150 Dynamic Theory. 



arises from the sufficiency of one to do the work and the consequent 

 uselessness of the other. This happens in cases where after periods of 

 activity in which high development is reached, a tribe is forced into 

 inactivity. As a prosperous citizen with twelve children builds a house 

 with fifteen rooms. But when the children are married and gone he 

 and his old dame have use for only three. As seen under the subject 

 of rudimentary organs, the vertebrate body is full of these super- 

 numerary and useless, because unused, parts. Other examples will be 

 given presently. The external tube of the vertebrate body therefore is 

 like the union of two individuals, necessary to each other, yet antago- 

 nistical and in severe competition and, like two animals, both depend- 

 ent on the common vegetative internal tube for nourishment. 



It is to be expected that the two sides of this bilateral body should 

 be so joined by nerve connections as that each side being influenced by 

 the action of the other side, the two should act in some degree of har- 

 mony or correspondence. Accordingly we find that each segment in 

 worms and the vertebral segments in the vertebrates are supplied with 

 nerves, so that the limbs and organs on one side are always automatically 

 co-ordinated with those on the other, and this co-ordination is further 

 carried from end to end by the spinal cord. The spinal cord originating 

 from the two-sided body is itself duplex, and what may be called its 

 extensions and additions, constituting the brain; viz., the medulla ob- 

 longata, the cerebellum and the optic lobes, the optic thalami, the cor- 

 pora striata and the cerebrum, are all likewise double. 



Under the universal law that habit and use increase and strengthen a 

 part or organ, while disuse has the contrary effect, to diminish and 

 abort it, when either side possesses activities not shared or balanced by 

 activities of the other side, the two will become unequally developed 

 and there will be, to a certain extent, a Want of Symmetry between them. 

 Our habit of using the right hand more than the left has been followed 

 by its greater development in size and dexterity, and by sympathy the 

 greater size and dexterity of the right leg and foot. And this differ- 

 ence in the bilateral activities of the exterior, react upon the internal 

 anatomy, causing asymmetry in bilateral organs there. Thus the two 

 lungs, which are very directly connected with and essential to external 

 activity, differ materially from each other, the right lung being the 

 largest and divided into three lobes, the left smaller and divided into two. 



The right lobe of the liver is also much larger than the left. The 

 brain also is very directly involved in external activities, the cerebral 

 hemispheres being devoted to the correlation of the conscious and intel- 

 ligent movements. In consequence of the crossing of the nerves in the 

 medulla oblongata, the left hemisphere becomes the organ of the right 

 side of the body and the right hemisphere that of the left side. Ac- 



