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THE AMEBIC AX NATURALIST [Vol. L 



Experiment B 



In 1912 I started, on a small scale, an experiment which 

 had for its object the modification of the agouti pattern 

 by repeated crossing with black (non-agouti) animals. 

 It was thought that if the gametes of agouti animals 

 showed quantitative variation in the factor underlying 

 the agouti pattern repeated crossing with non-agoutis 

 would succeed in decreasing the amount of the agouti 

 character. It follows that if this was accomplished by 

 contamination between the agouti gamete and the non- 

 agouti (black) gamete, the non-agouti animals should 

 show increasing traces of the agouti character as the 

 agouti animals showed a diminution of that character. 



The method followed was to cross black agouti animals 

 derived from a sooty yellow stock with blacks from the 

 same stock and selecting the blackest agoutis from each 

 litter repeat the process. All the agoutis used were thus 

 heterozygous for the agouti factor. This process was re- 

 peated for more than seven generations during which 

 approximately 400 young were recorded. From the out- 

 set two facts were evident. First, the agoutis grew dis- 

 tinct!?/ blacker: second, there ivas no corresponding sign 

 of contamination on the part of the non-agouti animals, 

 but they too greiv blacker. It appears, therefore, that 

 there was no sign of modification of the factor underly- 

 ing the agouti pattern; but that a race was isolated by 

 selection which showed a distinct increase in depth of pig- 

 mentation. In this connection it is interesting to note that 

 while the agouti animals grew blacker and the yellow 

 areas in the hair decreased in extent, the yellow pigment 

 itself was a deeper richer yellow than that of the ordi- 

 nary black agoutis. The yellow pigment had been in- 

 creased in depth while the black pigment was being in- 

 creased in depth and in extent, two entirely distinct 

 processes. 



AH the agoutis up to the third generation were gray- 

 bellied agoutis usually having more dark pigment on the 

 ventral surface than is found in wild mice. In the third 

 generation there occurred an agouti with a distinctly yel- 



