No. 602] SHORTER ARTICLES AND DISCUSSIONS 



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of subjects ; for such a discussion would in most instances lead 

 directly into physics and chemistry, and into a study of the 

 physiological processes of the animals affected by each of these 

 factors. I should, however, like to dispose at once of one of the 

 "factors" listed, and which I hear and see repeatedly cited as 

 a cause of restriction in distribution— particularly in that of 

 birds. 



Many people claim to see in the facts of distribution only the 

 operation of a preference on the part of each animal — by virtue 

 of which, if a heterogeneous lot of animals were introduced into 

 an area presenting diverse conditions, each species would cJioose 

 its ''natural" surroundings and rapidly allocate itself in a nor- 

 mal way. I grant that such a choice would almost certainly be 

 made. In fact the hypothesis is being proved continually all 

 over the country in connection with the migration of birds. 

 Scores of species travel north in the spring to countries for a 

 preceding interval unoccupied; and while, roughly speaking, 

 they travel together, and arrive together, they segregate them- 

 selves, immediately on their arrival, and repair to separate sorts 

 of ground, each species by itself: the pipits to the prairie, the 

 water-thrushes to the streamside thicket, the black-poll war- 

 blers to the spruce forest, and so on. We have here an obvious 

 choice exercised in the selection of habitats. But does this 

 segregation of species by exercise of "individual preference" in 

 a uniform direction change the nature of the problem in any 

 fundamental way ? Should we not here recognize merely a char- 

 acter in the cerebral equipment of each race, which, like every 

 external peculiarity in its structure, is in considerable measure 

 the result of protracted impress upon the organism from the 

 environmental complex of factors to which the race has been 

 subject through past time ? There is no other additional factor 

 than those environmental ones (plus the intrinsic fixedness of 

 the species, within certain limits of plasticity, and the "evolu- 

 tionary momentum") to be called into account. 



As to the mechanism of geographic limitation, the adjustments 

 to the various critical factors are inevitably forever in process, 

 though reduced to a minimum at times of slow environmental 

 change. The refined method of individual "preference" or 

 "choice" is superior to the wasteful process of wholesale de- 

 struction which would be experienced by individuals finding 

 themselves out of place as the result of a haphazard selection of 



