CHAPTER IV 

 THE PULSE OF LIFE 



RICHARD SWANN LULL 



PROFESSOR OF VERTEBRATE PALEONTOLOGY IN YALE UNIVERSITY 



The stream of life flows so slowly that the imagination fails 

 to grasp the immensity of time required for its passage, but 

 like many another stream, it pulses as it flows. There are 

 times of quickening, the expression points of evolution, and 

 these are found to be coincident with geologic change. These 

 coincidences are so frequent and so exact that the laws of 

 chance may not be invoked to account for them. They stand 

 to each other in the relation of cause and effect. 



This does not imply the acceptance of any one philosophical 

 factor of evolution, for whether the creature is directly modi- 

 fied by environmental change, or indirectly through induced 

 habit, or whether nature merely sets a standard to which the 

 organism must attain if it would survive, matters not; the 

 fact remains that changing environmental conditions stimulate 

 the sluggish evolutionary stream to quickened movement. 

 Whenever it has been possible to connect cause and effect, the / 

 immediate influence is found to be generally one of climate, 

 back of which lies, as the main cause, earth shrinkage and a 

 consequent warping of the crust, with the elevation and spread | 

 of the lands and the formation of mountain ranges. In addi- 

 tion to this mundane cause, there are the complex rhythms in 

 solar energy and the consequent variation in the amount of/ 

 solar-derived heat. For example, the most generally accepted 



