672 THE AMERICAN NATURALIST. [VoL. XXXII. 
Another effect of change of climate due to the further 
upheaval, drainage, and drying up of lakes and river sources in 
the central portions of all the continents was the destruction 
of forests resulting from the drying up of the lakes and streams, 
the formation of vast internal desert regions, with the desert 
floras and faunz and saline animals peculiar to them; these 
are the last steps in geological history of the origination of 
species, and have been taken almost under the observation 
of man. In the origin of species adapted to desert areas and 
to salt lakes, faune relicte of the lakes on the elevated plains 
of Asia, South America, Africa, Sweden, and the Great Lake 
region, we see that geographical isolation and the absence of 
competition are the primary factors in the case. 
In conclusion, it is, from the nature of the case, notwith- 
standing the imperfection of the geological record, apparent 
that the fullest, most complete and convincing proof of organic 
evolution is derived from the past history of life, from paleon- 
tology, which involves the fact of geological succession. 
Looking back for half a century, we see that organic evolution 
is a fact, and is grounded and dependent on geological evolu- 
tion, and the latter on cosmical evolution. Should we ever 
have to abandon the principle of evolution, we should also have 
to give up the theory of gravitation, the principle of the cor- 
relation of physical forces, and also the conception of the unity 
of nature. All of these principles are interdependent, and 
form the foundation stones of our modern science. | 
The rapid summary we have given of the successive changes 
and revolutions in the earth’s history, and the fact that they 
are accompanied or followed by the process of the extinction 
of the unadapted, and their replacement by the more special- 
ized and better adapted, show that there is between these two 
sets of phenomena a relation of cause and effect. 
Moreover, it cannot be denied that the formation of our solar 
system in the manner outlined by the founders of the nebular 
hypothesis, that the progressive changes in geology and the 
earth’s topography, the gradual building up or evolution of 
the continents, and the increasing fitness and intelligence of the 
life on its surface, the final outcome being man, whose physical 
