THE CHANGING WORLD OF LIFE 549 



teristics, yet they both had their origin as a wild dog that became do- 

 mesticated in the distant past. 



There seems to be almost no limit to the extent of changes that may 

 be induced through artificial selection. Fig. 34.7 shows the effects of 

 three generations of selection on size of eggs in poultry. Artificial selec- 



Photo by Winchester 



Fig. 34.7. Artificial selection for egg size. The eggs at the top are the average size 

 laid by a flock of chickens raised at Mt. Hope Farm, Williamstown. Mass. After 

 three generations, in which only the largest eggs were selected for incubation, the 

 third-generation hens laid eggs noticeably larger. The eggs at the bottom are ex- 

 amples of the average size of such eggs. 



tion can go faster than natural selection because there can be a more 

 absolute control of the breeding individuals, yet the principles involved 

 are the same. 



Mutation and Selection 



If there were no way in which genes could be changed, selection 

 would necessarily be limited. An animal could advance so far in one 

 direction, but when the genes available in the species were exhausted, 

 advance would have to stop. There is a means of gene change, how- 

 ever, that makes advance unlimited. This method is called mutation. 



