Note 7 — Chapter VI

I am not aware that the details of this striking case have ever been worked out. It should be noted that the green and blue forms are not due to simple modification of the red pigment; for these colours, due to interference, fork over the area occupied by the red lines. The distinctions between these forms cannot therefore be simply chemical, as we may suppose them to be, for instance, in the case of many red and yellow forms, and the genetic relationships of the Heliconid varieties would raise many novel problems and be well worth studying experimentally.