66 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
mollusks and crustaceans are represented in almost all rock deposits 
which contain any kind of fossil remains.” —Jordan and Kellogg. 
[The study of geology teaches us that the earth’s outer zones have 
undergone within the period of vertebrate history numerous profound 
changes which in general we may term climatic changes. There have 
been periods of continental subsidence, accompanied by ocean-floor 
elevations, during which great continental plains have been covered 
with comparatively shallow seas. The marine faunas of the seas have 
migrated into these shallows and representatives of them have been 
buried in sediment. When the reverse change has occurred and the 
continental plain has been again elevated, the sedimentation of the 
shallow-sea period forms a great rocky stratum laden with marine 
fossils. Between periods of subsidence millions of years elapsed, and 
therefore a break in the continuity of the entombed fossils is to be 
expected. Discontinuity between the fossil faunas in adjacent strata 
is the invariable rule. Were it not for this periodicity of subsidence 
and elevation there would be no boundaries between consecutive 
geologic strata. 
In addition to the methods of fossilization mentioned, a few others 
deserve notice. Many animals of the arid plains have been fossilized 
by becoming imbedded in dust or sand drifts which have piled up 
against rocky outcrops or have filled in dried-up arroyos. Some very 
valuable fossils have been recovered from asphaltic deposits as the 
result of animals falling into liquid or semiliquid lakes or pools of 
asphalt. 
Not only are external organs preserved with precision, but even 
delicate internal structures, such as the brains or the viscera of verte- 
brates, have been found in such a perfectly natural shape that the 
comparative anatomy could be worked out with confidence. 
On the whole, then, we must conclude that the earlier pessimism 
regarding the inadequacy and insufficiency of fossil data is giving way 
before a steadily increasing optimism, due to the very rapid advance 
in technique and the surprisingly abundant discoveries of the modern 
palaeontologist. The more enthusiastic of the new school of fossil- 
hunters do not despair of ultimately bringing to light all of the really 
essential links in the chain of evidence necessary to place the evolution 
theory beyond the reach of controversy.—Ep.] 
* From D. S. Jordan and V. L. Kellogg, Evolution and Animal Life (copy- 
right 1907). Used by special permission of the publishers, D. Appleton & Company. 
