You may define hardiness different that others do. In the same way that pure breeds can be breed for increased laying capacity, increased body weight, faster growth rates, etc. They also can be breed for resistance to diseases, resistance to parasites, reduced physical defects etc. While haphazard breeding will not produce better layers, will not produce larger birds, will not produce faster growth rates, will not produce disease resistance, will not produce parasite resistance, etc. Well bred lines however will. My hybirds lay a lot better than my breeds do but my breeds are a much easier keep (i.e. fewer losses, fewer illnesses, etc.). 
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