APPLICATION TO METAZOA. 125 



high degree of independence morphologically, and 

 are different both in tissue and in function. It is 

 difficult at first glance to conceive how the results 

 of irritability and sensibility can be transmitted 

 from one organ to another so perfectly as in the 

 case of Hydra or the single-celled organism. But 

 we find that all the organs of the mammal are 

 united by particular nerve-tracts to a central ner- 

 vous system, which is affected by nervous impulses 

 from every part of the body, and in its turn is 

 capable of affecting every part of the body. To 

 make good the comparison between the Hydra and 

 the mammal, we must try to conceive how the 

 nervous system of the latter has been evolved 

 from the generally unstable condition of the former, 

 and for this I think no other explanation can be 

 given than that usually set forth, — to the effect 

 that nervous molecular impulses passing through a 

 body would seek the channels of least resistance ; 

 by repeatedly passing through the same channels 

 they would cause a differentiation of the tissue, 

 thus forming definite nerve-tracts. Whether we 

 refer it to the principle of natural selection or the 

 principle of improvement by repetition of action, in 

 either case the facts warrant our conclusion that 

 the complex nerve-tracts perform their office more 

 perfectly than the undifferentiated body tissue. In 



