1!»4 Ciiin rsity of California Publications in Zoology [Vol. 10 



species: Melospiza m. pusillula, Melospiza m. samuelis, Melospiza 

 in. maxillaris, Reithrodontomys raviventris, Reithrodowtomya 

 halicoetes, Microtus edax, Sorex sinuosus. In each ease the 

 habitat of the upland near relative adjoins that of the salt or 

 brackish marsh form, the palustrine conditions being in some 

 cases actually confluent from fresh to salt water. 



It would appear that we find here a form of isolation not 

 referable to the ordinary geographic kind, but dependent upon 

 some factor other than separation by long distance or interposi- 

 tion of impassable harriers. There seems to be existent a strong 

 preferment on the part of individuals for the fresh-water condi- 

 tions on the one hand or the salt-water on the other; and this 

 preferment is a racial trait. 



The halophilous species of all three differentiation areas show 

 community in one character: depth of color, that is, quantity of 

 pigmentation, as compared with their near relatives. Only one 

 exception presents itself, that of the clapper rail. In matter of 

 size, all four of the species of the Suisun district (3) are giants 

 as compared with their near relatives; while around the south 

 arm of San Francisco Bay (1) four of the palustrine species are 

 distinctly dwarfs. The only obvious allocation of cause and 

 effect is shown in the association of the dark coloration with 

 halicoetic habit. But we find without exception that the blackest 

 species are in the Suisun area where the water of the marshes 

 is least salt ! 



These facts are called to the attention of students of speeiation 

 with the hope that parallels may be reported from elsewhere. 

 Through the cataloging of instances of such interrelationships, 

 especially where conspicuous physical peculiarities of the environ- 

 ment are involved, it is to be confidently expected that much 

 clearer insight is to be gained into the behavior of species in 

 nature. 



