The Physiological Unit. 1 7 1 



thing are all traceable to modifications of these units. 

 Each one of them is thus complex beyond the power 

 of thought to imagine, and the multiformity in their 

 intricate structure is beyond the power of figures to 

 express. Yet these varieties of character are necessary 

 to the evolutionist's explanation of the diversity seen 

 in organized nature. The explanation is not less 

 complex than the thing explained. 



5. The physiological units have "a more or less 

 distinctive character." " The form of each species of 

 organism is determined by a peculiarity in the struc- 

 ture of its units." These units " have a special struc- 

 ture in which they tend to arrange themselves." They 

 have " an innate tendency to arrange themselves into 

 the shape of the organism to which they belong." " A 

 plant or animal of any species is made up of special 

 units, in all of which there dwells the intrinsic apti- 

 tude to aggregate into the form of that species." They 

 show a " proclivity towards a particular arrangement."* 

 These characteristics are ancestral : they are inherited. 

 The extreme modifiability of organic aggregates of 

 molecules is the property most frequently brought 

 into view by Mr. Spencer. He has been at great 

 pains to illustrate the instability of the organic com- 

 pounds. Yet each physiological unit, however mobile, 

 has stability enough to retain the impress of its 

 ancestry, and to perpetuate it by reproduction. At 



* Biolbgy, Vol. I., 65. 



