognized independently in 
Europe and North America and separate names were given for 
each stage on the two continents (See Table 2). Each advance 
of the ice was followed by a long interglacial period during which 
the ice melted and the climate as shown by the fauna and flora 
