CHAPTER VI 



THE RELATIONSHIP OF EXISTING ORGANISMS 



(Continued) 



4. PHYSIOLOGY 



We have so far dealt primarily with the structure of living 

 things but the action of the parts composing an individual organ- 

 ism, the processes which are even more evidently essentials of life, 

 are no less significant in evolution. The structure of organisms is 

 a more tangible evidence of relationship but it is important to 

 remember that it is only one of the three essential factors in the 

 existence of living things. While environment, response, and 

 hereditary structure are distinct, we cannot have life or individual 

 lives without the intimate correlation of all three. The response 

 of the organism or its functional activity constitutes the subject 

 matter of physiology. 



Fundamental Physiological Properties. Among the character- 

 istics of living matter we find that a certain few l^elong to this 

 category. While chemical composition and definite form are 

 distinctive, the qualities of irritability and contractility are no 

 less so, yet they are of an entirely different nature since their 

 existence depends both on the things which the organism has re- 

 ceived from its ancestors and upon the conditions under which it 

 lives. These physiological properties include irritability, con- 

 ductivity, contractility, metabolism and reproduction. They may 

 be known equally well as the adaptive properties since it is through 

 them that the organism is adjusted to its environment. With the 

 exception of reproduction alone all are involved in the correlation 

 of the individual and its environment, and the excepted property 

 is, of course, essential to the adaptation of species. 



It is in this group of characters that the kingdoms of organisms 

 are chiefly distinct from each other, although there are species 

 which are both plant and animal even in physiological processes. 



In the single-celled organisms, where we can observe the proc- 

 esses of life at their simplest, it is obvious that all of these things 

 are active in any cell, be it plant or animal. Such cells as the little 



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