62 THE DATA OF BIOLOGY. 



Nutrition, Circulation, Eespiration, and Secretion, in their 

 many subdivisions, go on simultaneously. 



Thus, however well each of these definitions expresses 

 the phenomena of life under some of its aspects, no one of 

 them is more than approximately true. It may turn out, that 

 to find a formula which will bear every test is impossible. 

 Meanwhile, it is possible to frame a more adequate formula 

 than any of the foregoing. As we shall presently find, 

 these all omit an essential peculiarity of vital changes in 

 general — a peculiarity which, perhaps more than any other, 

 distinguishes them from non-vital changes. Before specify- 

 ing this peculiarity, however, it will be well to trace our way, 

 step by step, to as complete an idea of Life as may be reached 

 from our present stand-point : by doing which, we shall both 

 see the necessity for each limitation as it is made, and ulti- 

 mately be led to feel the need for a further limitation. 



And here, as the best mode of determining what are those 

 general characteristics which distinguish vitality fi'om non- 

 vitality, we shall do well to compare the two most unlike 

 kinds of vitality, and see in what they agree. Manifestly, 

 that which is essential to Life must be that which is common 

 to Life of all orders. And manifestly, that which is common 

 to all forms of Life, will most readily be seen on contrasting 

 those forms of Life which have the least in common, or are 

 the most unlike.* 



§ 25. Choosing assimilation, then, for our example of 

 bodily life, and reasoning for our example of that life 

 known as intelligence ; it is fii'st to be observed, that they 

 are both processes of change. Without change, food cannot 

 be taken into the blood nor transformed into tissue : without 



* This paragraph replaces a sentence that, in The Principles of Psiichology, 

 referred to a preceding chapter on " Method ; " in which the mode of procedure 

 here indicated, was set forth as a mode to be -systematically pursued in the choice 

 of hypotheses. Should opportunity ever permit, this chapter on Method will be 

 embodied, along with other matter on the same topic, in a General Introduction 

 to First Principles. 



