ORGANIC MATTER. 5 



arrangement. Hence this frequency of its occurrence among 

 the components of organic matter is significant as implying 

 a further kind of molecular mobility. 



One more fact, that is here of great interest for us, must 

 be set down. These four elements of which organisms are 

 almost wholly composed, exhibit certain extreme unlike- 

 nesses. While between two of them we have an unsurpassed 

 contrast in chemical activity; between one of them and the 

 other three, we have an unsurpassed contrast in molecular 

 mobility. While carbon, until lately supposed to be infusible 

 and now volatilized only in the electric arc, shows us a de- 

 gree of atomic cohesion greater than that of any other known 

 element, hydrogen, oxygen, and nitrogen show the least 

 atomic cohesion of all elements. And while oxygen dis- 

 plays, alike in the range and intensity of its affinities, a 

 chemical energy exceeding that of any other substance (un- 

 less flourine be considered an exception), nitrogen displays 

 the greatest chemical inactivity. Now on calling to mind 

 one of the general truths arrived at when analyzing the pro- 

 cess of Evolution, the probable significance of this double 

 diiference will be seen. It was shown {First Principles, 

 § 163) that, other things equal, unlike units are more easily 

 separated by incident forces than like units are — that an in- 

 cident force falling on units that are but little dissimilar 

 does not readily segregate them; but that it readily segre- 

 gates them if they are widely dissimilar. Thus, the sub- 

 stances presenting these two extreme contrasts, the one be- 

 tAveen physical mobilities, and the other between chemical 

 activities, fulfil, in the highest degree, a certain further con- 

 dition to facility of diiferentiation and integration. 



§ 2. Among the diatomic combinations of the three ele- 

 ments, hydrogen, nitrogen and oxygen, we find a molecular 

 mobility much less than that of these elements themselves ; at 

 the same time that it is much greater than that of diatomic 

 compounds in general. Of the two products formed 



