Vol. X, No. G ] The Gallbladder in certain Rodents. 21& 



[NJ3.] 



■ 



and its close allies, it is not absent from all the Muridae, since 

 it is present in the genus Gerbillus. 



But perhaps the most interesting fact is the curious dis- 

 tribution of the organ among the squirrels. It is present in 

 some species but not in others. 



It is surprising to find that the gall-bladder may be pre 

 sent or absent within the narrow limits of a single genus ', since 

 this organ has been established in the vertebrate series for a 

 longer time even than the limbs, if we are to believe the evi- 

 dence afforded by the Cyclostomata which have a gall-bladder 

 but no limbs. 



Most explanations of organic phenomena that have hitherto 

 been given have started from the idea of utility. Both Teleo- 

 logy and the Selection theory have this common origin. It i- 

 therefore worth while to consider any observations that bear 

 on this subject. 



It seems obvious that within the same genus the presence 

 and absence of the gall-bladder cannot both be advantageous 

 at the moment. It is however possible to imagine that there 

 was a time in the past history of the squirrels when absence of 

 the gall bladder might have been of advantage to onu branch 

 of the genus. It has been shown lately that the gall bladder 

 has a pathological importance. Major E. D W. Greig found 

 that both the typhoid and the cholera bacilli persisted in the 

 gall-bladder long after they had otherwise been eliminated 

 from the body. The ascertained fact that two distinct kinds 



of bacilli have a special predilection for the gall bladder suggest 

 that in certain circumstances it might be advantageous to a 

 race ot animals to lose this organ. 



Having demonstrated that the absence of the gall-bladder 

 might be of advantage, the Selectionist regards its absence as 

 thereby explained. In regard to that explanation we may say 

 definitely, that it is satisfying to some but not to others. 



But let us leave this problem and return again to the 

 facts. It must be admitted, 1 think, that the gall-bladder has 

 dropped out of the Rodent series on more than on * occasion 

 and perhaps on several occasions. If we were to believe that the 

 loss had occurred on one occasion only we should have to believe 

 that the genus Mus was derived from one branch of the squirrels, 

 that which had lost the gall-bladder, and there is no reason 

 for making such an assumption on general anatomical grounds. 

 It seems evident then that Seiurus and Mus lost their gall- 

 bladder on oifferent occasions and more facts would probably 

 show that the organ must have been lost on several occasions 

 among the Rodents alone. Tue loss of the organ In other 

 pans of the vfirtahprate series, in the horse and the saw fish 



1 Seiurus carolinensis is still left in Seiurus by Miller. See U. S. Nat. 

 Miw. Bull. 79. p. 332 (1912). 



