424 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1909. 
Gaudry, after having drawn a magnificent picture of Miocene 
nature, concluded that it was characterized by the great development 
of herbivorous animals, and demonstrated that the mammals, on 
account of their complexity, had undergone much greater variations 
than the invertebrates. 
He developed also another very important idea, which had been 
entertained by Cuvier, and which to-day furnishes valuable results. 
I refer to migrations. 
Notable differences are often found between two successive forms 
in superimposed strata, and these differences can only be explained 
by the modification in the habitat of the animals. But these modifi- 
cations were caused by invasions or recessions of the ocean, by eleva- 
tion or depression of certain continental areas. Briefly stated, the 
modifications of a biological nature in a region are dependent on 
modifications of a physical nature, the faunal changes bearing a close 
relation to the geographical changes. ‘“ When I say that the differ- 
ence between the two substages of the Upper Miocene results from 
modifications in the habitat of the animals, I do not consider that I 
am pointing out an isolated fact in the history of the development 
of living beings. There is reason to suppose that the whole organic 
world has proceeded in a continuous manner, and that if geologists 
encounter sudden appearances of fossil species in passing from one 
stage to another, it is because they have, in general, placed the bound- 
aries of the stages at points where displacements of faunas have taken 
place. The paleontologist who does not believe in migrations and 
in local extinctions can not admit the relatedness of ancient forms of 
hfe. He encounters appearances, disappearances, and revivals which 
he is unable to understand.” (Gaudry.) 
Thus are explained the close, inseparable bonds between geology 
and paleontology. A paleontologist, no matter how eminent he may 
be, should be a geologist in order to know the faunas which he studies, 
and a geologist who studies sedimentary formations is obliged to be 
a paleontologist. 
3. Primitive reptiles and the theory of the archetype. 
After having considered the evolution of mammals at the close of 
Miocene times, and having drawn the important conclusions already 
noticed, Gaudry undertook the study of amphibians and reptiles, 
specimens of which had been found in the Permian schists of Autun. 
Before this epoch primary amphibians and reptiles were almost 
unknown in France. In describing the curious forms of Autun— 
Protriton, Actinodon, Euchirosaurus, Stereorachis, etc—and pointing 
out their affinities, Gaudry filled a gap which existed in the history 
of the primitive vertebrates, and was led to discuss the question of 
