Answer:
Breeder, traits, vary, modify, generations, Artificial selection
Step-by-step explanation:
Darwin looked at the changes plant/animal Breeders produced. The breeders knew traits of individual species vary hence they could modify the species over many generations.
When individuals with desired traits are selected to mate together, This is called Artificial selection (If changes can happen through artificial selection, then it wold make sense that the same thing could happen naturally)