THE AMPHIBIA OF OKLAHOMA 



excitement conditioned by social factors during breeding, (e) a greater 

 tolerance of heat among developing tadpoles in pools, and (f) inherent 

 faster rate of tadpole development (shown clearly only by the spade- 

 foots). The mesic pattern shows the opposite tendencies although in 

 degrees varying with individual species and sometimes with special 

 local conditions. 



It is to be noted that these patterns of behavior are subject to indi- 

 vidual and specific variations over a considerable range; they are not 

 to be taken as absolutely rigid and fixed in all situations, but rather as 

 traits of behavior, whose relative constancy in the total pattern in any 

 one species is notable. 



(9) Breeding habits, habit patterns, and todpole behavior are useful to a 

 limited degree as aids in taxonomy and identification. 



(10) These facts are open to the following generalizations: 



(a) From the nature of the Amphibia, we expect the mesic pattern to 

 be the more primitive. 



(b) On this assumption, the species of grassland and desert arose from 

 mesically adapted ancestors. 



(c) In the dispersal to progressively more xeric environments, the vari- 

 ious factors in the xeric pattern were developed in evolution where 

 partial barriers in the environment were reached. 



(d) Those groups (genera or species) that became xerically adapted 

 tended (with some exceptions) to be restricted to xeric environ- 

 ments thereafter; those that became only partially adapted (B. w. 

 woodhousii) often were able, later, to have a wider geographic 

 range than their more closely adapted relatives; those that failed 

 to become xerically adapted were either stopped in their dispersal 

 by the environmental barrier (e.g. B. t. americanus, H. v. versicolor, 

 B. w. jowleri) so that their geographic range coincided with eco- 

 logical range; or they continued to spread to, and to live in, eco- 

 logically favorable islands within larger, more xeric, ecological com- 

 munities (e.g. Rana catesbeiana, R. clam/ tans, Acris crepitans), and 

 thereby extended their geographic range much more widely than 

 their ecological range would otherwise permit. 



There are some points which are still unexplained by the observations 

 above reported. Why should some xerically adapted forms (e.g. Bnjo cog- 

 natus, B. insidior, B. compact/Us, Pseudacrls clar]{ii) not spread eastward 

 into woodland? If xeric adaptation prohibits this in some groups, why not 

 in all.'' The genus Scaphiopus as a whole manifests the xeric pattern in its 

 extreme form yet two of its species [holbroohji and hiirtern) have invaded 

 and are now limited to non-xeric environments. It is easy to understand why 

 progressive dryness might stop the dispersal of Amphibia in places, but why 

 progressive moistness (or conditions associated with this) should do so in 

 the other direction is still unexplained. 



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