370 THE JOURNAL: OF GEOLOGY. 
genera varies greatly in successive periods. These changes have 
been due to migration; each rising of a seacoast, or transgres- 
sion of sea on land areas, or connection of basins that before 
were separated, has added to the confusion of faunas, bringing 
in new elements, while many of the old were forced to migrate, 
or else were unable to survive the new conditions and increased 
natural competition with vigorous immigrants. 
Marine invertebrates migrate chiefly through their young, 
and along shore lines, where the successive generations can find 
the same conditions suitable to their perpetuation, and the young 
are especially sensitive to changes in temperature and food- 
supply. Therefore marine currents along continental borders 
are very favorable to migration, since by them the same condi- 
tions necessary for life are spread far out of their usual range. 
If the naturalist would trace out the life history of genera 
and species, he must not confine his studies to a single region, 
for there the history is made up of disconnected episodes. He 
must seek the regions from which the faunas came and follow 
out their migrations into regions to which they departed, must 
find out what regions contributed certain new elements to the 
population and must learn the order of appearance of a fauna in 
different parts of the earth. In this way alone can he get a true 
idea of the changes which forms have suffered, and the reason 
of these changes, the mutual working of the laws of natural 
selection and adaptation to surroundings. In this way, too, he 
can get a true history of the inhabitants of the earth, and of the 
changes in physical geography at various periods, the primary 
object of geologic investigation. . 
The study of the faunal relations of the various series of 
sedimentary rocks of California has proved exceedingly inter- 
esting and has thrown much light on past changes in physical 
geography. Some of the facts obtained seem to conflict with 
the theory of the permanency of continental plateaux and oceanic 
troughs. 
The writer has, therefore, undertaken to outline the faunal 
relations which California had with various regions during differ- 
