226 THE iinDUCtio:;s op biology. 



to assume its specific structure resulted from the coopera- 

 tion of its component cells, then a single cell, or the inde- 

 pendent homologue of a single cell, having no other to co- 

 operate with, could exhibit no structural traits. Not only, 

 however, do single-celled organisms exhibit structural traits, 

 but these, even among the simplest, are so distinct as to 

 originate classification into orders, genera, and species; and 

 they are so constant as to remain the same from generation 

 to generation. 



If, then, this organic polarity (as we might figuratively 

 call this proclivity towards a specific structural arrange- 

 ment) can be possessed neither by the chemical units nor 

 the morphological units, we must conceive it as possessed by 

 certain intermediate units, which we may term physiological. 

 There seems no alternative but to suppose that the chemical 

 units combine into units immensely more complex than them- 

 selves, complex as they are; and that in each organism the 

 physiological units produced by this further compounding 

 of highly compound molecules, have a more or less distinc- 

 tive character. We must conclude that in each case some 

 difference of composition in the units, or of arrangement in 

 their components, leading to some difference in their mutual 

 play of forces, produces a difference in the form which the 

 aggregate of them assumes. 



The facts contained in this chapter form but a small part 

 of the evidence which thrusts this assumption upon us. We 

 shall hereafter find various reasons for inferring that such 

 physiological units exist, and that to their specific properties, 

 more or less unlike in each plant and animal, various organic 

 phenomena are due. 



