664 GEOLOGY. 



poses of comparison with human history, and for forecasting the future 

 of man and of the whole biological kingdom. Moreover, floras and 

 faunas, as such, are used in the correlation of formations, and in this 

 application they give surer results than correlations by individual 

 species. A particular species may live far beyond the usual period 

 of a species, and if fossiUzed in one region in its early history and in 

 another in its late history, the two formations might be referred erro- 

 neously to the same stage. This is far less likely to happen with a 

 whole assemblage of forms. There is a similar liability to error in 

 interpreting migrations on the basis of a single or a few species, for a 

 single species or a few species may be transported by unusual or acci- 

 dental means, so to speak, when there is no normal pathway for general 

 migration, and when no systematic migration takes place. In most 

 of the great questions that arise concerning the connections and dis- 

 severances of the continents, and concerning the unions and separations 

 of the oceans, which are the fundamental causes of the migrations and 

 of the isolations of plants and animals, typical floras and faunas are to be 

 studied, rather than isolated species or sporadic forms. A brief sketch 

 of the leading causes and consequences of these special assemblages of 

 plants and animals may aid in appreciating the underlying significance 

 of floras and faunas, and in interpreting their meaning as they are met 

 in the study of the strata. A part of these grow out of the relations of 

 the organisms to one another, and a part out of the relations of the 

 organisms to their environment. 



(1) Assemblages InfMenced hy the Mutual Relations of Organisms. 



(a) Food relations. — The relations of food-supply are among the 

 most obvious reasons for assemblages. As animals are dependent 

 directly or indirectly on plants for their food, they must gather where 

 the plants grow, or in the currents in which the plant products are 

 borne. Whatever determines an assemblage of plants also causes, or 

 at least invites, an assemblage of animals. Whatever causes an assem- 

 blage of particular plants, invites an assemblage of the particular 

 animals that use these plants. Animals that feed on plants are in turn 

 preyed upon by other animals, and these in turn by others. A whole 

 train of organisms may, therefore, be gathered into a region by the con- 

 ditions that foster a certain kind of vegetation there. In interpreting the 

 physical significance of such a train, it is obvious that the head of the 



