THE FINAL ADJUSTMENTS 1 25 



do not know this for certain. In one species of fly, 

 also, we know of a gene which makes a wing grow 

 in the place of one of the other appendages, or a foot 

 instead of the antenna, and it certainly seems as if 

 this gene must have altered the evocators. 



There is more evidence that genes can affect 

 individuation fields; our original example of the 

 short-finger gene in man is an example, since clearly 

 this abnormality involves an alteration of the indi- 

 viduation field of the hand rudiment. In fact, all genes 

 which affect patterns, even if they are only patterns 

 of coloured hairs on the skin of a dog, may be said 

 to affect an individuation field. The differences 

 between the races of dogs, whose faces are shown in 

 Fig. 30, are probably, in some cases certainly, caused 

 by genes, and we may say that these genes have 

 altered, though rather slightly, the individuation 

 fields or patterns of their faces. The analogy with 

 the human faces suggests that this alteration has 

 been done by changing the amount of various 

 hormones in the blood-stream, which have then 

 caused alterations in the growth-rates of different 

 parts of the face. It may seem that this has solved 

 the problem, and has shown that an alteration of 

 pattern is only a special case of an alteration of the 

 quantity of a substance which is produced. This 

 may be correct in some cases though in others the 

 alterations produced by genes are more radical and 

 can hardly be explained so simply. Such an ex- 

 planation cannot, however, dispose of the problem 

 of how patterns arise. Because we have still got to 



