OltGAXISiVS AND THEIR MEDIA. 215 



ORGANISMS AND THEIR MEDIA. 



By H. CHAELTON BASTIAN, M. D., F. R. S. 



HEAT and light are physical influences to which even the lowest 

 units of living matter respond, whether their mode of life and 

 nutrition is most akin to that of plants or to that of animals. These 

 influences act on such organisms, either by stimulating, retarding, or 

 otherwise modifying the chemical changes naturally occurring in 

 their interior, and upon the existence of which their life depends. 

 "Where the vital processes of the organism are stimulated by these 

 physical agencies, their incidence may, in many instances, become the 

 cause of so-called " spontaneous movements." The term, however, 

 as applied to movements, is a bad one since all the movements oi 

 an organism are alike dependent upon a series of antecedent states of 

 contractile and other tissues. There is some sort of foundation, it is 

 true, for the popular mode of expression. A movement is not said to 

 be " spontaneous " if it follows immediately upon some external im- 

 pression as a cause ; the term is generally applied where the cause of 

 the movement is not distinctly recognizable. In some instances the 

 undetected or unconsidered external cause may be the incidence of a 

 diffused physical agent such as heat, which, by stimulating the vital 

 processes, seems to give rise to spontaneous movements. In other 

 cases so-called "spontaneous" movements are to be referred to inter- 

 nal states or changes, whose origin is even less distinctly traceable, 

 to impressions, it may be, which emanate from some of the internal 

 organs, and thence are transmitted to ganglia in direct relation with 

 some of the organs of locomotion. 



Heat mostly acts on organisms upon all sides alike, so that, 

 though it may stimulate their life-processes generally, and, in some 

 instances, give rise to movements, these movements are not deter- 

 mined in one, more than in another, direction. Thus, while heat 

 stimulates the " to-and-fro " or the gyratory movements of bacteria, 

 and also renders more striking and rapid those changes of form which 

 all amoeboid organisms are apt to display, the movements evoked are 

 random, and apparently devoid of all purpose. 



It is not altogether similar, however, with the influence of light. 

 This agent almost always, and necessarily, falls more on one side than 

 on another ; and consequently it often suffices to induce movements 

 to be made in definite directions, by the lower forms of life, just as it 

 causes definite and responsive movements to be executed by certain 

 parts of higher plants, which come fully under its influence. In each 

 case the movement, or altered position, is due to some nutritive 

 change; that is, to some alteration, whatever its nature, in the ac- 

 tivity of the life-processes taking place in the part impressed by the 



