J 40 THE CLIMATIC P^ACTOR AS ILLUSTRATED IN ARID AMERICA. 



do not cover the whole of a single one of the longer cycles. Yet long records are essential 

 if we would understand what has really happened in the past and thus put ourselves in a 

 position to determine the cause of j^resent changes and to arrive at the ability to predict 

 those of the future. Such a knowledge is equally essential if we would understand all tlie 

 varied and important effects which climatic pulsations, working through famine, migration, 

 and other causes, have produced upon man in the past and are likely to produce upon him 

 in the future. Only by such knowledge of the past, apparently, and by the ability to 

 predict the future, can we be sure to avoid calamities akin to some which have over- 

 whelmed certain portions of the earth at various historic periods. Seemingly, then, it is 

 not only of scientific interest, but of direct practical value, to be able to extend our climatic 

 records as far back as possible in all parts of the world. 



If the method of interpreting the growth of trees introduced by Douglass and amphfied 

 in this volume commends itself to the scientific world and is widely adopted, the work of a 

 decade or two, or at most of a generation, ought to give us curves of tree growth extending 

 back 300 years more or less in all the chief parts of the earth. A comparison of the later 

 portion of each curve with the local meteorological records for a few decades will show 

 what kind of weather conditions promote or retard growth and will enable us to determine 

 the kind of cells and the proportion of different kinds which grow in years of particular types. 

 If the curves of several species are available from the same area, and if the cellular structure 

 is carefully studied in each case, it will not only be possible to interpret each curve from 

 beginning to (nd, but it will also probably be feasible to draw curves representing changes 

 in special types of meteorological phenomena, such as the amount of rain, its distribution, 

 the length and temperature of the growing season, and the other factors which are of chief 

 importance in determining the rate of growth of vegetation. The curves thus obtained, 

 being derived from a great number of trees of several species scattered over a considerable 

 area, will represent average conditions in a way that is possible only with a large number 

 of meteorological stations. More important than this is the fact that the tree curves sum 

 up the effect of all sorts of meteorological conditions upon vegetation. This effect is of 

 paramount importance, as is clear from the fact that the primary object of the greatest 

 weather bureaus is to furnish statistics and predictions of weather for the benefit of agri- 

 culture. Various attempts have been made to combine precipitation, temperature, length 

 of seasons, evaporation, the monthly distribution of rainfall, and other factors into a single 

 curve which should simi up the effect of the weather upon plant life, but the results have 

 not been satisfactory. Tree curves, however, seem to furnish exactly what is wanted. Of 

 course large munbers of them are necessary, but the expense of obtaining them is not 1 per 

 cent as great as the expense of obtaining reliable meteorological records year after year. 



"When we have obtained rehable chmatic curves for various parts of the world covering 

 a period of two or three centuries, we shall probably be in a position to make most instructive 

 comparisons of one country with another. If meteorologists are right in thinking that no 

 great change in the circulation of the atmosphere, or in any of the other climatic elements, 

 can take place in one part of the world without a corresponding change of some sort in 

 other parts, it ought ultimately to be possible to say that a change of a certain sort in 

 California is indicative of sucli and such a change of quite a different kind in China, South 

 Africa, or some other part of the world. Bj^ this I do not mean that minor changes, such 

 as those of a single year, can be correlated in different regions, but only the main ones, those 

 that belong to long cycles (such as Bruckner's 35-year cycle) or, still more, to the much 

 greater cycles of which the studies of this volume seem to furnish evidence. Granting, 

 then, that a pronounced change can not take place in one part of the earth's atmosphere 

 without inducing some sort of variation in other parts, it seems to follow that from a single 

 long curve like that of Cahfornia we can work out the main changes in all parts of the world. 

 Other lines of evidence will furnish assistance, and little by little the curves of other trees 



