1916] HolUger: Anatomical Adaptations in the Pocket Gopher 489 



SUMMARY 



In the course of this study of structural specialization among 

 rodents, certain facts have become apparent which seem applicable 

 outside the limits of the relatively narrow field covered. We 

 therefore venture to suggest the following conclusions as generally 

 true in other forms, as well as among rodents : 



(1) In proportion to their size, fossorial forms have a stronger 

 mechanical equipment than forms adapted to either the cursorial or 

 the arboreal mode of life. The ground being the densest medium 

 in which animals live, those forms which live in it must be provided 

 with the heaviest and strongest mechanism of motion. And we find 

 this mechanism developed in a definite direction, to the end that a 

 particular set of movements may be easily and powerfully per- 

 formed. 



(2) The bony levers found in the animal body are accurately 

 adjusted to the strength of the muscles which operate them, and to 

 the kind of movement which is most needed. The gopher needs to 

 make short, powerful strokes, while the rabbit requires long, swift 

 movements. The bony levers are so arranged that in the former 

 type force is gained at the cost of speed, while the latter speed 

 is gained by sacrificing force. 



(3) The idea that irregularity of outline of bones indicates 

 strength of the osseo-muscular machine is correct. We find the 

 most irregular bones associated with power in the digging types, 

 especially in the gopher. On the other hand, smooth bones are 

 associated with swiftness of movement in the cursorial types. 



(4) Volume of bones is not a criterion of strength of body. 

 The relative volume of the bones in the jack rabbit is larger than 

 in any of the other forms examined. 



(5) The particular sets of muscles which are most used are 

 either actually largest, for example, the flexors of the forearm of 

 the chickaree, or they are made functionally strongest by the 

 arrangement of the bony levers, as in the extensors of the forearm 

 in the gopher. Hence the force of a given set of muscles depends 

 both on its size and on its point of attachment to the mechanical 

 system. 



(6) Domestication reduces specialization. The typical cursorial 

 modifications have either disappeared or have been much reduced in 

 the Belgian hare. 



Transmitted May 1, i914. 



