504 
GEOLOGY: J. BARRELL 
ganisms must possess at the same time an instability of germ plasm, 
permitting variations or mutations to arise. These would under the 
law of probabilities be more often degenerative than ascensive, but the 
strenuous environmental conditions cut off unsparingly all but the fa- 
vored, few which showed increased efficiency in meeting the critical fac- 
tor in the environment. The rising death rate owing to recurrences of 
seasons of marked aridity was the ultimate cause back of the rise of 
air-breathing vertebrates. Natural selection, by the survival of the 
fittest, is shown thus, by matching the advance in life to the necessi- 
ties of environment, to be the overarching cause which led the verte- 
brates to the possession of the land, their future theater of high evolution. 
It is doubtful in view of this conclusion if vertebrates could have attained 
their present dominance of the land without the driving effect of cli- 
matic adversities such as are found in the Devonian. 
The biologist who studies variations, mutations, Mendelian factors, 
hybridization; the paleontologist who studies orthogenetic variations, 
the budding and expansion of phyla, are looking at the expressions of 
internal forces. These, however, do not constitute a complete picture 
of the cause and effects of organic evolution any more than the observer 
of a running antelope could fully explain its flight as the result of the 
coordination of nervous and muscular actions on a skeletal framework. 
He must also take into account the ultimate cause — the carnivore behind, 
and also the carnivores which pursued its ancestors through millions 
of years. The ultimate controls of evolutions are found in a study of the 
geologic record, though the possibilities of evolution must be latent 
within the organism. Natural selection, although discredited as a 
cause determining specific variations, appears nevertheless to be a major 
factor in evolution, the driving cause in association with changes in 
environment, which has forced the great advances in organic progress. 
^ This paper was given orally by the author before the American Society of Vertebrate 
Paleontology, December 26, 1907 [Loomis, F. B., Secretary, The American Society of Ver- 
tebrate Paleontology, Science, 27, 254-256 (1908)], and also before the Geological Society of 
America at Washington, December 28, 1915, and the complete paper will be published in 
the Bulletin of that Societ}' in 1916. Some discussion of the subject has also been given by 
Professor Schuchert in his recent text on Historical Geology (Part II of Pirsson-Schuchert, 
Text Book of Geology. 
2 Chamberlin, T. C, On the Habitat of the Early Vertebrates, /. Geol., 8, 400-412 (1900). 
2 Walcott, C. D., Preliminary Notes on the Discovery of a Vertebrate Fauna in Ordo- 
vician Strata, Bull. Gtol. Soc. Amer., 3, 153-172 (1892). 
