No. 480] 



EXTJ.\( r/().\ OF MAMMALIA 



855 



results have been de^^eneracy or extinction. On tlie other hand 

 plants and animals of unspecialized habits have survived." ' 



Dominant Cliaractcrs. — Characters which have reached an 

 extreme stage so as to demand a larger share of the sum total of 

 bodily nutrition than their general or apparent utility justilics 

 may be known as dont'mant (mjam. 'Fhey appear to violate the 

 law of economy of growth, or the maximum combination of favor- 

 able characters by the subservience of each part to the whole. 



The great horns of the Titanotheres, the tooth of the nai whal, 

 the tusks of the Babirussa, the horns of tiie Irish deer, Megaeeros, 

 the tusks of the mammoths, E. primigenim, E. rolunih/, and /;. 

 imperator, are cases in point. The tusks of the elephants, however, 

 serve a variety of useful purposes. 



Overdevelopment of such organs has long been considered 

 among the possible causes of extinction. The overdeveloj)nient 

 itself has recently been explained by F. B. Loom is ^ as follows: 



"The abov^e are selected examples in which a feature once 

 useful has been developed beyond its maxinnmi utility. "Many 

 others e(|nally strikinir nn'o-ht he cited, the explanation of all o> 

 which is extremely .lifficult unle>s <urh a factor as momentum is 

 called in. In the light of this factor, however, a logical and ap- 

 parent cause is found. Momcntimi also exj)lains why a character 

 that originated in accordance with the environment develops so 



surroundings, it still goes on beyond a perfect adjustment. It 

 may be laid down as a rule then that a ranafioii starivd al(>n<i an// 

 line fends to carry that line of drvclopuiott to its ultimate, hritn/ 

 driven hj momentum, if the feature is detrimental the group 

 dies out; if, however, it is merely a minor feature it makes a handi- 

 cap. A line of development may be stopped and its momentum 

 overcome but the tendency is to keep right on." 



Sr/rctio/i of ('srlcss Dominant Organs.— Auoihev explanation 

 may be offered for certain male .lonn-nant eharacters. namely, 

 tha't by sexual selection <.r compc^tition between the males for 



