Bilateralism. 145 



it is reduced to a mere button hidden in a mass of fat. In the Manx 

 Cat, in the higher Apes, and in Man, it is a subcutaneous rudiment 

 which never penetrates the skin, except in some rare cases of reversion 

 or arrested development. 



CHAPTER XXI. 



BILATERALISM. 



Bilateralism, or the possession of two complementary sides, which 

 has reached a considerable degree of perfection among the vertebrates, 

 is a subject of development as much as the differentiation of an inside 

 from an outside. Many of the lowest invertebrates are not bilateral, as 

 Hydra-polyps, Bryozoa, Medusae, Star animals, Echinoderms, &c. Lo- 

 comotion in these animals, where there is any, is by means of limbs ra- 

 diating at equal angles from a common center the mouth or stomach. 

 Most mollusks are, in part, bilateral, as are all the articulated animals, 

 including insects, worms and vertebrates. The development of comple- 

 mental sides seems a most obvious mechanical necessity and result, 

 when we consider the nature of locomotion. The muscles of the tail of 

 a fish, for example, must be able to flap the tail as much to the left side 

 as to the right, or else what motion the fish would have would be ex- 

 tremely one-sided and impracticable. All locomotion depends upon the 

 exertion of muscles, that is, their alternate contraction and relaxation 

 as stimulation reaches them or ceases to do so. Such stimulus causing 

 the contraction of the muscle fibres on one side of a worm, for example, 

 would cause that side to move, but it could not go far without the other 

 side and the motion would tend to be in a circle and without purpose, 

 till the other side should receive like stimulus. Then a definite locomo- 

 tive progression becomes possible. To this necessarily alternate reac- 

 tion of the right and left sides against the stimulus of the contact of the 

 outside environment (which to the earliest organism was water) is, 

 without doubt, to be attributed first, the differentiation from the gen- 

 eral sensibility of the organism, the sense of touch or contact; followed 

 by the further differentiation of the other sense organs from it ; and 

 contemporaneously the differentiation from the general contractility of 

 the organism, of a special and superior contractility to be thenceforward 

 located in the muscles of the sides. Under the special stimulus thus 

 directed to localized points, the muscle fibre has received its peculiar 

 properties and at the same time in connection with the muscle the nerve 

 fibres have been specialized by use for more perfectly conveying the 

 stimulation from a part in direct contact with the stimulus to a remote 

 part not in such direct contact. Any peculiarity in the application of 



