148 THE AMERICAN NATURALIST [Vol. LVI 



ford,' working independently, have shown that very slight 

 structural differences that distinguish closely related 

 species of amphipods and tiger-beetles are correlated 

 with distinct habitat preferences. The structure and 

 physiology of animals are modified by environment— the 

 structures and activities of the systems are changed. 

 Different species may possess almost identical structures, 

 but show specificities of behavior in relation to environ- 

 ment. 



Darwin made much of the struggle for existence among 

 animals, pointing out that many species hold their places 

 on the earth through wide dissemination and selective 

 survival. One who has seen the strangler trees gaining 

 a foothold in the tropical forest, the fiddler crabs fighting 

 to hold a favorable place on an ocean beach, or the oysters 

 in an overplanted area striving to survive, can not 

 doubt that there is such a struggle. More animals are 

 produced than can find a place to exist, and in general 

 those survive that are best suited to the environment that 

 is available. 



Animals are not always obliged to adjust themselves 

 to the environment or struggle for a favorable place to 

 live in it. They migrate from situations where their sys- 

 tems can not well carrj- on activities to some spot where 

 conditions are more propitious. In such migrations ani- 

 mals have very definite relations to the environment. 

 They are limited by their reaction pattern to certain 

 habitats; they must disperse from their "centers of 

 origin" through ''highways," and are prohibited from 

 migration into certain regions called "barriers." Bar- 

 riers are areas where certain environmental factors vary 

 beyond the limit of toleration for a species. A "center 

 of origin" as usually understood by geographers, may be 

 the real place of origin of a species or it may merely rep- 

 resent the locality where the most environmental factors 

 are favorable. In general a uniform environment cover- 

 ing a wide range of territory permits the species suited 

 to such an environment to have a wide geographic range. 



T Biol. Bull, 1911. 



