482 University of California Publications in Zoology [Vol. 12 



two propositions (geographic isolation a condition to divergence in 

 higher vertebrates; classes of vertebrates descended from one or a 

 very few original stocks) extensive invasions and re-invasions, each 

 geographical locality would now be occupied by one mammal, or one 

 bird, or one reptile, only. 



Following out this thought it would appear that there have been 

 as many mammalian invasions or re-invasions for each locality as 

 there are species or subspecies of mammals within it — unless so 

 large a locality is taken as to contain barriers which would serve 

 to isolate portions of the stocks from each other. This would seem 

 to afford a confirmation from the standpoint of the theoretical 

 necessities of isolation, of the axiom, stated by C4rinnell (1914, p. 

 98) to be a necessary result of the ever-shifting location geograph- 

 ically of associational, faunal, and zonal conditions ; this axiom is 

 that "every single element or line of descent, now represented in 

 the biota of any locality, must have come either in its present form 

 or in some antecedent one from somewhere else." 



In almost every locality there are ecologic niches which are 

 unoccupied. In many instances there exist animals which could 

 occupy these niches if they could get to them ; but barriers of one 

 sort or another effectually prevent their getting to them. This con- 

 stitutes an important line of negative evidence which is comple- 

 mentary to the positive line. If an organic stock were capable of 

 divarication in a single locality, given geologic time all occupiable 

 niches ought to have been filled. 



Of course the species resident in a locality might and probably 

 would become modified monotypically through a progressively 

 changing environment or from some other cause or causes, but there 

 would never ensue any differentiation. To increase the number of 

 species or subspecies of mammals in the locality further invasions 

 or re-invasions are called for. 



Tentative Suggestions Regarding the Manner in Which Geographic Isola- 

 tion Acts in the Process of Speciation 



Although in most cases laboratory work is done with forms low 

 in the organic scale, and it is questionable how broadly the conclu- 

 sions based upon such work may be applied, some of the results of 

 experimental work are exceedingly suggestive as to what may be 

 taking place in nature in widely separated classes of organisms. 

 As Thomson says (1909, p. 329), "The world of organisms is very 



