356 THE EVOLUTION THEOKY 



and through several generations, who have a white lock of hair, 

 in a particular spot, on an otherwise dark-haired head. This cannot 

 be referred to external influences, it must depend on a difference in 

 the germ, on one, too, which does not affect the whole body, not even 

 all the hairs of the body, but only those of a particular spot on the 

 surface of the head. It is a matter of indifference whether the white 

 colouring of the hair-tuft is produced by an abnormal constitution of 

 the matrix of the hair, or by other histological elements of the skin, 

 as of the blood-vessels or nerves. It can only depend ultimately on 

 a divergently constituted part of the germ-plasm, which can only affect 

 this one spot on the head, and alter it, if it is itself different from 

 what is usual. On this account I call it the determinant of the 

 relevant skin-spot and hair-group. In Man such minute local 

 variations are usually lost after a number of generations, but in 

 animals there are innumerable phenomena which prove to us that 

 single minute deviations can become permanent. Thus there lives in 

 Central Europe a brown ' blue butterfly,' Lyccena agestis, which has 

 a little black spot in the middle of its wing. The same species also 

 occurs in Scotland, but there, instead of the black spot, it has a 

 milk-white one, and so-called ' eye-spots ' on the under surface of the 

 wing have also lost their black centres. The species has thus varied 

 transmissibly, but only in regard to these particular spots on the wing. 

 A slight variation must therefore have taken place in the germ-plasm 

 which only affects these few parts of the body, or, to express it 

 otherwise, the germ-plasms of the ancestral species and of the variety 

 can only be distinguished by a difference which determines exclusively 

 the scale colour of these spots. The two germ- plasms differ, I should 

 say, only as regards the determinants or' these wing-scales. 



We know from the artificial selection to which Man has subjected 

 and still subjects his domesticated animals and useful plants, that 

 any spots and parts of the body which he chooses can be hereditarily 

 altered, if the desired variations which present themselves are always 

 selected for breeding, and that this does not necessarily cause variation 

 in other parts of the body. When, for instance, in the case cited 

 by Darwin, the comb of a Spanish cock which had previously hung 

 downwards was made to stand upright because a prize had been 

 offered for this character, or when a certain breed of hens was 

 ' furnished with beards,' the results were permanent variations affecting 

 only the parts on which the fancier's attention had been fixed. In 

 the same way, when the tail feathers of the Japanese cock are 

 lengthened to three feet the rest of the plumage does not alter, still 

 less any other part of the body. Of course there are numerous 



