But we know from analogy that under such circumstances the kangaroo's ears, like 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 funnels of cartilage and skin, they are directed this way and that to catch 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 further clear, since, in this case, variations due to functional activity cannot be individually acquired, they cannot be transmitted, 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
