EVOLUTION IN NATURE 153 



creek, "but to his surprise and dismay he found 

 them corresponding exactly to the markings of 

 the English brook trout." Jordan cites further 

 instances as follows: "Dr. Day speaks of the 

 Loch Leven trout as changing into ordinary brook 

 trout, when planted in streams of Gloucester or 

 Guildford, the colors of the Loch Leven trout 

 being seen on exceptionally well fed individuals 

 only. In Australia, according to Day, fine ex- 

 amples of the Great Lake trout, Salmoferox, weigh- 

 ing twenty pounds, have been reared from eggs of 

 Salmo fario [English brook trout], taken in Hamp- 

 shire and Buckingham." Here we see that actual 

 species of the accepted classification, not mere geo- 

 graphic races, owe their differences to the condi- 

 tions under which they live. 



In contrast with the case of the trout, Sumner 

 has found that the subspecies of California deer- 

 mice {Peromyscus maniculatus) , are germinally 

 fixed, regardless of the conditions under which they 

 are reared.^ Mice of the subspecies rubidus, 

 sonoriensis, and gambeli, from Eureka, the Mohave 

 Desert, and Berkeley and La Jolla, respectively, 

 were studied for correlation of their characters with 

 the conditions of their environments, and later 

 specimens of sonoriensis were reared at Berkeley. 

 ^^ Neither the originally introduced animals nor their 

 offspring, nor their grandchildren, have thus far 



7 Am. Nat., Vol. XLIX, pp. 688-701, 1915. 



