238 DARWINISM TO-DAY. 



their possessors little by little to gain on the parent stock, 

 so that a new species would be established side by side with 

 the old, or on whether a wide fluctuation or mutation would 

 give rise to a new species which would hold its own in com- 

 petition with its parent. In theory, either of these condi- 

 tions might exist. In fact, both of them are virtually un- 

 known. In nature a closely related distinct species is not 

 often quite side by side with the old. It is simply next to it, 

 geographically or geologically speaking, and the degree of 

 distinction almost always bears a relation to the importance 

 or the permanence of the barrier separating the supposed 

 new stock from the parent stock. 



"A flood of light may be thrown on the theoretical prob- 

 lem of the origin of species by the study of the probable 

 actual origin of species with which we may be familiar, or 

 of which the actual history or the actual ramifications may 

 in some degree be traced." 



Dr. Jordan then proceeds to relate and analyse our pres- 

 ent actual knowledge of the make-up of certain local faunae, 

 of the migrations and distribution of certain well-known 

 animal species (especially in the phyla of birds and fishes, 

 in which groups our knowledge of the present status in. 

 North America of species and varieties and their distribu- 

 tion is nearly exhaustive), and of the climatic, topographic, 

 and general geographic barriers 8 which determine this dis- 

 tribution, in a way most convincing to unprejudiced minds. 

 He brings to the support of his own statements of fact and 

 opinion the testimony (contained in personal letters an- 

 swering direct queries from himself) of many well-known 

 American students of systematic and faunistic zoology. 

 Jordan sums up the results of his display of North Ameri- 

 can faunal conditions in various paragraphs, from among 

 which the following are quoted : 



"In regions broken by few barriers, migration and inter- 

 breeding being allowed, we find widely distributed species,. 



