ORGANISMS AND THEIR MEDIA. 223 



which common sensibility is most frequently called into play. And 

 just as this common sensibility is a crude or general sense of touch, so 

 are the several special senses only more or less highly-refined modes ol 

 the same sense-endowment. In the case of special tactile organs, of 

 organs of taste and organs of smell, the several contacts between the 

 animal and the body which impresses it, though differing in their deli- 

 cacy or refinement, are still immediate ; while in the case of the or- 

 gans of hearing and the organs of vision the contact between the sensi- 

 tive surfaces and the impressing body is mediate, by the intervention 

 in the one case of vibrations transmitted through water or air, and, 

 in the other, of vibrations from the often far-distant luminous body, 

 through an intermediate and all-pervading ether. 



The movements of locomotion, or of parts of the organism which 

 become established in correspondence with these various impressions, 

 slowly increase in number, definiteness, and complexity. Such re- 

 sponsive movements, however, are found, as a general rule, to liave 

 the effect of prolonging the action of any influences which previous 

 individual or race experiences have proved to be favorable to the life 

 and well-being of the organism ; and, on the other hand, of cutting 

 short or avoiding influences which past individual or race expei'iences 

 have proved to be contrary to its general well-being. The capture 

 and swallowing of food are ends to which a very large proportion 

 indeed of the definite motions of most of the lower organisms are 

 directed ; and this direction of their energies is only a special case to 

 be included under the rule above indicated; just as efforts to escape 

 from predatory neighbors are other, though opposite, instances of the 

 same rule. 



In addition to the various modes of impressibility by external in- 

 fluence which we have hitherto been considering, there are certain 

 internal modes of impressibility due to changes in the condition of 

 internal parts of the organism. These are commonly spoken of as 

 divisible into two categories : 1. The impressions derivable from, or 

 in some way attendant upon, the contractions of muscles ; and, 2. Im- 

 pressions emanating from one or other of the various sets of internal 

 organs, such as the alimentary canal and its appendages, the respira- 

 tory organs, the genital organs, or other internal parts. 



With the first set of impressions we have at present nothing to do. 

 They differ altogether from others, whether of external or internal 

 origin, by the fact that they follow or accompany movements whose 

 intensity they are supposed to measure, and do not themselves lead 

 to movements. Granting that such impressions may have a real 

 existence, it is obvious we can know nothing about them among 

 invertebrate animals, if they have only a subjective existence, and do 

 not cause an efflux of molecular movements along outgoing nerve- 

 fibres. 



The second category of internal impressions those emanating 



