No. 484] CLOSELY RELATED SPECIES 



219 



of distribution brought forward for several groups of mammals 

 are valuable. Dr. Merriam considers the geographic relations 

 of certain American rats, chipmunks, and ground squirrels; and 

 refers besides to other groups. His representation of specific 

 distribution agrees with that held by Wagner, with a qualification. 

 Merriam shows that the mammals in question occupy distinct 

 areas with very little exception, but that the areas often overlap, 

 and that the overlaps are likely to constitute narrow transition 

 zones characterized by the presence of intergrades. Actual phy- 

 sical barriers are often wanting. 



President David Starr Jordan has also discussed the Mutation 

 question from the standpoint of organic geography and assembled 

 from his own experience and that of others a considerable body 

 of evidence regarding birds, while he himself speaks for fishes 

 (Jordan, :05). His paper, which appeared in Science a little 

 more than a year ago, contains some extraordinarily sweeping 

 assertions. He says: . . . .Moritz Wagner (1868) first made 

 it clear that geographical isolation (niumliche Sonderung) was a 

 factor or condition in the formation of every species, race, or 

 tribe of animal or plant we know on the face of the earth." The 

 principles set forth by Wagner ''have never been confuted,^ 

 scarcely even attacked, so far as the present writer remembers, 

 but in the literature of the present day they have been almost 

 universally ignored." The question is much discussed whether 

 minute variations may serve to establish a new species in the 

 presence of a parent species, or whether wide fluctuation or muta- 

 tion may do so. "In theory either of these conditions might 

 exist. In fact both of them are virtually unknown. In nature 

 a closely related distinct species is not often found quite side by 

 side with the old. It is simply next to it, geographically or geolog- 

 ically 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." "The 

 contention is not that species are occasionally associated with 



' See the works of Darwin (72), Romanes, Weismann (72), and Niigeli cited 

 in the Bibhography. Weismann's paper relying upon the case of Planorbis 

 multiformis in the Steinheim chalk should be considered in connection with 

 Hyatt's Memoir on the same form ('80). 



