=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, however, 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 modified, 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 modification just described, is nicely illustrated by the balsam-spruce formation at Minnehaha. Of the initial gravel slide stage, the relicts are _Vagnera stellata_ and _Galium boreale_, the one modified into _Vagnera leptopetala_, and the other into _G. boreale hylocolum_. The thicket stage is represented by _Holodiscus dumosa_, greatly changed in form and branching, and in the shape and structure of the leaf. The most striking relict of the aspen formation is the facies itself, _Populus tremuloides_. The tall slender trunks of dead aspens are found in practically every balsam-spruce forest. In many places, living trees are still found, with small, straggling crowns, which are vainly trying to outgrow the surrounding conifers. Of the aspen undergrowth, _Rosa sayii_, _Helianthella parryi_, _Frasera speciosa_, _Zygadenus elegans_, _Castilleia confusa_, _Gentiana acuta_, and _Solidago orophila_ remain more or less modified by the diffuse light. It is still a question whether the aspen stage passes directly into the balsam-spruce forest, or whether a pine forest intervenes. The presence of both _Pinus ponderosa_ and _P. flexilis_, which are scattered more or less uniformly through the formation, furnishes strong evidence for the latter view. [Illustration: Fig. 71. Relict spruces and aspens, showing the character of the succession immediately preceding the burn succession now developing.]