No. 484] CLOSELY RELATED SPECIES 



221 



the ^Mutation Theory, but is drawn from the geographic evidence. 

 It is, however, true that the evidence is rather scanty. ]More- 

 over there are some exceptions to the general law of distribution, 

 and if these exceptions should, upon further research become 

 very numerous, the prejudicial force of the law would be much 

 diminished. But from the evidence at hand we may infer the 

 very general truth that animal species are distril)iit(Ml according 

 to Jordan's law of geographic isolation; that when exceptions 

 occur, the exceptional species are taken over into some other 

 category of isolation. The nearly universal patch-work char- 

 acter of specific chorolog}' — as at present depicted in the works 

 of zoologists — strongly suggests the gradual spreading out of 

 individuals over the surface of the earth, their settlement here 

 and there in isolated districts or topographically (h'stincr stations, 

 where shielded from promiscuous intercrossing they hav(> under- 

 gone transformations, which have been different in the (htlVrcnt 

 areas; transformations wliieh, advancing l)y whatt'ver force> or 

 conditions, whetlier tho>e of Xatiu'al SehM tif)n or of ortlion-cne-^is, 



history is that which forms itself in the imagination of most students 

 of animal geography and has appealed most strongly to me as I 

 have reviewed the literature of the subject. 



The Distribution of Plants. 



Turning now to the vegetable kingdom we find, first, that 

 there have been few or no exhaustive (vssays (U'ah'ng with the 

 question of specific (hstrihniion in n'htiion to the theorv of evohi- 

 tion. In the second j)hice, it may be >ai(l at once that when 

 botanists have tm-ned their attention in thi> (h'rection their \ iews 

 generally do not coincide with tlio.->e of the zoologists as to the 

 nature of the facts. 



NiigeH (73) opposed Wagner in a paper of uhich the purport 

 is succinctly ex|)resse(] in the iith\ "Die gt^scllschaftHche Mntstch- 

 ung neuer Spezies," the social oriyln of new species. This 



acter of specific distribution, to the study of which he gave much 

 time in the field for several years, lie calls particular attention 



