The 'Genesis sound' (''what ever that means'', as has already been pointed out) has always been evolutionary. If one listens to (or reads about) what the band themselves over the years have said, it is that originally each member brought whatever ideas and themes they had to the practise sessions and these were worked through and bits and pieces accumulated, built up & broken down. Most of the post FGTR/ Pre- Trespass material stored up through this process was recycled over the 3lps that were to come. SEBTP didn't contain any of those older pieces and themes and was definitely a sea-change in how they composed (or not..) as a group. The Lamb did contain some recycled pieces from the '69 period such as 'Anyway' (which appeared with completely different lyrics on the 'Genesis plays Jackson' sessions) and lilywhite Lilith. I personally don't think there is a 'locked down' Genesis sound but even up until Wind & Wuthering they had a very definable characteristic which established very early on. I think 'Yes' are a perfect parallel in that the first Yes lp was very definitely 'Yes' in sound even if it had a journey to go on before they reached 'Close to the edge'.. playing both lps back to back shows that apart from technological development and compositional maturity, you are definitely listening to one band with one definite sound.
Just another life-long fans personal view

