26 ELEMENTS OF BIOLOGY 



elements in protoplasm are arranged into extraordinarily complex 

 molecules that react toward each other because of the elements they 

 contain, and the whole is arranged in a colloidal condition, with 

 all the properties that characterize the colloidal state. 



From the foregoing it is plain that no very complete description 

 of protoplasm as a reaction system is at present possible. It is the 

 purpose of modern Biology to study the reactions by inference 

 from models and reactions in non-living systems, and by experiment 

 on living protoplasm, and thereby to evaluate the roles played by 

 these various physical and chemical phenomena in the energy trans- 

 formations which are described as the characteristics of life. 



Since protoplasm is alive because of its peculiar organization, a 

 very great difficulty is placed before the technical processes involved 

 in analyzing the physico-chemical nature of life. If one trifles with 

 this intricate system with any violence, the organization is de- 

 stroyed and the observer is left with a non-living mass from which 

 the very characters he hoped to study have disappeared. 



Life and Machines. Living organisms are often compared 

 to man-made machines. The analogy is convenient but not very ac- 

 curate. In the sense that both are composed of parts that by trans- 

 formations of energy do work, the comparison is justified. But a 

 living organism possesses powers that Man has thus far been unable 

 to introduce into a machine. No man-made machine can repair 

 itself nor secure its own sources of energy. Furthermore, no machine 

 directs its activities from within itself, nor can it reproduce itself 

 unless directed and controlled by plans that do not originate from 

 its own work, from its own expenditure of energy. 



General Consideration. In the chapters to follow, many 

 phenomena of animal life will be described and discussed. Only 

 occasionally will it be possible to relate or to describe them in terms 

 of the principles of Chemistry and Physics that the present-day 

 biologist knows to be their fundamental nature. With a background 



