ORGANIC EVOLUTION — THE FACTORS' 103 



But we know from analogy that under such circum- 

 stances the kangaroo's eai-s, hke all structures which 

 become useless or less useful, would undergo retrogression. 

 Therefore since reversed selection could not cause the 

 retrogression, it is clear that the cause that would must 

 be some other cause. Only two other theories remain 

 available ; we are driven to attribute this hypothetical 

 retrogression either to Cessation of Selection or to 

 Cessation of Use — to the lapsing of inborn variations or 

 to the lapsing of acquired variations. 



The external ears of the kangaroo, to which, obviously, 

 Mr. Spencer alone refers, are passive structures, moved 

 by muscles perhaps, but not owing any part of their 

 spread to those muscles. Mere fuimels of cartilage and 

 skin, they are directed this way and that to cat«h and 

 conduct sounds, but on them as distinguished from the 

 middle and internal ear the loudest sound makes no 

 greater impression than it does on the animal's tail. It 

 is clear therefore that increase or decrease of functional 

 activity cannot cause them to increase or decrease in size 

 or vary in any other way. Whence it is fm-ther clear, 

 since, in this case, variations due to functional activity 

 cannot be individually acquired, they cannot be trans- 

 mitted, and therefore evolution or retrogression in the 

 external ear cannot, in the slightest degree, be due to 

 use or disuse, to the accumulation or the lapsing of 

 acquired variations, but must be attributed wholly to 

 the accumulation or the lapsing of inborn variations. 



As regards the eye of the proteus, we cannot prove 

 as positively as in the case of the external ear, that its 

 evolution and subsequent retrogression were due wholly 

 first to the accumulation and subsequently to the 

 lapsing of inborn variations, for here variations may 

 be acquired by the individual, and in the individual 

 acquired variations may be lapsed. Nevertheless since 

 the accumulation of inborn variations is sufficient to 



