Bilateralism. 155 



about since the race began to make more use of the right hand than the 

 left. Occasionally the whole six nerves are developed, a circumstance 

 due to arrested development. 



The Narwhal, a cetacean mammal which grows to be twenty or thirty 

 feet long, has in infancy the germs of two horns, one from each inter- 

 maxillary bone. As a rule, however, one of these the right one is 

 suppressed, its gelatinous core being supplanted and smothered by a 

 growth of ivory. The left one, however, is developed into a straight 

 horn from six to ten feet long, grooved spirally. 



Certain others of the Cetacea the Cachelots are unsymmetrical. 

 They have only one blow hole, the left one, instead of the usual two, 

 and the left eye is often much smaller than the right, so that whalers 

 always try to attack that blind side. Many snakes possess but one 

 lung. In some cases where there are two, one is large and useful and 

 the other rudimentary and functionless. This obviously retrograde 

 character of these organs is, as might be expected, associated with 

 degeneracy of the bilateral limbs. One genus of the Orvets (Pseudo- 

 pus) has no visible limbs except two little bones which form small 

 prominences where the thigh bones should be. But under the skin they 

 have pelvis and shoulder bones. Their lungs are double, but one is one- 

 fourth shorter than the other. Others of this family have no vestige of 

 outer limbs, a circumstance accompanied by a still greater disparity in 

 the lungs. The ear also appears to follow the gradations of the lungs. 

 In the best developed, the tympanum lies upon the outside, but in the 

 most retrograde it is sunk in the head and permanently covered with, 

 skin. Other serpents have rudimentary limbs under the skin, and in 

 some, small hook-like hind limbs project outside the skin. In these the 

 lungs are very unequal in size and in many there is only one lung. 



In most of the Amphibians the lungs are equal in size, but in the 

 members of the snake-like group Gymnophiona, the right lung is much 

 reduced and these animals are destitute of limbs. The same character- 

 istics extend into the snake-like genera of the saurian reptiles. The 

 Seps, the Dipodes, Chalcides and Chirotes are all mutilated in their 

 limbs, and some of them are destitute of hind limbs, others of fore 

 limbs. In some the organ of hearing is retrogressive. In all, the lungs 

 are irregular and unequal, and in some one lung is reduced to a mere 

 rudiment. It appears from this that the loss of the bilateral limbs is 

 accompanied with the loss of the bilateral quality of the lungs and con- 

 firms the position taken above that the conversion of single internal 

 organs into double ones is due to the action of the two-sided activities of 

 limbs, &c. , in the first place. 



The case of the Chameleon shows still another peculiar relationship. 

 This reptile has only one lung, and it is an immensely large one, but it 



