Febkuaby 16, 1906.] 



SCIENCE. 



255 



the change in the species is gradual; and 

 conversely, where the geographic change is 

 abrupt the change in the species is abrupt. 

 Changes in a north-and-south direction are 

 likely to be gradual; those in an east-and- 

 west direction are likely to be more abrupt. 

 In both cases, the controlling power of en- 

 vironment is easily recognized. It follows 

 that one species does not change here, an- 

 other there; the rule — to which there are 

 exceptions— is that all change in a common 

 belt or area. It thus comes about that 

 American students of species, from Baird's 

 time to the present, have learned to recog- 

 nize certain geographic areas, usually in 

 the form of belts, where the transition from 

 one set of animals and plants to another set 

 takes place. These belts of intergradation, 

 broad or narrow as the case may be. are 

 always flooded with intergrades, which, 

 though the bane of the museum man, are 

 of great importance to the student of vari- 

 ation and evolution. ' 



In order to make the matter perfectly 

 clear — for it is one of no small importance 

 to evolutionists— let me cite a few ex- 

 amples. 



The common prairie ground squirrel or 

 striped spermophile {Citellus tridecemlme- 

 atus) splits up into four geographic forms 

 or subspecies. The range of the species as 

 a whole extends from the plains of the 

 Saskatchewan on the north to the coast of 

 Texas, and from the extreme eastern edge 

 of the prairie country in the Great Lake 



"Among the belts of this kind are the overlap- 

 ping boundaries of the life zones and certain other 

 lines which mark the change from one physi- 

 ographic or physiognomic type to another. 

 Among these may be mentioned a strip along the 

 western edge of the deciduous forests, where wood- 

 land gives place to prairie ; a belt near the ninety- 

 ninth meridian, where the Immid prairies change 

 to the arid plains; a belt along the east base of 

 the Rocky Mountains, where plains forms change 

 to mountain forms; and similar or corresponding 

 areas in the Great Basin and in the interior of 

 California. 



region westerly to and a little beyond the 

 Rocky Mountains (see map. Fig. 3). Typ- 

 ical tridecemlineatus occupies the eastern 

 and more humid part of this area. In 

 ranging westward it undergoes a change, 

 becoming paler and smaller as it enters the 

 arid plains. The change occurs between 

 the one-hundredth and one hundred and 

 first meridians, from South Dakota to 

 middle Texas, and the resulting pallid form 

 (subspecies pallidiis) continues westerly to 

 the foot of the Rocky Mountains. In pass- 

 ing southward' from the Upper Austral to 

 the Lower Austral zone tridecemlineatus 

 develops another form (subspecies texen- 

 sis), which occupies a broad belt in Okla- 

 homa and eastern Texas, east of the range 

 of subspecies pallidus. But this is not all, 

 for the plains form (pallidus) in pushing 

 westward over some of the passes of the 

 Rocky Mountains gives off still another sub- 

 species (parvus), which occupies the Green 

 River Basin in Wyoming, and extends 

 thence southerly in an irregular and inter- 

 rupted belt along the border between Colo- 

 rado and Utah, and occurs in isolated col- 

 onies in New Mexico and extreme eastern 

 Arizona. Between these several forms are 

 the belts of intergradation, as shown on the 

 accompanying map (Pig. 3). 



The case is one in which a well-marked 

 species splits into four geographic forms 

 or subspecies in conformity with the cli- 

 matic and physical features of the region 

 inhabited. Bach form is fairly constant 

 throughout the major part of its range and 

 develops intergrades in the transitional belt 

 between it and the next form. 



Among North American mammals and 

 birds there are hundreds of such cases, and 

 more than 1,000 species and subspecies, 

 connected with other forms by series of 

 intergrades, might be enumerated. 



It may be set down as a general law that 

 wherever a species or subspecies passes into 

 another, intergrades occur, not side by side 



