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THE AMEBIC AN NATURALIST [Vol. XLVIII 



In the third category of distributional control there is a con- 

 spicuous association of the majority of so-called adaptive struc- 

 tures of animals (often of high taxonomic value) with certain 

 mechanical, or physical, features of their environment. An 

 animal may thus intimately depend upon certain inorganic or 

 organic peculiarities, or both, of a given area, and be unable to 

 maintain existence beyond the limits of occurrence of those 

 features of the environment. Tracts of relatively uniform en- 

 vironmental conditions, including their inanimate as well as 

 living elements, are here called associations. 



After a consideration of all the birds and mammals occurring 

 both within the state of California and elsewhere as far as the 

 writer's knowledge goes, associational restriction appears to be 

 governed by the following three factors, of relative importance 

 in the order named. 



1. Kind of food-supply afforded, with regard to the inherent 

 structural powers of each of the animals concerned to make it 

 available. 



2. Presence of safe breeding places, adapted to the varying 

 needs of the animals, in other words depending upon the respect- 

 ive inherent powers of construction, defence and concealment 

 in each species concerned. 



3. Presence of places of temporary refuge for individuals, 

 daring daytime or nighttime, or, while foraging, when hard- 

 pressed by predatory enemies, again correlated with the respec- 

 tive inherent powers of defence and concealment of each species 



It is believed that the geographical distribution of any animal 

 is correctly diagnosed in terms of each of the three main group- 

 ings here suggested. In other words an animal belongs simul- 

 taneously to one or more zones, to one or more faunas, and to 

 one or more associations. No one of these groupings can be 

 stated in terms of the other, any more than a person can com- 

 pute liquids by candle-power, or weight in miles. The constit- 

 uent species within each of these groupings always belong to 

 the other two. To illustrate: the southern white-headed wood- 

 pecker inhabits the coniferous forest association of the San 

 Bernardino fauna of the Transition zone; the Abert towhee be- 

 longs to the mesquite and the quail-brush associations of the Colo- 

 rado Desert fauna, of the Lower Sonoran zone ; the Pacific shrew 

 belongs to the upland riparian association of the northern coast 

 redwood fauna of the Transition and Boreal zones. 



