METHODS OF INTERPRETATION. 281 



decisively upon the latter, as well as indirectly through vegetation. Finally, 

 animals and man react not only upon plant life, as shown above, but they may 

 modify or control the habitat itself to a marked degree. As a consequence, 

 the interpretation of the geological and biological conditions of a particular 

 period or era is greatly simplified by two things. First, every fact to be used 

 as evidence has its proper place in the primary sequence indicated above, and 

 second, each bit of evidence can be read in two directions. This last statement 

 is true at least of all the biotic evidence, which constitutes such an indispen- 

 sable part of the record, and of a large pa;rt of the geological evidence as well. 

 Thus, within the limitations of the method, it is clear that grasses indicate the 

 presence of a dry or drier climate, as well as the existence of grazing animaJs. 

 Conversely, the discovery of fossil bison in a certain horizon presupposes a 

 grassland vegetation and a corresponding climate. Similarly, the accumula- 

 tion of plant remains as peat or coal is universally regarded as indicating the 

 existence of swamps due to topographic or climatic causes. Moreover, the 

 actual accumulation of plant remains at such periods further suggests the 

 reactions found in swamps and bogs to-day, and the resulting responses of bog 

 communities. Finally, in the case of man, Huntington's (1914) brilliant work 

 upon the relation of Mayan civiKzations to chmatic and vegetational changes 

 furnishes an outstanding example of the value of the evidence derived from 

 himian populations. Hewitt, Henderson and Robbins (1913) have used the 

 same method to obtain most interesting suggestions as to the disappearance of 

 the Pueblan populations of New Mexico. 



Although the application of this method of causal sequence is barely begun, 

 there is no doubt of the fascinating future before it. An instance of its 

 remarkable possibiUties is afforded by the French Revolution. The relation 

 of human events to vegetation and climate is clearly seen in Stoddard's account 

 (1904 : 8) of the insurrection of October 1789: 



" The summer of 1788 was one of extraordinary dryness. The crop was an 

 almost total failure. Then there came a winter of such unusual severity as 

 had never been known in France since the beginning of history. 'On the 

 31st of December, 1788, the Reaumur thermometer registered 18 degrees 

 below freezing in Paris. Frosts began on the 24th of November and the 

 Seine was frozen from the 26th.' Prices continued to rise and the suffering 

 from cold and scarcity was terrible." 



The significance of this is seen by a scrutiny of Himiphreys's graphs (1913 : 34) 

 of sun-spot cycles, temperature departures, and volcanic activities. The year 

 1788 is seen to fall at a sun-spot maximum, and to follow closely upon the 

 greatest reduction in the mean temperature known since the beginning of 

 reliable records — a reduction coincident with a period of great volcanic activity. 



The method of succession is the direct outcome of the cardinal principle in 

 the interpretation of vegetation. It is based upon the conviction that climax 

 vegetations existed during former geological eras, and that they showed a 

 development and structure similar in all important respects to those of the 

 climax formations of to-day. Bare areas must have arisen in consequence 

 of the action of topographic, climatic, and biotic initial causes. Seres and 

 coseres must have developed in these bare areas much as they do to-day, 

 passing through initial, medial and final consocies to the stable climax. 

 Then, as now, the pioneer life-forms must have been algae and Uchens, and 



