140 SUCCESSION 



quantitatively the physical factors of the initial stages, and 

 the reactions produced by the subsequent stages. This 

 should be done by automatic instruments for humidity, 

 light, temperature and wind, in order that a continuous 

 record may be obtained. "Water content and precipitation 

 readings are taken daily or even less frequently, while soil 

 properties, and physiographic factors, altitude, slope, sur- 

 face and exposure, are determined once for all. It is equally 

 needful to determine the development and structure of 

 each stage with particular reference to the adjacent for- 

 mations, to the stage that has just proceeded, and the one 

 that is to follow. For this, the use of the permanent 

 quadrat is imperative, as the sequence and structure of the 

 stages can be understood only by a minute study of the 

 shifting and rearrangement of the individuals. In the ex- 

 amination of successions, since cause and effect are so 

 intimately connected in each reaction, it is especially im- 

 portant that general and superficial observations upon 

 structure and sequence be replaced by precise records, and 

 that vague conjectures as to causes and reactions be sup- 

 planted by the accurate determination of the physical fact- 

 ors which underly them. 



The period of time through which a primary succession 

 operates is usually too great to make its complete study 

 possible within a single lifetime. Secondary successions 

 run their course much more quickly, and a decade will some- 

 times sufKce for stabilisation, though even here the period 

 is normally longer. The longest and most complex suc- 

 cession, however, may be accurately studied in a region, 

 where several exataples of the same succession occur in 

 different stages of development. In the same region, the 

 physical factors of one example of a particular succession 

 are essentially identical with those of another example in 

 the same stage. If one is in an initial stage, and the other 

 in an intermediate condition, the development of the former 

 makes it possible to reestablish more or less completely the 

 life history of the latter. The same connection may be 



