CHAPTER II. 
THE GEOLOGICAL EXPRESSION OF FAUNAL MIGRATIONS. 
The association of specific difference in plants and animals with 
geographical distribution, involving difference in climate, altitude, 
and general difference in environment, has been noticed by natural- 
ists for centuries. It was a problem of geographical distribution, 
more than anything else, which suggested to Darwin the accounting 
for difference in organisms b} 7 evolution through the agency of nat- 
ural selection. In a letter to Moritz Wagner, Darwin wrote, in 1876, 
"It was such cases as that of the Galapagos Archipelago which chiefly 
led me to study the origin of species."" 
The geologist, however, for whom the record of change in fossils is 
more sharply apparent on passing vertically through successive strata, 
is accustomed to associate change with sequence of time, neglecting 
the part which migration and associated change of environmental 
conditions may play in the modification of the specific composition of 
fossil faunas. 
It is commonly known that great thicknesses of limestone, repre- 
senting immense periods of geological time, are dominated from bot- 
tom to top by the same fauna; while shales and sandstones, indicat- 
ing rapid accumulation of sediment and change in conditions of the 
sea bottom, present series of faunas in which not only species but 
genera differ. If the rate of evolution during the long periods of 
time represented by the limestone indicates the steadiness with which 
organisms reproduce their kind under uniform conditions of environ- 
ment, then either the changes of environment coincident with change 
of sediments must be the occasion of the modification of the organ- 
isms observed in the successive faunas of the second case, or else the 
faunas have shifted with the change, and the observed difference is 
due to migration of new species into the region whose conditions have 
changed, with only slight immediate change in the character of the 
species. 
If we adopt the first assumption, viz, that the rapid changes of 
environment are coincident with rapid evolution, the irregularity in 
rate of evolution in different parts of the globe must have resulted 
in great diversity of organisms, and Huxley's view, that likeness of 
fossils in widely distant portions of the globe does not indicate time 
equivalency, must be accepted as substantially correct. If, on the 
other hand, we adopt the second inference, viz, that coincident with 
"Life and Letters, Vol. II, p. 338, New York, D. Appleton & Co., 1898. 
Bull. 210—03 3 33 
