402 JOHN H. GEROULD 



of a given generation (cf. the 'pitch'); genes act differentially 

 upon individuals, determining the particular intensity exhibited 

 by each as compared with its brothers and sisters. 



INADEQUACY OF THE PRESENCE-ABSENCE HYPOTHESIS TO 

 EXPLAIN GRASS-GREEN VS. BLUE-GREEN BLOOD COLOR 



The reader is now asked to return from this digression to the 

 main subject of this paper, pigments derived from chlorophyl, 

 which have little in common with the yellow-orange urates just 

 discussed except that both are dissolved in hemolymph. 



A xanthophylloid yellow element is present in the blood of 

 the dominant grass-green (body and blood color) caterpillar 

 and absent from the recessive blue-green, but it is clearly evident 

 that the dominant character grass-green is produced not by the 

 dominant gene, as the presence-absence theory would imply, 

 but is supplied by the food. We are safe, I believe, in accepting 

 the conclusion of Poulton, Mayer, and later observers that the 

 yellow component of green blood pigment in plant-eating cater- 

 pillars is modified xanthophyl that has left the plastids of the 

 food plant and combined with proteids in the blood plasm. 

 The active factor in this pair of allelomorphs is the recessive, 

 an inhibitor or deteolorizer that attacks and destroys xanthophyl 

 on its way into the hemolymph, or otherwise suppresses the 

 xanthophylloid pigment of normal grass-green blood. The 

 dominant hereditary factor representing the normal grass-green 

 is probably in this case a blank, an absence of the recessive gene, 

 for the physical basis of the dominant yellow-green is furnished 

 by the food. Thus, to fit this case, the presence-absence 

 hypothesis must be inverted. 



ELIMINATION OF THE BLUE-GREEN MUTANTS BY NATURAL 



SELECTION 



English sparrows in the summer of 1920 w^ere a serious hin- 

 drance in my breeding experiments, boldly entering the green- 

 house through the open sash and feeding on caterpillars or pupae 

 that could be reached by pecking through the screening of the 

 cages. Outdoor cultures suffered still worse from the ravages 

 of the birds. 



