186 MICHIGAN SURVEY, 1908. 



6. Contimiity and directness of individual variations. The continu- 

 ity amdi directness or definiteness of individual variations along routes 

 of dispersal may give very definite information as to tlie direction of 

 origin. This is perhai»s not of universal application but carries much 

 weight under certain conditions. For example, continuity of variations, 

 as dwarfing or increasing size, have a certain definiteness which clearly 

 points in a limited number of directions, when correlated with liighways 

 of dispersal (cf. Horn 72, p. .383). This is particularly so when a route 

 is of a restricted character, as a drainage line, or a valley. If these 

 variations were entirely promiscuous along lines of dispersal, there could 

 be no idea of direction ; but by taking into consideration the entire range, 

 as one is perfectly justified in doing, continuity and directness clearly 

 point in a given direction. It is mainly when the animals along a route 

 are uniform or promiscuous that direction cannot be determined by the 

 character of the variation. This criterion, as restated, like most other 

 criteria should not be used independently. Compare Tower '06, pp. 

 12-13. 



7. Geographical centers and climax associations. To apply this 

 ecological criterion it is necessary to understand the principles which 

 underlie the succession of beetle associations or societies. By a beetle 

 association is meant that combination of beetles which occur asso- 

 ciated in the same breeding habitat. As the environmental conditions 

 upon which beetles depend change, the beetles also change and thus a 

 succession is produced. The same general principle holds for a. beetle 

 association. Thus as the conditions change the association also 

 changes and a aur-rcfision of hcetlc associations is produced. When, 

 however, a relatively complete adjustment or equilibrium is acquired, 

 and changes become slight, a self perpetuating or climax association or 

 formation has become established. Areas occupied by formations, 

 through their abundance and dominance, become centers of dispersal, 

 although tliey are probably more productive or originative, at an in- 

 termediate stage, before the dominance of the climax association is 

 fully established. 



Members, therefore, of such climax associations may be expected to 

 point in the direction of such centers as include their associated species. 

 If such a relation is valid, the various characteristics of climax associa- 

 tions will aid in the determination or location of centers of origin and 

 dispersal. Such criteria may have more value in determining centers 

 of dispersal than those of origin. This criterion will probably a]>ply to 

 secondary societies, but with attenuated force. 



8. Dominance. This is a fundamental criterion in the determination 

 of ecological associations. The tendency for certain associated species 

 to obtain exclusive possession of any given area implies the abundance 

 of individuals and their dominance. This idea is prevalent and funda- 

 mental in ecologic studies. This is also a relative term, and like all 

 other criteria, has its limitations. Dominance in a desert must in 

 general have a difl'erent meaning than in a humid area. 



9. Bicgeographical or ecological affinities. In its broader applica- 

 tion this criterion is applicable to general biotic relations and to large 

 areas. It is one of the oldest criteria used in the determination of faunal 

 and floral affinities. In some respects it is closely related to No. 7. This 



