DISCUSSION 201 



muscles, and a number of other instances may be recalled. 

 Nevertheless, in considering comparative differences between 

 groups of mammals as closely related as are the three sub- 

 genera of wood rats, it seems logical to consider that but a 

 small proportion of the osteological variation is due to stimuli 

 other than mj^ological, although some of these have been 

 indirect, while others were direct. 



It has been claimed above that the largest muscles need 

 not always be the strongest. In a comparison of mammals 

 of such near relationship, it is to be expected that most 

 muscles of one subgenus are usually, but not always, strictly 

 comparable with the same muscles of the others on size 

 alone. If a process or ridge upon which muscles are attached 

 is more prominent in one subgenus, it is extremely likely 

 that this is because the attached muscles are either stronger, 

 or what amounts to the same thing, better situated for the 

 work to be performed by them. Hence, because it was not 

 apparent during dissection that a muscle of one animal was 

 larger than the same muscle of another, it should not be 

 deemed that appreciable difference in the process to which 

 it was attached is without important myological import. 



For the purpose of comparing the significance of the 

 myological differences with the osteologial ones, a condensed 

 table (table 2) of the latter is here presented. As with the 

 table for muscular variations, Homodontomys is arbitrarily 

 selected as the basic tj^e with which to compare the other 

 two subgenera. Some of the differences listed are too 

 slight to be of definite import to us, while others, perhaps 

 of greater consequence, are not included in the table because 

 of their complexity. 



Many of the difi'erences of percentages in the table im- 

 mediately preceding are too slight to be of value for they 

 are not greater than the coefiicient of error; but the correla- 

 tion of some of these differences and the trends shown 

 thereby are highly interesting and significant. 



