SECT, xiii.] INTRODUCTION. 73 



this it is known that Variation may be discontinuous, I submit 

 that it is easier to suppose that the change from red to purple 

 was from the first complete, and that the choice offered to Selec- 

 tion was between red and purple ; and that the tints of the purple 

 and of the red were determined by the chemical properties of 

 the body to which the colour is due. This case is a particularly 

 interesting one in the light of the fact that, as Mr F. G. Hopkins 

 has lately shewn me, this purple colour, dissolved in hot water, 

 leaves on evaporation a substance which gives the murexide 

 reaction and cannot as yet be distinguished from the substance 

 similarly derived from the orange or yellow colouring matters 

 of Pieridce in general. As was stated above, Mr Hopkins has 

 shewn that these yellows are acids, allied to mycomelic acid, a 

 derivative of uric acid, and therefore of the nature of excret- 

 ory products. Whether the purple body is related to the yellow 

 or to the orange as a salt is to an acid, or otherwise, cannot yet 

 be affirmed ; but if the difference between them is a chemical 

 difference, which can hardly be doubted, there is at least a pre- 

 sumption that the discontinuity of these colours in the several 

 species, is an expression of the discontinuity of the chemical 

 properties of this body. The possibility that from such bodies 

 a series of substances might perhaps by suitable means be pre- 

 pared in such a way as to represent many or even all intermediate 

 shades, does not greatly affect the suggestion made ; for even in 

 such series it is almost certain that points of comparative stability 

 would occur, and Discontinuity would be thus introduced. 



The case of Colour has been taken in illustration because it 

 is the simplest and most intelligible example of the possibility 

 that the Discontinuity of some Substantive Variations is deter- 

 mined by the Discontinuity of the chemical processes by which 

 the structures are produced. It is true that perhaps no species 

 has been rightly differentiated by colour alone, but colour is 

 still one of the many characters which go to the distinguishing 

 of a species, and it is precisely one of the characters whose 

 significance and delimitation by Natural Selection is most 

 obscure. Moreover by the fact that in the case of these yellow 

 and red Pieridse the colours are of an excretory nature, we are 

 reminded that Variation in colour may be an index of serious 

 changes in the chemical economy of the body, and that when an 

 animal is said to be selected because it is red or because it is 

 purple, the real source of its superiority may be not its red colour 

 or its purple colour, but other bodily conditions of which these 

 colours are merely symptoms. By those who have attempted to 

 reconcile the phenomena of Colour with the hypothesis of Natural 

 Selection this fact is too often overlooked. 



But though it may reasonably be supposed that much of the 

 Discontinuity of Variation and some of the Discontinuity of 



