Sentence:  The Henny strain never breeds a long-tailed cock unless crossed. The contrary has been stated by many persons, but the mistake has arisen from various birds which have not been <WSD>true-bred</WSD> Henny, but only the product of some cross; such will, of course, breed both kinds of plumage, but the <WSD>true-bred</WSD> Henny  never does. 
Meaning of the word marked with <WSD> tags:  Purebred; genetically pure.