144 SOCIAL EVOLUTION 



governing power fell into the hands of vigorous northern 

 barbarians. The civilization of Rome declined.-^ ^ 



We have now examined two kinds of climatic change, 

 latitudinal and pulsatory. It remains to consider the 

 influence of more local changes, what we commonly call 

 the weatj_ier. Careful investigation has confirmed the 

 popular belief that clear, cool weather is invigorating. 

 Dexter, in his book, "Weather Influences," has made 

 a study of the influence of various meteorological con- 

 ditions upon the conduct of school children, upon the 

 occurrence of crime, and upon the number of errors made 

 by bank clerks. In damp, muggy weather people feel 

 disagreeable and suppose themselves ready to do all sorts 

 of evil things. As a matter of fact they do not do them, 

 for the vital functions are so far depressed that there 

 is no surplus energy to spend in doing anything very 

 active, either good or bad. fixy, windy days stimulate 

 the vital processes, unless it be exceptionally warm, and 

 create a surplus of energy which finds expression in work 



31 Tlie evidence employed to substantiate the theory of pulsatory climatic 

 changes is roughly of four kinds. The first kind consists of physiographic 

 phenomena such as river terraces, lake strands, denuded mountain 

 slopes, desiccated springs, and rivers whose salinity has increased. A second 

 kind consists of archeological phenomena, ruins of great cities in places 

 whose supply of water is not now one-tenth large enough to support such 

 a population as once existed. A tliird kind of evidence consists of his- 

 toric accounts of famines, of old roads across tlie desert which to-day are 

 impassable. Finalh', evidence is based upon plant life. The thickness of 

 the rings of annual growth in old trees has been found to be proportional 

 to the amount of rainfall. Huntington measured the rings of annual growth 

 of 450 of the Big Trees of California, Sequoia gigantea, and plotted the 

 curve of climatic pulsation indicated by variation in these rings. The 

 trees which were measured were from 230 to 3200 years old. Eighty were 

 over 2000 years old and three more than 3000 years old. The /iata was, 

 therefore, quite comprehensive. The curve showed a remarkable verifica- 

 tion of the theory of pulsatory climatic change. The dry periods corre- 

 sponding with the periods of desiccation shown by other phenomena. See 

 "Changes of Climate and History." 



