414 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



the preservation of their bones and brought them into competition with 

 the piscine inhabitants of such regions. 



Evaluation of Causes in the Eise of Amphibians 



ACTION AND REACTION BETWEEN ENVIRONMENT AND ORGANISM 



Organisms are hedged in on all sides by organic and inorganic bounds. 

 This environment presses in on and tends to restrict more or less insist- 

 ently the limits of action of the individual, forming a "pressure of en- 

 vironment" against which the species must exert itself to prevent extinc- 

 tion. There is here an equality between action and reaction. As long as 

 a balance is maintained, the species holds its place unchanged. But time 

 brings to pass either restriction, retrogression, or extinction of the species 

 on the one hand or an expansion and evolution on the other. When such 

 change occurs, is the ultimate cause to be found in modifications of the 

 external world or of the internal nature of the organisms? Which is 

 action and which the reaction, or is there an interaction requiring a con- 

 currence of both external and internal causes in order to accomplish 

 those great leaps which have occurred in the upward progress of evolu- 

 tion? The vertebrate paleontologist has most commonly studied the re- 

 sults without attempting to sift out the controlling cause. The biologist 

 has come to place most weight on changes in the germ plasm ; perhaps 

 because the field of his observation does not include those changes in 

 enveloping nature which mark the progress of geologic time. The geo- 

 logic record shows that the transformations of faunas are profound and 

 rapid in proportion to the greatness of the terrestrial changes in climate 

 and geography. This correspondence indicates that the time of the 

 greater evolutionary advances is determined by external causes. Through 

 such environmental changes an intense struggle for existence is set up, 

 the death rate rises high and eliminates less hardy and adaptable types. 

 At such times the advanced mutations, if such can come to exist, have a 

 relatively higher chance of survival than in less strenuous times, and are 

 less likely to remain submerged within the common average of the species. 

 Such are the general geological indications. Let attention be given, 

 however, to individual factors. 



ENEMIES OF THE WATERS, AN INOPERATIVE CAUSE 



It has happened with many forms that when the pressure of enemies 

 has become too great, this pressure has gradually forced them to some 

 degree into a new environment and a new mode of life. Flying fish leap 

 into the air. Beavers and muskrats dive to safety. Many rodents escape 

 into trees or burrow into the earth. Could this pressure on fishes in their 



