'2,y2 THE FORMATION 



tion, though even here the period is normally longer. The longest and most 

 complex succession, however, may be accurately studied in a region, where 

 several examples of the same succession occur in different stages of de- 

 velopment. 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 interme- 

 diate condition, the development of the former makes it possible to re- 

 establish more or less completely the life history of the latter. The same 

 connection may be made between intermediate and ultimate stages, and it 

 is thus possible to determine with considerable accuracy and within a few 

 years the sequence of stages in a succession that requires a century or more 

 for its complete development. In the Rocky mountains, gravel slides (talus 

 slopes) are remarkably frequent. They occur in all stages of development, 

 and the alternating slides of different ages furnish an almost perfect record 

 of this succession. This method lacks the absolute finality which can be 

 obtained by following a succession in one spot from its inception to final 

 stabilization, but it is alone feasible for long successions, i. e., those extending 

 over a score or more of years. When it comes to be universally recognized 

 as a plain duty for each investigator to leave an exact and complete record 

 in quadrat maps and quadrat photographs of the stages studied by him, it 

 will be a simple task for the botanists of one generation to finish the investi- 

 gations of succession begun by their predecessors. 



330. The relict method of studying succession is next in importance 

 to the method of alternating areas. The two in fact are supplementary, 

 and should be used together whenever relicts are present. This method 

 is based upon the law of successive maxima, viz., the number of species and 

 of individuals in each stage constantly increases up to a certain maximum, 

 after which it gradually decreases before the forms of the next stage. In 

 accordance with this, secondary species usually disappear first, principal 

 species next, and facies last of all. There are notable exceptions to this, how- 

 ever, and the safest plan is to use the relict method only when principal 

 species or facies are left as evidence. An additional reason for this is that 

 secondary species are more likely to be common to two or more formations. 

 In the majority of cases, the relict is not modified, and is readily recognized 

 as belonging properly to a previous stage. This is true of herbs in all the 

 stages of grassland, and in the initial ones of forest succession. The herbs 

 and shrubs of earlier stages, which persist in the final forest stages, are 

 necessarily mollified, often in such a degree as to become distinct ecads, or 

 species. The facies of the stages which precede the ultimate forest are rarely 

 modified. The application of the relict method, together with the modifica- 



