488 University of California Puhlicatians in Zoology [Vol. 13 



It is evident from the above table that the force of the move- 

 ment produced by muscles of group 5 is considerably greater in 

 the gopher than in any of the other forms, notwithstanding its 

 smaller relative size. The ground squirrel ranks second in the 

 available work developed by these muscles. Of course, what is 

 gained in force is lost in speed of movement, but speed is not so 

 essential in the digging forms as is strength. In the jack rabbit, 

 however, rapidity of movement is essential, and we find that in this 

 animal force is sacrificed for speed. The three rabbits stand lowest 

 as regards the real force of these muscles, but highest in the speed 

 of movement produced. It is not apparent why group 5 in the 

 chickaree should be so strong as it appears in table 2. 



While complete accuracy is not claimed for the figures in table 

 2, they serve to bring out the fact that the efficiency of a muscle 

 or group of muscles is to a great extent dependent upon its point 

 of insertion. 



What has been worked out in detail in this' particular case is 

 only an example of what could undoubtedly be found in every part 

 of the mechanical structure of these forms. In the fossorial type 

 we should find speed sacrificed for force, and in the cursorial 

 forms force sacrificed for speed. 



Group 6, the flexors of the forearm, as shown by table 1, are 

 relatively larger in the chickaree than in any of the other rodents. 

 This is what would be expected, since these muscles are of great 

 importance in climbing trees. Of the other rodents this group is 

 smallest in the rabbits. 



Group 7, the extensors of the hand, are largest in the two 

 squirrels and smallest in the rabbits. This is another case in which 

 the correlation between habit and anatomy is not clear. In all 

 these cases there are undoubtedly some influencing factors which 

 we have not taken into account. 



Group 8, the flexors of the hand, we should expect to find 

 largest in the digging and climbing forms, and this is what table 1 

 actually indicates. The group is relatively largest in the gopher, 

 and in this animal its force is increased by the fact that the largest 

 muscle in the group, the flexor carpi ulnaris, is inserted on the high 

 pisiform bone. The chickaree ranks next in development of this 

 group, and the ground squirrel third. In the rabbits these muscles 

 are small. 



