One of the best ways of showing evolutionary continuity between species and geographic races is to demonstrate that hybridization still occurs between closely related species. In Victorian times and early this century, naturalists were very interested, like "stamp-collectors", in freaks of nature, including rare hybrids between species. Between the 1930s and about 1980, there was decreased interest about the peculiarities of nature, and increased emphasis on the "fundamental" biological "realities" of animal species. (Hybridization between plant species is so abundant and easily shown, of course, that this rather myopic view of "pure", "good" species has never really caught on among botanists). Widely used field guides from this period often omit pictures even of common hybrids of birds and butterflies that can be seen in the wild, while treating much rarer or even extinct species in the same book. Recently, however, there has been renewed interest in biodiversity in all its glory, and it is possible to discern a return of interest in exceptions ("bad species") as well as the good species. In some beautiful recent books, often thorough world treatments of particular groups of butterflies and birds, hybridization between species is again becoming well-documented.
Thanks for taking time to look and for ranking/comments