322 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



irenetifists are now at work, with the handicap that their studies 

 look forward to chan«::es that may be crt'ected, not backward to ancient 

 responses to shifting environment. 



In the study of any species whatever, we encounter four factors: 

 Two intrinsic — heredity and variation; two extrinsic — selection and 

 isolation or separation, with its accompanying segregation, the one 

 insuring the nonsurvival of the nonadaptable, the other leading to 

 mating by propinquity, through biological friction Avhich prevents 

 wide crossing by interrupting the fluidity of life. Factors other 

 than these four may exist, but in the history of every individual of 

 which a species is composed, each of these has been potent. With 

 that fact in mind, in view of the great range of investigation cover- 

 ing these matters, one may affirm that no wide-reaching biological 

 problem is more completely explained than that of "the origin of 

 species.'' 



The historic origin of individual species of living organisms runs 

 closely parallel with the origin of individual words in a language. 

 One may trace the derivation of thousands of words, while yet hesi- 

 tating or " expressing agnosticism " as to the origin of language. 

 In like fashion, we may trace back to their original stocks thousands 

 of anin)al or plant species and still hesitate about or " express 

 agnosticism " as to a complete definition of biological origins. For 

 after all each one has its own liistory, including vicissitudes of 

 migration, selection, and separation; and the theoretical generaliza- 

 tion can be only an inductive summing up of all evidence obtained. 



In the study of species as related to geographical conditions, one 

 is most impressed by the recognition of " twin " species, forms closely 

 related but nevertheless distinct, separated from each other by some 

 kind of barrier. To similar parallel forms I gave, in 190S, tlie name 

 of "geminate species." These agree wnth each other in generic 

 structural traits. In all matters of adaptation to environment, pre- 

 sumably results of selection, they may be absolutely identical, as 

 also in habits unless confronted by some novel condition. They 

 differ in minor regards, presumably of later origin than the generic 

 traits. 



I indicated the "law of geminate species" as follows: Given 

 any species in any region, the nearest related species is not likely to 

 be found in the same region, nor in a remote region, but in a neigh- 

 boring district separated from the first by a barrier of some sort or 

 at least by a belt of country the breadth of which gives the effect 

 of a barrier. 



The ornithologist. Dr. Joel A. Allen, accepted this generalization 

 and called it "Jordan's Law," though of course it rests on the 



