96 University of Califortiia Publications in Zoology [Vol. 12 



one another. Incidentally all other associations nia.y be crossed, back 

 and forth. Ardea Jicrodias trcgamai foraged far and wide in the 

 river association, but reared its young in restricted parts of the willow- 

 cottonwood association where lofty trees provided safe nesting sites. 

 Aeronautes melanoleucus found safe rendezvous in crevices of cliffs, 

 but foraged far over the lowlands; so also with certain bats. The 

 relations between animals and their environments are manifold and 

 complicated; and these interrelations depend both upon the inherent 

 peculiarities of the entire organism in the case of the animal and upon 

 the physical nature of the environment. 



After a consideration of all the birds and mammals mentioned in 

 the present report, as occurring both in the region specifically treated 

 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 struc- 

 tural 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 respective inherent 

 powers of construction, defence and concealment in each species con- 

 cerned. 



3. Presence of places of temporary refuge for individuals, during 

 day time or night time, or while foraging, when hard pressed by 

 predatory enemies, again correlated with the respective inherent powers 

 of defence and concealment of each species involved. 



