]914] Grinnell: Mammals and Birds of the Colorado Valley 109 



far short of coast line.s. The barriers here concerned are intangible 

 but nevertheless powerful. By their action the spread of species, 

 genera, and families is held in check as surely as by any tangible 

 obstruction. 



By these invisible barriers the individual may not necessarily be 

 stopped at all, as with animals of free locomotion ; but the species is 

 affected. For example, the mockingbird (Mimus pohjglottos leucop- 

 terus) in its Calif ornian distribution is closely confined to those parts 

 of the state possessing certain definite climatic features; but vagrant 

 individuals, especially in autumn, occur beyond the limits of these 

 restrictive conditions. Carnivorous mammals are well known to lie 

 subject to sporadic wanderings on the part of individuals, but the 

 species is kept in set bounds by some potent but invisible set of factors. 

 The very fact that individuals are quite capable of temporarily trans- 

 gressing these bounds and yet do not overstep them en masse 

 emphasizes all the more the remarkable potency of this category of 

 barriers as regards species and higher groups. 



Our geographic studies lead us to designate among these relatively 

 intangible barriers: (1) increase or decrease in prevailing temperature 

 beyond certain critical limits, according to the species concerned; (2) 

 increase or decrease in prevailing atmospheric humidity beyond cer- 

 tain limits; (3) modification in food-supply and appropriate breeding 

 and foraging ground as regards the inherent structural equipment of 

 each animal considered. In these three sorts of barriers will be recog- 

 nized the zonal, faunal and associational delimitation as discussed in 

 previous chapters. 



CLASSIFICATION OF BARRIERS TO SPECIES AS REGARDS BIRDS 

 AND MAMMALS 

 Barriers: 



A. Tangible (mechanical) 



(a) Land to aquatic species 



(b) Bodies or streams of water to terrestrial species 



B. Intangible (non-mechanical) 



(a) Zonal (by temperature) 



(b) Faunal (by atmospheric humidity) 



(c) Associational 



(1) By food-supply 



(2) By breeding places 



(3) By temporary refuges 



(Each of these three with regard to the inherent struc- 

 tural characters of each species concerned). 



