976 Dynamic Theory. 



bodies, we commonly suppose are not accompanied by sensibility (al- 

 though it would be difficult to prove it ). But at the other extreme, in 

 the highest organism, man, we find precisely the same rays of light and 

 heat produce these chemical and other physical effects and at the same 

 time set up sensation. This shows that between these extremes there 

 has occurred an evolution of organs so differing in character, that the 

 energ} 7 which results in the organism from the impact of the suns rays 

 on the outside, is split up, part of it arousing sensibility and the rest 

 effecting other sorts of changes in brain cells and other tissues. If a 

 beam of light strikes the face it tans the skin, a chemical effect ; it also 

 raises the temperature, a physical effect ; it contracts the pupil of the 

 eye, a mechanical effect ; it excites the rods and cones sending currents 

 into the brain which subdivide, some going to the organs of sensation 

 and producing a sensible effect ; while others rearrange organs in the 

 internal sense region and produce a mental effect. A different pair of 

 sensible and mental effects are also produced by currents sent to the 

 brain from the heated skin, a secondary effect. Now these various ef- 

 fects of the apparently simple energy are not exhibited until various 

 sorts of organs have become differentiated. But this differentiation be- 

 gins and makes progress before it shows itself on the outside, and the 

 distinction between impression and sensibility may become sketched 

 out further down in the scale of being than we are apt to imagine. All 

 intermolecular spaces are filled with ether, and its agitation by any ex- 

 ternal force has some effect upon the body enclosing it. Such effect 

 under certain conditions is in the direction of organization, so that we 

 may say that ether ensconced in denser matter and subject to the in- 

 fluence of external energy, furnishes conditions which include the poten- 

 tiality of sensibility as well as impression. Both of these functions are 

 performed distinctly, as it appears to me, in all animals possessed of 

 ganglions and nerves, and especially all that exhibit signs of pleasure 

 and pain ; and among these may be included all that make an effort to 

 get food. "We have evidence of impression being expanded to thought 

 in molusks and articulates, and certainly whenever the memory is ex- 

 hibited it is sufficient evidence that this differentiation has been accom- 

 plished; for recollection is thought. In all probability there has been a 

 a pari passu specialization of thought, and sensibility from mere im- 

 pression, so that we shall find an approximate correspondence between 

 the two wherever they occur; but not necessarily an exact one, because 

 it is possible for the development of either to proceed in a limited de- 

 gree independently of the other. 



As there is a progressive development of sensibility from lower to 

 higher organisms, so there is reason to believe there is a different de- 

 gree of it in different tissues of the same organism. In the inverte- 



