MILES ON NEWTON'S THIRD LAW. 93 



The most satisfactory definitions of "life" and "living matter" are 

 alike in making - continuous molecular changes in response to external 

 impressions the essential characteristic. 



Tseviranus at the beginning of the century defined life as "consisting 

 in the reaction of the organism to external influences," and fifty years 

 later, after an extended discussion of the subject, Herbert Spencer tells 

 us that "the broadest and most complete definition of life will be the 

 continuous adjustment of internal relations to external relations," which 

 is in effect a restatement of the definition of Tseviranus. 



Foster, still later, from the standpoint of the physiologist, defines lift- 

 ing matter as "not a thing or body of a particular chemical composition, 

 but matter undergoing a series of changes" which he likens to a "com- 

 plex whirl" or an "intricate dance" in which chemical composition and 

 histological structure are the figures, and he compares the whole body 

 of. man to a fountain of water. "As the fountain remains the same, 

 though fresh water is continually rising and falling, so the body seems 

 the same, though fresh food is always replacing the old man, which in 

 turn is always falling back to dust, and the conception we are now urg- 

 ing is one which carries an analogous idea into the study of all the 

 molecular phenomena of the body." 



From these definitions of life we may look upon biological processes 

 and activities as modes of motion involving transformations of energy 

 in accordance with Newton's third law which must then be accepted 

 as an important factor in organic evolution. 



As formulated by Newton, this law appears to be alike applicable to 

 organic and inorganic processes. "To every action there is always an 

 equal and contrary reaction; or the mutual actions of any two bodies are 

 always equal and oppositely directed." 



The struggle for existence implies the sum of the actions and reactions 

 of competing species with one another and with their environment, and 

 the fittest to survive are those that most readily conform in habits and 

 requirements to the constantly changing conditions so that an equilib- 

 rium in the conflicting forces with which they have to deal may be main- 

 tained. 



The surviving individuals and species must then be endowed with 

 a plasticity or elasticity of organization that enables them to adapt their 

 functional activities to varying conditions of the environment. 



It follows from this that the first step in the development of a new 

 species must be a physiological or functional adjustment of the organs of 

 certain individuals to changes in the environment, and when these func- 

 tional adaptations are well established by frequent repetition morphologi- 

 cal or structural peculiarities may be developed that are recognized as 

 specific characters, but which are in fact manifestations of the functional 

 adaptions that have preceded them. 



These preliminary steps in the development of a new species can only 

 be observed in living organisms, and in our laboratory methods of study- 

 ing dead organisms we niay fail to distinguish species that are clearly 

 distinct in habits, functional activities and powers of adaption from their 

 resemblance in external or morphological characters. 



In the geographic distribution of species there must be a balance of 

 organisms in conformity with the mutual needs of competing species 



