vni] Discussion of Colour-Evidence 139 



(a fine dark red), on the petals just external to the yellow 

 eye (see Plate VI). These spots are never formed unless 

 the stigma is red. When such a type is crossed with one 

 having a green stigma, F^ has a green stigma and no spots 

 on the petals. In 7% there are of course some with green 

 stigmas and some with red, some with spots and some with 

 no spots. But the distribution of these two characters 

 shows that the combination green stigma + spot on petals 

 does not occur. The stigma may be red though no spot be 

 formed, but if the stigma be green, the spot is absent, though 

 the factor for it may exist in the individual. 



Formerly such cases might have been regarded as 

 examples of "correlation," but that term is only applicable 

 to them in a loose and quite incorrect sense. 



Nothing so fully demonstrates the fundamental signifi- 

 cance of colour in the economy of plants and animals as the 

 strange series of phenomena that have been discovered in 

 regard to the complex inter-relations between the genetic 

 behaviour of certain kinds of pigmentation on the one hand 

 and certain structural features on the other. In the chapters 

 dealing with gametic coupling and with the heredity of Sex 

 it will be shown that not only the factors governing structure, 

 but also the factors which are the ultimate cause of sexual 

 differentiation, may be distributed among the germ-cells 

 according 'to systems which are modified and ordered in 

 inter-dependence on the distribution of the factors for colour. 



Summary and Discussion of the Evidence as to the 

 Genetics of Colour and Colour-Patterns. 



Since we have abundant proof that the development of 

 colour and even of particular colours may be bound up with 

 other features of morphological or physiological importance, 

 it is clearly impossible to regard the genetics of colour- 

 characters as apart from the rest. A summary of the 

 chapters dealing with that subject will nevertheless be useful 

 at this point, and with this may be combined a brief dis- 

 cussion of essential points. 



In many animals and plants colour has been shown 

 experimentally to behave as if due to a single allelomorphic 



