72 CHEMICAL. [ixtrod. 



that of colour, we are familiar with the fact that the colours of 

 many organic substances undergo definite changes when chemi- 

 cally acted on by reagents, and it is not suggested that the 

 definiteness and discontinuity of the various colours assumed is 

 dependent on anything but the definiteness of the chemical 

 changes undergone. The changes of litmus and many vegetable 

 blues to red on treatment with acids, of many vegetable yellows 

 to brown on treatment with alkalies, the colours of the series 

 of bodies produced by the progressive oxidation of biliverdin are 

 familiar examples of such definite colour-variations. 



With facts of this kind in view, the conclusion is almost 

 forced on us that the definiteness of colour-variation is a conse- 

 quence of the definiteness of the chemical changes undergone. 

 No one doubts that the orange colouring matter of the variety 

 of the Iceland Poppy (P. nudicaule) is a chemical derivative 

 from the yellow colouring matter of the type. It is not ques- 

 tioned that in such cases a definite alteration in the chemical 

 conditions in which the pigment is produced determines whether 

 the flower shall be orange or yellow ; and I think it is reasonable 

 to expect that the frequency with which the flowers are either 

 yellow or orange as compared with the rarity of the intermediate 

 shades is an expression of the fact that the yellow and orange forms 

 of the colouring matter have a greater chemical stability than the 

 intermediate forms of the pigment, or than a mixture of the two 

 pigments. If then it should happen, as we may fairly suppose it 

 might, that the orange form were to be selected and established 

 as a race, it would owe the definiteness of its orange colour and 

 the precision of its tint, not to the precision with which Selection 

 had chosen this particular tint, but to the chemical discontinuity of 

 which the originally discontinuous Variation was the expression. 



( r To pass from the case of a sport to that of Species, it is well 

 known that of the many S. African butterflies of the genus 

 Euchloe ( = Anthocharis, Orange-tips), some have the apices or 

 tips of the fore-wings orange-red (for example, E. danae), while 

 in others they are purple (for example, E. ione). Upon the 

 view that the transition from orange to purple, or 2iice_jversa f 

 had been continuously effected by the successive Selection of 

 minute variations, we are met by all the difficulties we know so 

 well. Why is purple a good colour for this creature ? If purple 

 is a good colour and red is a good colour, how did it happen that 

 at some time or other all the intermediate shades were also good 

 enough to have been selected ? and so on. These and all the 

 cognate difficulties are opened up at once, and though they have 

 been met in the fashion we know, they have scarcely been over- 

 come. But at the outset this view assumes that every inter- 

 mediate may exist and has existed, an assumption which is 

 gratuitous and hardly in accordance with the known fact that 

 chemical processes are frequently discontinuous. When besides 



