One of the most famed progressive rock bands out there originally started off as a blues rock band. Yes, that sentence can describe Rush, but it also describes Jethro Tull. Yet, even back then, when their music was mostly made up of classic blues progressions, Jethro Tull had something extra about them. For one, Ian Anderson and his famous flute, which he uses to great effect on several of the record's best tracks. For two, their knowledge of jazz and their decision to include a cover of Roland Kirk's "A Serenade To A Cuckoo", which proves to be one of the best songs on the album. Through these things, you can see the beginnings of something special. It was because Mick Abrahams leaving of the band, as he wanted to continue in blues, that Ian Anderson was allowed creative control over the band, which eventually led to them becoming the progressive rock giants they are known as today.