XV. THE INVESTIGATION OF SUCCESSION. 



Primary methods. — ^There are three primary metiiods of investigating suc- 

 cession: (1) by inference; (2) by sequence, (3) by experiment. Investigation 

 by inference consists in piecing together the course of development from the 

 associes and consocies found in a region. From the very nature of succession, 

 this method was necessarily the first one to be employed, and its use still 

 predominates to the practically complete exclusion of the other two. This 

 is easily vmderstood when one recalls that it is the only method that can be 

 applied in studies lasting but one or two seasons. Moreover, the interpreta- 

 tion .of successional evidence has reached a point where inference often yields 

 fairly conclusive results, and regularly furnishes the working hypotheses to 

 be tested by the methods of sequence and experiment. In a complete system 

 of investigation, inference can only furnish the preliminary outline, which 

 must be subjected to thoroughgoing test by means of sequence and experiment 

 before the course of succession can be regarded as established. However, it 

 must be recognized that the value of inference, even when used alone, must 

 steadily increase in just the degree that it is confirmed by the other two 

 methods. Successional studies have been slow in making their way in ecology, 

 in spite of their fundamental value, because of the labor and time demanded 

 even by the method of inference. The adoption of the more conclusive 

 and exacting methods of sequence and experiment will be slower stUl, but there 

 would seem to be no serious doubt of their final and complete acceptance. 



The method of sequence consists in tracing the actual development of one 

 or more communities in a definite spot from year to year. In short, it is the 

 direct study of succession itself as a process. It is clear that sequence must 

 furnish the basic method of study, and that the value of inference and experi- 

 ment depends upon the degree to which they reveal the sequence itseK. If 

 the whole course of development from bare area to climax required but a few 

 years, or a decade or two at most, the method of sequence would give us a 

 complete account of succession. But even the shortest of secondary seres 

 require a decade or longer, and most of them demand more than the working 

 period of a life-time. Primary seres rarely if ever complete their development 

 within a century, and the large majority of them last through several centuries, 

 or even millenia. As a result, the method of sequence can not be applied 

 directly by a single investigator to the whole course of development from the 

 pioneer colonies in water or on rock to the final grassland or forest climax. 

 Three possible solutions present themselves, however. He may carry his 

 studies of a particular commvmity as far as possible, and then turn his records 

 of the development over to a younger investigator, who will carry the record 

 through another life-time. Such a method requires concerted action such as 

 is unknown at present, but there can be little question that continuous investi- 

 gations of this nature will soon be organized by great botanical institutions. 

 In fact, an approach to it has already been made by the Desert Laboratory of 

 the Carnegie Institution of Washington and by some of the experiment stations 

 of the United States Forest Service. So far, however, research is chiefly a 

 fimction of the individual investigator, and he will seek one or both of the other 

 solutions. The most obvious one is to make a simultaneous study of the 



423 



