300 



THE AM ERIC AX XATT'RAJJST 



[Vol. XLV 



Looked at this way, the physical basis of heredity could 

 not be considered a series of equipotent units, but rather 

 it must be regarded as being composed of systems of 

 units of different orders of organization and different 

 degrees of coordination. Alterations in the configura- 

 tion, constitution or relative positions of the unit con- 

 stituents which represent the links of the main protein 

 chain or ring, for instance, would precipitate much 

 deeper-seated changes than would replacement of side- 

 chains by those of different type, and such replacements 

 would, in turn, doubtless appear objectively as differ- 

 ences of greater degree than those resulting from shifts 

 in the composition or configuration of the individual 

 side-chains. 



Our whole scheme of natural classification, in fact, 

 demands just such a physical basis as is depicted for the 

 structure of the protein molecule. For morphological 

 characters are not all equivalent. In any large group 

 certain characters are more conservative than others and 

 represent . more fully the organization, as a^ whole, while 



down to less and le^s inclusive ones until the trivial fea- 

 tures which make up species differences and varietal 

 traits are reached. However, this parallel between the 

 make-up of the protein molecule and the natural classifi- 

 cation of living organisms can be looked on only as a 

 suggestive illustration because in addition to proteins 

 other things often enter into the construction of what we 

 term characters in plants and animals. These characters, 

 indeed, are frequently blends of the effects of numerous 

 influences. 



But as an example of how changes in different parts 

 of the protein molecule might work out visibly in the 

 organism let us see how such alterations actually work 

 out in simpler and better known compounds. In the 

 familiar benzene ring compounds, for example, there 

 exists (1) the main framework or more stable com- 

 ponent, the so-called ring itself, and (2) innumerable 

 substitution groups which can be attached to the "ring" 



