VAEIATIONS IN POTENCY 



this case be carried no further than the brown 

 stage, because of the lack of some oxidizing 

 agency necessary to the last stage in pigment 

 production. The production of yellow is prob- 

 ably a first or early step in the oxidation 

 process preliminary to the production of brown 

 or black, yet all yellow animals, so far as 

 known, are able to take the further steps ; they 

 retain the capacity to form either brown or 

 black pigment to some extent, if only in the 

 eye. 



The variations thus far described are what 

 De Vries has called retrogressive, i. e. due to 

 loss or modification. A much rarer sort of 

 variation has been called by De Vries progres- 

 sive, i. e. due to gain, acquisition of some char- 

 acter not before possessed by the race. I can 

 call to mind very few cases which certainly 

 fall in this category. One which it would seem 

 must belong here is the rough or resetted con- 

 dition of the hair in guinea-pigs, a variation 

 similar in nature to the reversed plumage of 

 birds, seen, for example, in the Jacobin pigeon. 

 The rough coat of guinea-pigs is surely not 

 an ancestral condition, yet it behaves as a 



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