All modern chicken breeds are the results of a cross. a long time ago befor commercial farming ...Farmers would have a direct work plan to improve egg laying , or meat production, or good all purpose bird ... in orpington chickens ,a farmyard favorite but eggs were small for size of chicken and they wanted to increase egg production so they knew what to cross to get that. Australorps set one world record after another with no artificial lighting, the worlds record being 364 eggs in 365 days...the crosses worked.. the old timers being much closer to the farm and animals than we are today understood if you cross this breed with orpingtons you will increase egg production..RIRs bred for winter hardy all around good farm bird.. buckeye with smaller comb cold hardy birds all are crosses .ect...ect... they usually had a plan or need befor they started a project. Once a project bird started breeding true they could call it a breed and ask it be inducted into SOP.