144 THE AMEBIC AN NATURALIST [Vol. XLVI 



not wholly, in the more specialized portion of any stock 

 and in some cases at least appear to be associated with 

 the fading-out or senescence of that particular branch. 

 This suggests the possibility that the potency of a biotype 

 ultimately alters, even though there is no visible or tan- 

 gible evidence of change. 



A second problem which puzzles the systematist is the 

 variability in the value of a character for distinguishing 

 species, genera and even higher groups. Color is a fa- 

 miliar example of this. It is of real value among birds 

 and in numerous other cases, but is almost worthless 

 among many invertebrates. Does the knowledge of the 

 existence of biotypes help us to understand why this is? 

 At first thought one might say that here again the inequi- 

 potency of the biotypes was the explanation of the phe- 

 nomenon, but further consideration will show that this is 

 not the case, for of course the potency of a biotype will 

 involve all of its characteristic determinants and not 

 merely that or those associated with the character in 

 question. It is clear then that the value of any char- 

 acter for distinguishing species from each other— in 

 other words, its value for systematic work — depends on 

 the actual determinants in the genoplastic groups com- 

 posing those species. The variability in systematic value 

 shown by a given character is due then, not to the 

 potency, but to the composition of the biotypes involved. 

 Thus if all the biotypes contain identical color determi- 

 nants, then color will be an absolutely constant char- 

 acter in that species, but the greater the diversity in the 

 color determinants of the biotypes the more variable will 

 the color of the species be and the less useful the color 

 be as a distinguishing character. Conversely, we may 

 say that the value of color in systematic work will depend 

 on the degree of identity in color-determinants among 

 the biotypes composing the species concerned. If this is 

 so, the study of systematic characters and the measur- 

 ing of their diversity may suggest some characteristics of 

 biotypes as yet unsuspected. Thus biometrical work 



