490 University of California Publications in Zoology [Vol. 12 



the only effectual environmental impact is that which is received 

 by the germ-cells; therefore it is not opposed to such a conclusion, 

 already reached experimentally in widely different groups of 

 organisms. 



17. There is evidence from vertebrate zoogeography that indi- 

 cates the cumulative character of the differentiation associated with 

 geographic isolation. 



18. The different ecologic niches in the same locality, so far as 

 they are occupied by a given class of organisms, would seem to 

 have been filled, not through processes of differentiation and adapta- 

 tion in that single locality, but through those of migration, geo- 

 graphic isolation, differentiation, concurrent adaptation to different 

 niches, and final invasion or re-invasion of the locality in que'stion — 

 to occupy different niches there. If geographic isolation is a con- 

 dition essential to speciation in the higher vertebrates, and if each 

 class of the higher vertebrates is derived from one or a few ances- 

 tral stocks, it follows that the tracing of these processes is highly 

 important if not indispensable, either to an adequate understanding 

 of the course of development of the class, or to a thorough concep- 

 tion of any particular facies of the class. Among these processes 

 the principles of invasion and re-invasion, which are apparently of 

 great significance in explaining the class-assemblage of a given 

 locality, have been less emphasized than their importance deserves. 



Transmitted November 18, 1914. 



LITERATURE CITED 



Allex, J. A. 



1877. "Castoridae" in "Monographs of North American Rodentia" in 

 Report of the U. S. Geol. Surv. of the Terr., 11, no. 6, pp. 

 427-454. 



Bailey, V. 



1905. Biological survey of Texas. U. S. Dept. Agric. Bureau Biol. 



Surv., N. Amer. Fauna, 25, 1-222, pis. i-xvi, 24 figs, in text. 

 1913. Two new subspecies of North American beavers. Proc. Biol. 

 Soc. Wash., 26, 191-193. 



Baker, M. 



1906. Geographic dictionary of Alaska. U. S. Geol. Surv., Bull. 299 



(ed. 2, Washington, Government Printing Office), 1-690. 



Bangs, 0. 



1913. The land mammals of Newfoundland. Bull. Harvard Mus. 

 Comp. Zool., 14, 509-516, 1 fig. in text. 



