300 The Unity of the Organism 



being. On the other hand, the concept "cell" being com- 

 mitted to a single type to which it strives to reduce what- 

 ever being it approaches, is locked, as one might say, against 

 the vast diversity that actually exists in the living world. 

 Not only has the cell-theory, strictly understood, no ex- 

 planation of the structural variety of living nature, but 

 by its very essence it tends to minimize the significance of 

 that variety, and to divert attention from it. That the 

 theory always has been and is now narrowing in its influ- 

 ences no candid student will try to deny. 



The injury that the science of microorganisms has suf- 

 fered and to which attention has been called in preceding 

 pages is a notable instance of the tendency of the cell- 

 theory here criticized. 



But docs almost limitless diversity in the conception of 

 "organism" deprive it of value as a generalization? Has 

 it any of the unifying quality upon which the usefulness of 

 a scientific theory depends? The answer to this is two-fold. 

 In the first place, there is just so much unity in the concept 

 organism as there is in the larger and smaller groups of 

 the plant and animal kingdoms. On this side the conception 

 is rooted in comparative anatomy and physiology and tax- 

 onomy. 



But tlie conception's unifying quality par excellence is 

 seen in another direction, namely, that of the elements-in- 

 common of organization in all organisms whatever, so far 

 as their structure is known to us. In all living beings from 

 the largest and most complex to the smallest and simplest, 

 if still visible, the body is differentiated into a surface layer 

 or coat somewhat firmer and denser than the underlying 

 more voluminous parts. We know of no living thing without 

 a skin of some sort. Frequently this is spoken of as a mere 

 lifeless protective structure. But as a matter of fact it is 

 the organism's organ of contact with its environment, and 

 so of the utmost importance for the nutritional, respiratory 



