72 CHEMICAL. [IXTROD. 



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

 many organic substances und.-rg.. definite changes when chemi- 

 1 on by reagents, and it is not suggested that the 

 d. Unite,,, .ss and discontinuity of the various colours assumed is 

 dependent on anything but the defiriiteness of the chemical 

 changes undergone. The changes of litmus and many vegetable 

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



i.rown on treatment with alkalies, the colours of the series 

 of Udies produced by the progressive oxidation of bihverdm are 

 tamiliar examples of such definite colour-variations. 



\\ ith facts of this kind in view, the conclusion is almost 



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



quence of the defiuiteness of the chemical changes undergone. 



\ one doubts that the orange colouring matter of the variety 



the Iceland Poppy (P. midicaule) 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 

 tli.- llow.T shall be orange or yellow; and I think it is reasonable 

 to i that the frequency with which the flowers are either 



;..\\ or orange as compared with the rarity of the intermediate 

 >|,;. ,11 expression of the fact that the yellow and orange forms 



bhe colouring matter have a greater chemical stability than the 

 int. iin.ili.it> forms of the pigment, or than a mixture of the two 

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

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

 as it would owe the definiteness of its orange colour and 



tin- ]>ivri>i.>n of its tint, not to the precision with which Selection 

 had eh"s.-n this part ienlar tint, but to the chemical discontinuity of 

 \\liidi th. . .li-inally di-i-oiit innous Variation was the expression. 



To |':i~-- troin ili.- case of a sport to that of Species, it is well 

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

 /.' * J ntlinclni //>-, < )range-tips). some have the apices or 

 tips of th.- foiv -\\in-s orange-red (tor example, E. danae), while 

 in othe^ th'-y are purple (tor example, /,'. lone}. Upon the 



-s that tin- transition from orange to purple, or vice versa, 

 had li.-. n e.nit innoiisly effected liy 1 he successive Selection of 

 niinutf \ariatioii-. \\.- arc met by all tin.- difficulties we know so 



\\i-ll. \Vhy is pnrplr a - I colmir for this creature ? If purple 



i-~ a ^on.l c..lonr and n-d i> a -oml culmir. ho\v did it happen that 

 at -omr tini.' or other all tin- intermediate shades were also good 

 .ii-H-h to ha\e been selected? and so on. These and all the 



grnate ilith'cnlti.- are o|-n.'d up at once, and though they have 

 l'ii met iii the fa-hion we know, they have scarcely been over- 

 come. Hut at tin- outset thU view assumes that every inter- 

 ini-diate may e\i>t and has existed, an assumption which is 



ituit'.iis and hardly in armrdanre with the known fact that 

 eh. miral \<i<- at.- 1 1 , , pie] , i ] \ disron t inuous. When besides 



