AMERICAN BATS — HANDLEY 



147 



Table 5. — Seasonal distribution of P. mexicanus and P. townsendii in Mexico, as 

 determined from specimens for which collection date is known 



species, reacting to one another as species rather than as related 

 subspecies, however, occurs v/ith some frequency in mammals, al- 

 though it is more conspicuous in birds. It might apply in this instance 

 if it could be proved that both P. mexicanus and the Mexican popula- 

 tions of P. townsendii (P. t. australis) intergrade to the northward 

 with other populations of P. townsendii (P. t. pallescens). Then it 

 could be assumed that the extremely arid environment of eastern 

 Chihuahua and western Coahuila, where big-eared bats are not known 

 to occur, has served as a barrier, sufficiently isolating related popula- 

 tions to the east and v/est of it so that they reacted to each other as 

 species when both segments invaded areas to the south of the deserts. 

 Mexican populations of P. townsendii intergrade northward with 

 other populations of P. townsendii. The zone of intergradation is 

 broad, extending from northern Coahuila, through the Texas Pan- 



